diff options
Diffstat (limited to 'doc/src/sgml/runtime.sgml')
| -rw-r--r-- | doc/src/sgml/runtime.sgml | 2773 |
1 files changed, 2773 insertions, 0 deletions
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml new file mode 100644 index 0000000..2a34716 --- /dev/null +++ b/doc/src/sgml/runtime.sgml @@ -0,0 +1,2773 @@ +<!-- doc/src/sgml/runtime.sgml --> + +<chapter id="runtime"> + <title>Server Setup and Operation</title> + + <para> + This chapter discusses how to set up and run the database server, + and its interactions with the operating system. + </para> + + <para> + The directions in this chapter assume that you are working with + plain <productname>PostgreSQL</productname> without any additional + infrastructure, for example a copy that you built from source + according to the directions in the preceding chapters. + If you are working with a pre-packaged or vendor-supplied + version of <productname>PostgreSQL</productname>, it is likely that + the packager has made special provisions for installing and starting + the database server according to your system's conventions. + Consult the package-level documentation for details. + </para> + + <sect1 id="postgres-user"> + <title>The <productname>PostgreSQL</productname> User Account</title> + + <indexterm> + <primary>postgres user</primary> + </indexterm> + + <para> + As with any server daemon that is accessible to the outside world, + it is advisable to run <productname>PostgreSQL</productname> under a + separate user account. This user account should only own the data + that is managed by the server, and should not be shared with other + daemons. (For example, using the user <literal>nobody</literal> is a bad + idea.) In particular, it is advisable that this user account not own + the <productname>PostgreSQL</productname> executable files, to ensure + that a compromised server process could not modify those executables. + </para> + + <para> + Pre-packaged versions of <productname>PostgreSQL</productname> will + typically create a suitable user account automatically during + package installation. + </para> + + <para> + To add a Unix user account to your system, look for a command + <command>useradd</command> or <command>adduser</command>. The user + name <systemitem>postgres</systemitem> is often used, and is assumed + throughout this book, but you can use another name if you like. + </para> + </sect1> + + <sect1 id="creating-cluster"> + <title>Creating a Database Cluster</title> + + <indexterm> + <primary>database cluster</primary> + </indexterm> + + <indexterm> + <primary>data area</primary> + <see>database cluster</see> + </indexterm> + + <para> + Before you can do anything, you must initialize a database storage + area on disk. We call this a <firstterm>database cluster</firstterm>. + (The <acronym>SQL</acronym> standard uses the term catalog cluster.) A + database cluster is a collection of databases that is managed by a + single instance of a running database server. After initialization, a + database cluster will contain a database named <literal>postgres</literal>, + which is meant as a default database for use by utilities, users and third + party applications. The database server itself does not require the + <literal>postgres</literal> database to exist, but many external utility + programs assume it exists. There are two more databases created within + each cluster during initialization, named <literal>template1</literal> + and <literal>template0</literal>. As the names suggest, these will be + used as templates for subsequently-created databases; they should not be + used for actual work. (See <xref linkend="managing-databases"/> for + information about creating new databases within a cluster.) + </para> + + <para> + In file system terms, a database cluster is a single directory + under which all data will be stored. We call this the <firstterm>data + directory</firstterm> or <firstterm>data area</firstterm>. It is + completely up to you where you choose to store your data. There is no + default, although locations such as + <filename>/usr/local/pgsql/data</filename> or + <filename>/var/lib/pgsql/data</filename> are popular. + The data directory must be initialized before being used, using the program + <xref linkend="app-initdb"/><indexterm><primary>initdb</primary></indexterm> + which is installed with <productname>PostgreSQL</productname>. + </para> + + <para> + If you are using a pre-packaged version + of <productname>PostgreSQL</productname>, it may well have a specific + convention for where to place the data directory, and it may also + provide a script for creating the data directory. In that case you + should use that script in preference to + running <command>initdb</command> directly. + Consult the package-level documentation for details. + </para> + + <para> + To initialize a database cluster manually, + run <command>initdb</command> and specify the desired + file system location of the database cluster with the + <option>-D</option> option, for example: +<screen> +<prompt>$</prompt> <userinput>initdb -D /usr/local/pgsql/data</userinput> +</screen> + Note that you must execute this command while logged into the + <productname>PostgreSQL</productname> user account, which is + described in the previous section. + </para> + + <tip> + <para> + As an alternative to the <option>-D</option> option, you can set + the environment variable <envar>PGDATA</envar>. + <indexterm><primary><envar>PGDATA</envar></primary></indexterm> + </para> + </tip> + + <para> + Alternatively, you can run <command>initdb</command> via + the <xref linkend="app-pg-ctl"/> + program<indexterm><primary>pg_ctl</primary></indexterm> like so: +<screen> +<prompt>$</prompt> <userinput>pg_ctl -D /usr/local/pgsql/data initdb</userinput> +</screen> + This may be more intuitive if you are + using <command>pg_ctl</command> for starting and stopping the + server (see <xref linkend="server-start"/>), so + that <command>pg_ctl</command> would be the sole command you use + for managing the database server instance. + </para> + + <para> + <command>initdb</command> will attempt to create the directory you + specify if it does not already exist. Of course, this will fail if + <command>initdb</command> does not have permissions to write in the + parent directory. It's generally recommendable that the + <productname>PostgreSQL</productname> user own not just the data + directory but its parent directory as well, so that this should not + be a problem. If the desired parent directory doesn't exist either, + you will need to create it first, using root privileges if the + grandparent directory isn't writable. So the process might look + like this: +<screen> +root# <userinput>mkdir /usr/local/pgsql</userinput> +root# <userinput>chown postgres /usr/local/pgsql</userinput> +root# <userinput>su postgres</userinput> +postgres$ <userinput>initdb -D /usr/local/pgsql/data</userinput> +</screen> + </para> + + <para> + <command>initdb</command> will refuse to run if the data directory + exists and already contains files; this is to prevent accidentally + overwriting an existing installation. + </para> + + <para> + Because the data directory contains all the data stored in the + database, it is essential that it be secured from unauthorized + access. <command>initdb</command> therefore revokes access + permissions from everyone but the + <productname>PostgreSQL</productname> user, and optionally, group. + Group access, when enabled, is read-only. This allows an unprivileged + user in the same group as the cluster owner to take a backup of the + cluster data or perform other operations that only require read access. + </para> + + <para> + Note that enabling or disabling group access on an existing cluster requires + the cluster to be shut down and the appropriate mode to be set on all + directories and files before restarting + <productname>PostgreSQL</productname>. Otherwise, a mix of modes might + exist in the data directory. For clusters that allow access only by the + owner, the appropriate modes are <literal>0700</literal> for directories + and <literal>0600</literal> for files. For clusters that also allow + reads by the group, the appropriate modes are <literal>0750</literal> + for directories and <literal>0640</literal> for files. + </para> + + <para> + However, while the directory contents are secure, the default + client authentication setup allows any local user to connect to the + database and even become the database superuser. If you do not + trust other local users, we recommend you use one of + <command>initdb</command>'s <option>-W</option>, <option>--pwprompt</option> + or <option>--pwfile</option> options to assign a password to the + database superuser.<indexterm> + <primary>password</primary> + <secondary>of the superuser</secondary> + </indexterm> + Also, specify <option>-A scram-sha-256</option> + so that the default <literal>trust</literal> authentication + mode is not used; or modify the generated <filename>pg_hba.conf</filename> + file after running <command>initdb</command>, but + <emphasis>before</emphasis> you start the server for the first time. (Other + reasonable approaches include using <literal>peer</literal> authentication + or file system permissions to restrict connections. See <xref + linkend="client-authentication"/> for more information.) + </para> + + <para> + <command>initdb</command> also initializes the default + locale<indexterm><primary>locale</primary></indexterm> for the database cluster. + Normally, it will just take the locale settings in the environment + and apply them to the initialized database. It is possible to + specify a different locale for the database; more information about + that can be found in <xref linkend="locale"/>. The default sort order used + within the particular database cluster is set by + <command>initdb</command>, and while you can create new databases using + different sort order, the order used in the template databases that initdb + creates cannot be changed without dropping and recreating them. + There is also a performance impact for using locales + other than <literal>C</literal> or <literal>POSIX</literal>. Therefore, it is + important to make this choice correctly the first time. + </para> + + <para> + <command>initdb</command> also sets the default character set encoding + for the database cluster. Normally this should be chosen to match the + locale setting. For details see <xref linkend="multibyte"/>. + </para> + + <para> + Non-<literal>C</literal> and non-<literal>POSIX</literal> locales rely on the + operating system's collation library for character set ordering. + This controls the ordering of keys stored in indexes. For this reason, + a cluster cannot switch to an incompatible collation library version, + either through snapshot restore, binary streaming replication, a + different operating system, or an operating system upgrade. + </para> + + <sect2 id="creating-cluster-mount-points"> + <title>Use of Secondary File Systems</title> + + <indexterm zone="creating-cluster-mount-points"> + <primary>file system mount points</primary> + </indexterm> + + <para> + Many installations create their database clusters on file systems + (volumes) other than the machine's <quote>root</quote> volume. If you + choose to do this, it is not advisable to try to use the secondary + volume's topmost directory (mount point) as the data directory. + Best practice is to create a directory within the mount-point + directory that is owned by the <productname>PostgreSQL</productname> + user, and then create the data directory within that. This avoids + permissions problems, particularly for operations such + as <application>pg_upgrade</application>, and it also ensures clean failures if + the secondary volume is taken offline. + </para> + + </sect2> + + <sect2 id="creating-cluster-filesystem"> + <title>File Systems</title> + + <para> + Generally, any file system with POSIX semantics can be used for + PostgreSQL. Users prefer different file systems for a variety of reasons, + including vendor support, performance, and familiarity. Experience + suggests that, all other things being equal, one should not expect major + performance or behavior changes merely from switching file systems or + making minor file system configuration changes. + </para> + + <sect3 id="creating-cluster-nfs"> + <title>NFS</title> + + <indexterm zone="creating-cluster-nfs"> + <primary>NFS</primary> + </indexterm> + + <para> + It is possible to use an <acronym>NFS</acronym> file system for storing + the <productname>PostgreSQL</productname> data directory. + <productname>PostgreSQL</productname> does nothing special for + <acronym>NFS</acronym> file systems, meaning it assumes + <acronym>NFS</acronym> behaves exactly like locally-connected drives. + <productname>PostgreSQL</productname> does not use any functionality that + is known to have nonstandard behavior on <acronym>NFS</acronym>, such as + file locking. + </para> + + <para> + The only firm requirement for using <acronym>NFS</acronym> with + <productname>PostgreSQL</productname> is that the file system is mounted + using the <literal>hard</literal> option. With the + <literal>hard</literal> option, processes can <quote>hang</quote> + indefinitely if there are network problems, so this configuration will + require a careful monitoring setup. The <literal>soft</literal> option + will interrupt system calls in case of network problems, but + <productname>PostgreSQL</productname> will not repeat system calls + interrupted in this way, so any such interruption will result in an I/O + error being reported. + </para> + + <para> + It is not necessary to use the <literal>sync</literal> mount option. The + behavior of the <literal>async</literal> option is sufficient, since + <productname>PostgreSQL</productname> issues <literal>fsync</literal> + calls at appropriate times to flush the write caches. (This is analogous + to how it works on a local file system.) However, it is strongly + recommended to use the <literal>sync</literal> export option on the NFS + <emphasis>server</emphasis> on systems where it exists (mainly Linux). + Otherwise, an <literal>fsync</literal> or equivalent on the NFS client is + not actually guaranteed to reach permanent storage on the server, which + could cause corruption similar to running with the parameter <xref + linkend="guc-fsync"/> off. The defaults of these mount and export + options differ between vendors and versions, so it is recommended to + check and perhaps specify them explicitly in any case to avoid any + ambiguity. + </para> + + <para> + In some cases, an external storage product can be accessed either via NFS + or a lower-level protocol such as iSCSI. In the latter case, the storage + appears as a block device and any available file system can be created on + it. That approach might relieve the DBA from having to deal with some of + the idiosyncrasies of NFS, but of course the complexity of managing + remote storage then happens at other levels. + </para> + </sect3> + </sect2> + + </sect1> + + <sect1 id="server-start"> + <title>Starting the Database Server</title> + + <para> + Before anyone can access the database, you must start the database + server. The database server program is called + <command>postgres</command>.<indexterm><primary>postgres</primary></indexterm> + </para> + + <para> + If you are using a pre-packaged version + of <productname>PostgreSQL</productname>, it almost certainly includes + provisions for running the server as a background task according to the + conventions of your operating system. Using the package's + infrastructure to start the server will be much less work than figuring + out how to do this yourself. Consult the package-level documentation + for details. + </para> + + <para> + The bare-bones way to start the server manually is just to invoke + <command>postgres</command> directly, specifying the location of the + data directory with the <option>-D</option> option, for example: +<screen> +$ <userinput>postgres -D /usr/local/pgsql/data</userinput> +</screen> + which will leave the server running in the foreground. This must be + done while logged into the <productname>PostgreSQL</productname> user + account. Without <option>-D</option>, the server will try to use + the data directory named by the environment variable <envar>PGDATA</envar>. + If that variable is not provided either, it will fail. + </para> + + <para> + Normally it is better to start <command>postgres</command> in the + background. For this, use the usual Unix shell syntax: +<screen> +$ <userinput>postgres -D /usr/local/pgsql/data >logfile 2>&1 &</userinput> +</screen> + It is important to store the server's <systemitem>stdout</systemitem> and + <systemitem>stderr</systemitem> output somewhere, as shown above. It will help + for auditing purposes and to diagnose problems. (See <xref + linkend="logfile-maintenance"/> for a more thorough discussion of log + file handling.) + </para> + + <para> + The <command>postgres</command> program also takes a number of other + command-line options. For more information, see the + <xref linkend="app-postgres"/> reference page + and <xref linkend="runtime-config"/> below. + </para> + + <para> + This shell syntax can get tedious quickly. Therefore the wrapper + program + <xref linkend="app-pg-ctl"/><indexterm><primary>pg_ctl</primary></indexterm> + is provided to simplify some tasks. For example: +<programlisting> +pg_ctl start -l logfile +</programlisting> + will start the server in the background and put the output into the + named log file. The <option>-D</option> option has the same meaning + here as for <command>postgres</command>. <command>pg_ctl</command> + is also capable of stopping the server. + </para> + + <para> + Normally, you will want to start the database server when the + computer boots.<indexterm> + <primary>booting</primary> + <secondary>starting the server during</secondary> + </indexterm> + Autostart scripts are operating-system-specific. + There are a few example scripts distributed with + <productname>PostgreSQL</productname> in the + <filename>contrib/start-scripts</filename> directory. Installing one will require + root privileges. + </para> + + <para> + Different systems have different conventions for starting up daemons + at boot time. Many systems have a file + <filename>/etc/rc.local</filename> or + <filename>/etc/rc.d/rc.local</filename>. Others use <filename>init.d</filename> or + <filename>rc.d</filename> directories. Whatever you do, the server must be + run by the <productname>PostgreSQL</productname> user account + <emphasis>and not by root</emphasis> or any other user. Therefore you + probably should form your commands using + <literal>su postgres -c '...'</literal>. For example: +<programlisting> +su postgres -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' +</programlisting> + </para> + + <para> + Here are a few more operating-system-specific suggestions. (In each + case be sure to use the proper installation directory and user + name where we show generic values.) + + <itemizedlist> + <listitem> + <para> + For <productname>FreeBSD</productname>, look at the file + <filename>contrib/start-scripts/freebsd</filename> in the + <productname>PostgreSQL</productname> source distribution. + <indexterm><primary>FreeBSD</primary><secondary>start script</secondary></indexterm> + </para> + </listitem> + + <listitem> + <para> + On <productname>OpenBSD</productname>, add the following lines + to the file <filename>/etc/rc.local</filename>: + <indexterm><primary>OpenBSD</primary><secondary>start script</secondary></indexterm> +<programlisting> +if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postgres ]; then + su -l postgres -c '/usr/local/pgsql/bin/pg_ctl start -s -l /var/postgresql/log -D /usr/local/pgsql/data' + echo -n ' postgresql' +fi +</programlisting> + </para> + </listitem> + + <listitem> + <para> + On <productname>Linux</productname> systems either add + <indexterm><primary>Linux</primary><secondary>start script</secondary></indexterm> +<programlisting> +/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data +</programlisting> + to <filename>/etc/rc.d/rc.local</filename> + or <filename>/etc/rc.local</filename> or look at the file + <filename>contrib/start-scripts/linux</filename> in the + <productname>PostgreSQL</productname> source distribution. + </para> + + <para> + When using <application>systemd</application>, you can use the following + service unit file (e.g., + at <filename>/etc/systemd/system/postgresql.service</filename>):<indexterm><primary>systemd</primary></indexterm> +<programlisting> +[Unit] +Description=PostgreSQL database server +Documentation=man:postgres(1) +After=network-online.target +Wants=network-online.target + +[Service] +Type=notify +User=postgres +ExecStart=/usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data +ExecReload=/bin/kill -HUP $MAINPID +KillMode=mixed +KillSignal=SIGINT +TimeoutSec=infinity + +[Install] +WantedBy=multi-user.target +</programlisting> + Using <literal>Type=notify</literal> requires that the server binary was + built with <literal>configure --with-systemd</literal>. + </para> + + <para> + Consider carefully the timeout + setting. <application>systemd</application> has a default timeout of 90 + seconds as of this writing and will kill a process that does not report + readiness within that time. But a <productname>PostgreSQL</productname> + server that might have to perform crash recovery at startup could take + much longer to become ready. The suggested value + of <literal>infinity</literal> disables the timeout logic. + </para> + </listitem> + + <listitem> + <para> + On <productname>NetBSD</productname>, use either the + <productname>FreeBSD</productname> or + <productname>Linux</productname> start scripts, depending on + preference. + <indexterm><primary>NetBSD</primary><secondary>start script</secondary></indexterm> + </para> + </listitem> + + <listitem> + <para> + On <productname>Solaris</productname>, create a file called + <filename>/etc/init.d/postgresql</filename> that contains + the following line: + <indexterm><primary>Solaris</primary><secondary>start script</secondary></indexterm> +<programlisting> +su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data" +</programlisting> + Then, create a symbolic link to it in <filename>/etc/rc3.d</filename> as + <filename>S99postgresql</filename>. + </para> + </listitem> + </itemizedlist> + + </para> + + <para> + While the server is running, its + <acronym>PID</acronym> is stored in the file + <filename>postmaster.pid</filename> in the data directory. This is + used to prevent multiple server instances from + running in the same data directory and can also be used for + shutting down the server. + </para> + + <sect2 id="server-start-failures"> + <title>Server Start-up Failures</title> + + <para> + There are several common reasons the server might fail to + start. Check the server's log file, or start it by hand (without + redirecting standard output or standard error) and see what error + messages appear. Below we explain some of the most common error + messages in more detail. + </para> + + <para> +<screen> +LOG: could not bind IPv4 address "127.0.0.1": Address already in use +HINT: Is another postmaster already running on port 5432? If not, wait a few seconds and retry. +FATAL: could not create any TCP/IP sockets +</screen> + This usually means just what it suggests: you tried to start + another server on the same port where one is already running. + However, if the kernel error message is not <computeroutput>Address + already in use</computeroutput> or some variant of that, there might + be a different problem. For example, trying to start a server + on a reserved port number might draw something like: +<screen> +$ <userinput>postgres -p 666</userinput> +LOG: could not bind IPv4 address "127.0.0.1": Permission denied +HINT: Is another postmaster already running on port 666? If not, wait a few seconds and retry. +FATAL: could not create any TCP/IP sockets +</screen> + </para> + + <para> + A message like: +<screen> +FATAL: could not create shared memory segment: Invalid argument +DETAIL: Failed system call was shmget(key=5440001, size=4011376640, 03600). +</screen> + probably means your kernel's limit on the size of shared memory is + smaller than the work area <productname>PostgreSQL</productname> + is trying to create (4011376640 bytes in this example). + This is only likely to happen if you have set <literal>shared_memory_type</literal> + to <literal>sysv</literal>. In that case, you + can try starting the server with a smaller-than-normal number of + buffers (<xref linkend="guc-shared-buffers"/>), or + reconfigure your kernel to increase the allowed shared memory + size. You might also see this message when trying to start multiple + servers on the same machine, if their total space requested + exceeds the kernel limit. + </para> + + <para> + An error like: +<screen> +FATAL: could not create semaphores: No space left on device +DETAIL: Failed system call was semget(5440126, 17, 03600). +</screen> + does <emphasis>not</emphasis> mean you've run out of disk + space. It means your kernel's limit on the number of <systemitem + class="osname">System V</systemitem> semaphores is smaller than the number + <productname>PostgreSQL</productname> wants to create. As above, + you might be able to work around the problem by starting the + server with a reduced number of allowed connections + (<xref linkend="guc-max-connections"/>), but you'll eventually want to + increase the kernel limit. + </para> + + <para> + Details about configuring <systemitem class="osname">System V</systemitem> + <acronym>IPC</acronym> facilities are given in <xref linkend="sysvipc"/>. + </para> + </sect2> + + <sect2 id="client-connection-problems"> + <title>Client Connection Problems</title> + + <para> + Although the error conditions possible on the client side are quite + varied and application-dependent, a few of them might be directly + related to how the server was started. Conditions other than + those shown below should be documented with the respective client + application. + </para> + + <para> +<screen> +psql: error: connection to server at "server.joe.com" (123.123.123.123), port 5432 failed: Connection refused + Is the server running on that host and accepting TCP/IP connections? +</screen> + This is the generic <quote>I couldn't find a server to talk + to</quote> failure. It looks like the above when TCP/IP + communication is attempted. A common mistake is to forget to + configure the server to allow TCP/IP connections. + </para> + + <para> + Alternatively, you might get this when attempting Unix-domain socket + communication to a local server: +<screen> +psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such file or directory + Is the server running locally and accepting connections on that socket? +</screen> + If the server is indeed running, check that the client's idea of the + socket path (here <literal>/tmp</literal>) agrees with the server's + <xref linkend="guc-unix-socket-directories"/> setting. + </para> + + <para> + A connection failure message always shows the server address or socket + path name, which is useful in verifying that the client is trying to + connect to the right place. If there is in fact no server + listening there, the kernel error message will typically be either + <computeroutput>Connection refused</computeroutput> or + <computeroutput>No such file or directory</computeroutput>, as + illustrated. (It is important to realize that + <computeroutput>Connection refused</computeroutput> in this context + does <emphasis>not</emphasis> mean that the server got your + connection request and rejected it. That case will produce a + different message, as shown in <xref + linkend="client-authentication-problems"/>.) Other error messages + such as <computeroutput>Connection timed out</computeroutput> might + indicate more fundamental problems, like lack of network + connectivity, or a firewall blocking the connection. + </para> + </sect2> + </sect1> + + <sect1 id="kernel-resources"> + <title>Managing Kernel Resources</title> + + <para> + <productname>PostgreSQL</productname> can sometimes exhaust various operating system + resource limits, especially when multiple copies of the server are running + on the same system, or in very large installations. This section explains + the kernel resources used by <productname>PostgreSQL</productname> and the steps you + can take to resolve problems related to kernel resource consumption. + </para> + + <sect2 id="sysvipc"> + <title>Shared Memory and Semaphores</title> + + <indexterm zone="sysvipc"> + <primary>shared memory</primary> + </indexterm> + + <indexterm zone="sysvipc"> + <primary>semaphores</primary> + </indexterm> + + <para> + <productname>PostgreSQL</productname> requires the operating system to provide + inter-process communication (<acronym>IPC</acronym>) features, specifically + shared memory and semaphores. Unix-derived systems typically provide + <quote><systemitem class="osname">System V</systemitem></quote> <acronym>IPC</acronym>, + <quote><systemitem class="osname">POSIX</systemitem></quote> <acronym>IPC</acronym>, or both. + <systemitem class="osname">Windows</systemitem> has its own implementation of + these features and is not discussed here. + </para> + + <para> + By default, <productname>PostgreSQL</productname> allocates + a very small amount of System V shared memory, as well as a much larger + amount of anonymous <function>mmap</function> shared memory. + Alternatively, a single large System V shared memory region can be used + (see <xref linkend="guc-shared-memory-type"/>). + + In addition a significant number of semaphores, which can be either + System V or POSIX style, are created at server startup. Currently, + POSIX semaphores are used on Linux and FreeBSD systems while other + platforms use System V semaphores. + </para> + + <para> + System V <acronym>IPC</acronym> features are typically constrained by + system-wide allocation limits. + When <productname>PostgreSQL</productname> exceeds one of these limits, + the server will refuse to start and + should leave an instructive error message describing the problem + and what to do about it. (See also <xref + linkend="server-start-failures"/>.) The relevant kernel + parameters are named consistently across different systems; <xref + linkend="sysvipc-parameters"/> gives an overview. The methods to set + them, however, vary. Suggestions for some platforms are given below. + </para> + + <table id="sysvipc-parameters"> + <title><systemitem class="osname">System V</systemitem> <acronym>IPC</acronym> Parameters</title> + + <tgroup cols="3"> + <colspec colname="col1" colwidth="1*"/> + <colspec colname="col2" colwidth="3*"/> + <colspec colname="col3" colwidth="3*"/> + <thead> + <row> + <entry>Name</entry> + <entry>Description</entry> + <entry>Values needed to run one <productname>PostgreSQL</productname> instance</entry> + </row> + </thead> + + <tbody> + <row> + <entry><varname>SHMMAX</varname></entry> + <entry>Maximum size of shared memory segment (bytes)</entry> + <entry>at least 1kB, but the default is usually much higher</entry> + </row> + + <row> + <entry><varname>SHMMIN</varname></entry> + <entry>Minimum size of shared memory segment (bytes)</entry> + <entry>1</entry> + </row> + + <row> + <entry><varname>SHMALL</varname></entry> + <entry>Total amount of shared memory available (bytes or pages)</entry> + <entry>same as <varname>SHMMAX</varname> if bytes, + or <literal>ceil(SHMMAX/PAGE_SIZE)</literal> if pages, + plus room for other applications</entry> + </row> + + <row> + <entry><varname>SHMSEG</varname></entry> + <entry>Maximum number of shared memory segments per process</entry> + <entry>only 1 segment is needed, but the default is much higher</entry> + </row> + + <row> + <entry><varname>SHMMNI</varname></entry> + <entry>Maximum number of shared memory segments system-wide</entry> + <entry>like <varname>SHMSEG</varname> plus room for other applications</entry> + </row> + + <row> + <entry><varname>SEMMNI</varname></entry> + <entry>Maximum number of semaphore identifiers (i.e., sets)</entry> + <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry> + </row> + + <row> + <entry><varname>SEMMNS</varname></entry> + <entry>Maximum number of semaphores system-wide</entry> + <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry> + </row> + + <row> + <entry><varname>SEMMSL</varname></entry> + <entry>Maximum number of semaphores per set</entry> + <entry>at least 17</entry> + </row> + + <row> + <entry><varname>SEMMAP</varname></entry> + <entry>Number of entries in semaphore map</entry> + <entry>see text</entry> + </row> + + <row> + <entry><varname>SEMVMX</varname></entry> + <entry>Maximum value of semaphore</entry> + <entry>at least 1000 (The default is often 32767; do not change unless necessary)</entry> + </row> + + </tbody> + </tgroup> + </table> + + <para> + <productname>PostgreSQL</productname> requires a few bytes of System V shared memory + (typically 48 bytes, on 64-bit platforms) for each copy of the server. + On most modern operating systems, this amount can easily be allocated. + However, if you are running many copies of the server or you explicitly + configure the server to use large amounts of System V shared memory (see + <xref linkend="guc-shared-memory-type"/> and <xref + linkend="guc-dynamic-shared-memory-type"/>), it may be necessary to + increase <varname>SHMALL</varname>, which is the total amount of System V shared + memory system-wide. Note that <varname>SHMALL</varname> is measured in pages + rather than bytes on many systems. + </para> + + <para> + Less likely to cause problems is the minimum size for shared + memory segments (<varname>SHMMIN</varname>), which should be at most + approximately 32 bytes for <productname>PostgreSQL</productname> (it is + usually just 1). The maximum number of segments system-wide + (<varname>SHMMNI</varname>) or per-process (<varname>SHMSEG</varname>) are unlikely + to cause a problem unless your system has them set to zero. + </para> + + <para> + When using System V semaphores, + <productname>PostgreSQL</productname> uses one semaphore per allowed connection + (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process + (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background + process (<xref linkend="guc-max-worker-processes"/>), in sets of 16. + Each such set will + also contain a 17th semaphore which contains a <quote>magic + number</quote>, to detect collision with semaphore sets used by + other applications. The maximum number of semaphores in the system + is set by <varname>SEMMNS</varname>, which consequently must be at least + as high as <varname>max_connections</varname> plus + <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>, + plus <varname>max_worker_processes</varname>, plus one extra for each 16 + allowed connections plus workers (see the formula in <xref + linkend="sysvipc-parameters"/>). The parameter <varname>SEMMNI</varname> + determines the limit on the number of semaphore sets that can + exist on the system at one time. Hence this parameter must be at + least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal>. + Lowering the number + of allowed connections is a temporary workaround for failures, + which are usually confusingly worded <quote>No space + left on device</quote>, from the function <function>semget</function>. + </para> + + <para> + In some cases it might also be necessary to increase + <varname>SEMMAP</varname> to be at least on the order of + <varname>SEMMNS</varname>. If the system has this parameter + (many do not), it defines the size of the semaphore + resource map, in which each contiguous block of available semaphores + needs an entry. When a semaphore set is freed it is either added to + an existing entry that is adjacent to the freed block or it is + registered under a new map entry. If the map is full, the freed + semaphores get lost (until reboot). Fragmentation of the semaphore + space could over time lead to fewer available semaphores than there + should be. + </para> + + <para> + Various other settings related to <quote>semaphore undo</quote>, such as + <varname>SEMMNU</varname> and <varname>SEMUME</varname>, do not affect + <productname>PostgreSQL</productname>. + </para> + + <para> + When using POSIX semaphores, the number of semaphores needed is the + same as for System V, that is one semaphore per allowed connection + (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process + (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background + process (<xref linkend="guc-max-worker-processes"/>). + On the platforms where this option is preferred, there is no specific + kernel limit on the number of POSIX semaphores. + </para> + + + <variablelist> + <varlistentry> + <term><systemitem class="osname">AIX</systemitem> + <indexterm><primary>AIX</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + It should not be necessary to do + any special configuration for such parameters as + <varname>SHMMAX</varname>, as it appears this is configured to + allow all memory to be used as shared memory. That is the + sort of configuration commonly used for other databases such + as <application>DB/2</application>.</para> + + <para> It might, however, be necessary to modify the global + <command>ulimit</command> information in + <filename>/etc/security/limits</filename>, as the default hard + limits for file sizes (<varname>fsize</varname>) and numbers of + files (<varname>nofiles</varname>) might be too low. + </para> + </listitem> + </varlistentry> + + + <varlistentry> + <term><systemitem class="osname">FreeBSD</systemitem> + <indexterm><primary>FreeBSD</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + The default shared memory settings are usually good enough, unless + you have set <literal>shared_memory_type</literal> to <literal>sysv</literal>. + System V semaphores are not used on this platform. + </para> + + <para> + The default IPC settings can be changed using + the <command>sysctl</command> or + <command>loader</command> interfaces. The following + parameters can be set using <command>sysctl</command>: +<screen> +<prompt>#</prompt> <userinput>sysctl kern.ipc.shmall=32768</userinput> +<prompt>#</prompt> <userinput>sysctl kern.ipc.shmmax=134217728</userinput> +</screen> + To make these settings persist over reboots, modify + <filename>/etc/sysctl.conf</filename>. + </para> + + <para> + If you have set <literal>shared_memory_type</literal> to + <literal>sysv</literal>, you might also want to configure your kernel + to lock System V shared memory into RAM and prevent it from being paged + out to swap. This can be accomplished using the <command>sysctl</command> + setting <literal>kern.ipc.shm_use_phys</literal>. + </para> + + <para> + If running in a FreeBSD jail, you should set its + <literal>sysvshm</literal> parameter to <literal>new</literal>, so that + it has its own separate System V shared memory namespace. + (Before FreeBSD 11.0, it was necessary to enable shared access to + the host's IPC namespace from jails, and take measures to avoid + collisions.) + </para> + + </listitem> + </varlistentry> + + <varlistentry> + <term><systemitem class="osname">NetBSD</systemitem> + <indexterm><primary>NetBSD</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + The default shared memory settings are usually good enough, unless + you have set <literal>shared_memory_type</literal> to <literal>sysv</literal>. + You will usually want to increase <literal>kern.ipc.semmni</literal> + and <literal>kern.ipc.semmns</literal>, + as <systemitem class="osname">NetBSD</systemitem>'s default settings + for these are uncomfortably small. + </para> + + <para> + IPC parameters can be adjusted using <command>sysctl</command>, + for example: +<screen> +<prompt>#</prompt> <userinput>sysctl -w kern.ipc.semmni=100</userinput> +</screen> + To make these settings persist over reboots, modify + <filename>/etc/sysctl.conf</filename>. + </para> + + <para> + If you have set <literal>shared_memory_type</literal> to + <literal>sysv</literal>, you might also want to configure your kernel + to lock System V shared memory into RAM and prevent it from being paged + out to swap. This can be accomplished using the <command>sysctl</command> + setting <literal>kern.ipc.shm_use_phys</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><systemitem class="osname">OpenBSD</systemitem> + <indexterm><primary>OpenBSD</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + The default shared memory settings are usually good enough, unless + you have set <literal>shared_memory_type</literal> to <literal>sysv</literal>. + You will usually want to + increase <literal>kern.seminfo.semmni</literal> + and <literal>kern.seminfo.semmns</literal>, + as <systemitem class="osname">OpenBSD</systemitem>'s default settings + for these are uncomfortably small. + </para> + + <para> + IPC parameters can be adjusted using <command>sysctl</command>, + for example: +<screen> +<prompt>#</prompt> <userinput>sysctl kern.seminfo.semmni=100</userinput> +</screen> + To make these settings persist over reboots, modify + <filename>/etc/sysctl.conf</filename>. + </para> + + </listitem> + </varlistentry> + + <varlistentry> + <term><systemitem class="osname">HP-UX</systemitem> + <indexterm><primary>HP-UX</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + The default settings tend to suffice for normal installations. + </para> + <para> + <acronym>IPC</acronym> parameters can be set in the <application>System + Administration Manager</application> (<acronym>SAM</acronym>) under + <menuchoice><guimenu>Kernel + Configuration</guimenu><guimenuitem>Configurable Parameters</guimenuitem></menuchoice>. Choose + <guibutton>Create A New Kernel</guibutton> when you're done. + </para> + </listitem> + </varlistentry> + + + <varlistentry> + <term><systemitem class="osname">Linux</systemitem> + <indexterm><primary>Linux</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + The default shared memory settings are usually good enough, unless + you have set <literal>shared_memory_type</literal> to <literal>sysv</literal>, + and even then only on older kernel versions that shipped with low defaults. + System V semaphores are not used on this platform. + </para> + + <para> + The shared memory size settings can be changed via the + <command>sysctl</command> interface. For example, to allow 16 GB: +<screen> +<prompt>$</prompt> <userinput>sysctl -w kernel.shmmax=17179869184</userinput> +<prompt>$</prompt> <userinput>sysctl -w kernel.shmall=4194304</userinput> +</screen> + To make these settings persist over reboots, see + <filename>/etc/sysctl.conf</filename>. + </para> + + </listitem> + </varlistentry> + + + <varlistentry> + <term><systemitem class="osname">macOS</systemitem> + <indexterm><primary>macOS</primary><secondary>IPC configuration</secondary></indexterm> + </term> + <listitem> + <para> + The default shared memory and semaphore settings are usually good enough, unless + you have set <literal>shared_memory_type</literal> to <literal>sysv</literal>. + </para> + <para> + The recommended method for configuring shared memory in macOS + is to create a file named <filename>/etc/sysctl.conf</filename>, + containing variable assignments such as: +<programlisting> +kern.sysv.shmmax=4194304 +kern.sysv.shmmin=1 +kern.sysv.shmmni=32 +kern.sysv.shmseg=8 +kern.sysv.shmall=1024 +</programlisting> + Note that in some macOS versions, + <emphasis>all five</emphasis> shared-memory parameters must be set in + <filename>/etc/sysctl.conf</filename>, else the values will be ignored. + </para> + + <para> + <varname>SHMMAX</varname> can only be set to a multiple of 4096. + </para> + + <para> + <varname>SHMALL</varname> is measured in 4 kB pages on this platform. + </para> + + <para> + It is possible to change all but <varname>SHMMNI</varname> on the fly, using + <application>sysctl</application>. But it's still best to set up your preferred + values via <filename>/etc/sysctl.conf</filename>, so that the values will be + kept across reboots. + </para> + + </listitem> + </varlistentry> + + <varlistentry> + <term><systemitem class="osname">Solaris</systemitem></term> + <term><systemitem class="osname">illumos</systemitem></term> + <listitem> + <para> + The default shared memory and semaphore settings are usually good enough for most + <productname>PostgreSQL</productname> applications. Solaris defaults + to a <varname>SHMMAX</varname> of one-quarter of system <acronym>RAM</acronym>. + To further adjust this setting, use a project setting associated + with the <literal>postgres</literal> user. For example, run the + following as <literal>root</literal>: +<programlisting> +projadd -c "PostgreSQL DB User" -K "project.max-shm-memory=(privileged,8GB,deny)" -U postgres -G postgres user.postgres +</programlisting> + </para> + + <para> + This command adds the <literal>user.postgres</literal> project and + sets the shared memory maximum for the <literal>postgres</literal> + user to 8GB, and takes effect the next time that user logs + in, or when you restart <productname>PostgreSQL</productname> (not reload). + The above assumes that <productname>PostgreSQL</productname> is run by + the <literal>postgres</literal> user in the <literal>postgres</literal> + group. No server reboot is required. + </para> + + <para> + Other recommended kernel setting changes for database servers which will + have a large number of connections are: +<programlisting> +project.max-shm-ids=(priv,32768,deny) +project.max-sem-ids=(priv,4096,deny) +project.max-msg-ids=(priv,4096,deny) +</programlisting> + </para> + + <para> + Additionally, if you are running <productname>PostgreSQL</productname> + inside a zone, you may need to raise the zone resource usage + limits as well. See "Chapter2: Projects and Tasks" in the + <citetitle>System Administrator's Guide</citetitle> for more + information on <literal>projects</literal> and <command>prctl</command>. + </para> + </listitem> + </varlistentry> + + </variablelist> + + </sect2> + + <sect2 id="systemd-removeipc"> + <title>systemd RemoveIPC</title> + + <indexterm> + <primary>systemd</primary> + <secondary>RemoveIPC</secondary> + </indexterm> + + <para> + If <productname>systemd</productname> is in use, some care must be taken + that IPC resources (including shared memory) are not prematurely + removed by the operating system. This is especially of concern when + installing PostgreSQL from source. Users of distribution packages of + PostgreSQL are less likely to be affected, as + the <literal>postgres</literal> user is then normally created as a system + user. + </para> + + <para> + The setting <literal>RemoveIPC</literal> + in <filename>logind.conf</filename> controls whether IPC objects are + removed when a user fully logs out. System users are exempt. This + setting defaults to on in stock <productname>systemd</productname>, but + some operating system distributions default it to off. + </para> + + <para> + A typical observed effect when this setting is on is that shared memory + objects used for parallel query execution are removed at apparently random + times, leading to errors and warnings while attempting to open and remove + them, like +<screen> +WARNING: could not remove shared memory segment "/PostgreSQL.1450751626": No such file or directory +</screen> + Different types of IPC objects (shared memory vs. semaphores, System V + vs. POSIX) are treated slightly differently + by <productname>systemd</productname>, so one might observe that some IPC + resources are not removed in the same way as others. But it is not + advisable to rely on these subtle differences. + </para> + + <para> + A <quote>user logging out</quote> might happen as part of a maintenance + job or manually when an administrator logs in as + the <literal>postgres</literal> user or something similar, so it is hard + to prevent in general. + </para> + + <para> + What is a <quote>system user</quote> is determined + at <productname>systemd</productname> compile time from + the <symbol>SYS_UID_MAX</symbol> setting + in <filename>/etc/login.defs</filename>. + </para> + + <para> + Packaging and deployment scripts should be careful to create + the <literal>postgres</literal> user as a system user by + using <literal>useradd -r</literal>, <literal>adduser --system</literal>, + or equivalent. + </para> + + <para> + Alternatively, if the user account was created incorrectly or cannot be + changed, it is recommended to set +<programlisting> +RemoveIPC=no +</programlisting> + in <filename>/etc/systemd/logind.conf</filename> or another appropriate + configuration file. + </para> + + <caution> + <para> + At least one of these two things has to be ensured, or the PostgreSQL + server will be very unreliable. + </para> + </caution> + </sect2> + + <sect2> + <title>Resource Limits</title> + + <para> + Unix-like operating systems enforce various kinds of resource limits + that might interfere with the operation of your + <productname>PostgreSQL</productname> server. Of particular + importance are limits on the number of processes per user, the + number of open files per process, and the amount of memory available + to each process. Each of these have a <quote>hard</quote> and a + <quote>soft</quote> limit. The soft limit is what actually counts + but it can be changed by the user up to the hard limit. The hard + limit can only be changed by the root user. The system call + <function>setrlimit</function> is responsible for setting these + parameters. The shell's built-in command <command>ulimit</command> + (Bourne shells) or <command>limit</command> (<application>csh</application>) is + used to control the resource limits from the command line. On + BSD-derived systems the file <filename>/etc/login.conf</filename> + controls the various resource limits set during login. See the + operating system documentation for details. The relevant + parameters are <varname>maxproc</varname>, + <varname>openfiles</varname>, and <varname>datasize</varname>. For + example: +<programlisting> +default:\ +... + :datasize-cur=256M:\ + :maxproc-cur=256:\ + :openfiles-cur=256:\ +... +</programlisting> + (<literal>-cur</literal> is the soft limit. Append + <literal>-max</literal> to set the hard limit.) + </para> + + <para> + Kernels can also have system-wide limits on some resources. + <itemizedlist> + <listitem> + <para> + On <productname>Linux</productname> the kernel parameter + <varname>fs.file-max</varname> determines the maximum number of open + files that the kernel will support. It can be changed with + <literal>sysctl -w fs.file-max=<replaceable>N</replaceable></literal>. + To make the setting persist across reboots, add an assignment + in <filename>/etc/sysctl.conf</filename>. + The maximum limit of files per process is fixed at the time the + kernel is compiled; see + <filename>/usr/src/linux/Documentation/proc.txt</filename> for + more information. + </para> + </listitem> + </itemizedlist> + </para> + + <para> + The <productname>PostgreSQL</productname> server uses one process + per connection so you should provide for at least as many processes + as allowed connections, in addition to what you need for the rest + of your system. This is usually not a problem but if you run + several servers on one machine things might get tight. + </para> + + <para> + The factory default limit on open files is often set to + <quote>socially friendly</quote> values that allow many users to + coexist on a machine without using an inappropriate fraction of + the system resources. If you run many servers on a machine this + is perhaps what you want, but on dedicated servers you might want to + raise this limit. + </para> + + <para> + On the other side of the coin, some systems allow individual + processes to open large numbers of files; if more than a few + processes do so then the system-wide limit can easily be exceeded. + If you find this happening, and you do not want to alter the + system-wide limit, you can set <productname>PostgreSQL</productname>'s <xref + linkend="guc-max-files-per-process"/> configuration parameter to + limit the consumption of open files. + </para> + + <para> + Another kernel limit that may be of concern when supporting large + numbers of client connections is the maximum socket connection queue + length. If more than that many connection requests arrive within a very + short period, some may get rejected before the postmaster can service + the requests, with those clients receiving unhelpful connection failure + errors such as <quote>Resource temporarily unavailable</quote> or + <quote>Connection refused</quote>. The default queue length limit is 128 + on many platforms. To raise it, adjust the appropriate kernel parameter + via <application>sysctl</application>, then restart the postmaster. + The parameter is variously named <varname>net.core.somaxconn</varname> + on Linux, <varname>kern.ipc.soacceptqueue</varname> on newer FreeBSD, + and <varname>kern.ipc.somaxconn</varname> on macOS and other BSD + variants. + </para> + </sect2> + + <sect2 id="linux-memory-overcommit"> + <title>Linux Memory Overcommit</title> + + <indexterm> + <primary>memory overcommit</primary> + </indexterm> + + <indexterm> + <primary>OOM</primary> + </indexterm> + + <indexterm> + <primary>overcommit</primary> + </indexterm> + + <para> + The default virtual memory behavior on Linux is not + optimal for <productname>PostgreSQL</productname>. Because of the + way that the kernel implements memory overcommit, the kernel might + terminate the <productname>PostgreSQL</productname> postmaster (the + supervisor server process) if the memory demands of either + <productname>PostgreSQL</productname> or another process cause the + system to run out of virtual memory. + </para> + + <para> + If this happens, you will see a kernel message that looks like + this (consult your system documentation and configuration on where + to look for such a message): +<programlisting> +Out of Memory: Killed process 12345 (postgres). +</programlisting> + This indicates that the <filename>postgres</filename> process + has been terminated due to memory pressure. + Although existing database connections will continue to function + normally, no new connections will be accepted. To recover, + <productname>PostgreSQL</productname> will need to be restarted. + </para> + + <para> + One way to avoid this problem is to run + <productname>PostgreSQL</productname> on a machine where you can + be sure that other processes will not run the machine out of + memory. If memory is tight, increasing the swap space of the + operating system can help avoid the problem, because the + out-of-memory (OOM) killer is invoked only when physical memory and + swap space are exhausted. + </para> + + <para> + If <productname>PostgreSQL</productname> itself is the cause of the + system running out of memory, you can avoid the problem by changing + your configuration. In some cases, it may help to lower memory-related + configuration parameters, particularly + <link linkend="guc-shared-buffers"><varname>shared_buffers</varname></link>, + <link linkend="guc-work-mem"><varname>work_mem</varname></link>, and + <link linkend="guc-hash-mem-multiplier"><varname>hash_mem_multiplier</varname></link>. + In other cases, the problem may be caused by allowing too many + connections to the database server itself. In many cases, it may + be better to reduce + <link linkend="guc-max-connections"><varname>max_connections</varname></link> + and instead make use of external connection-pooling software. + </para> + + <para> + It is possible to modify the + kernel's behavior so that it will not <quote>overcommit</quote> memory. + Although this setting will not prevent the <ulink + url="https://lwn.net/Articles/104179/">OOM killer</ulink> from being invoked + altogether, it will lower the chances significantly and will therefore + lead to more robust system behavior. This is done by selecting strict + overcommit mode via <command>sysctl</command>: +<programlisting> +sysctl -w vm.overcommit_memory=2 +</programlisting> + or placing an equivalent entry in <filename>/etc/sysctl.conf</filename>. + You might also wish to modify the related setting + <varname>vm.overcommit_ratio</varname>. For details see the kernel documentation + file <ulink url="https://www.kernel.org/doc/Documentation/vm/overcommit-accounting"></ulink>. + </para> + + <para> + Another approach, which can be used with or without altering + <varname>vm.overcommit_memory</varname>, is to set the process-specific + <firstterm>OOM score adjustment</firstterm> value for the postmaster process to + <literal>-1000</literal>, thereby guaranteeing it will not be targeted by the OOM + killer. The simplest way to do this is to execute +<programlisting> +echo -1000 > /proc/self/oom_score_adj +</programlisting> + in the postmaster's startup script just before invoking the postmaster. + Note that this action must be done as root, or it will have no effect; + so a root-owned startup script is the easiest place to do it. If you + do this, you should also set these environment variables in the startup + script before invoking the postmaster: +<programlisting> +export PG_OOM_ADJUST_FILE=/proc/self/oom_score_adj +export PG_OOM_ADJUST_VALUE=0 +</programlisting> + These settings will cause postmaster child processes to run with the + normal OOM score adjustment of zero, so that the OOM killer can still + target them at need. You could use some other value for + <envar>PG_OOM_ADJUST_VALUE</envar> if you want the child processes to run + with some other OOM score adjustment. (<envar>PG_OOM_ADJUST_VALUE</envar> + can also be omitted, in which case it defaults to zero.) If you do not + set <envar>PG_OOM_ADJUST_FILE</envar>, the child processes will run with the + same OOM score adjustment as the postmaster, which is unwise since the + whole point is to ensure that the postmaster has a preferential setting. + </para> + + </sect2> + + <sect2 id="linux-huge-pages"> + <title>Linux Huge Pages</title> + + <para> + Using huge pages reduces overhead when using large contiguous chunks of + memory, as <productname>PostgreSQL</productname> does, particularly when + using large values of <xref linkend="guc-shared-buffers"/>. To use this + feature in <productname>PostgreSQL</productname> you need a kernel + with <varname>CONFIG_HUGETLBFS=y</varname> and + <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure + the operating system to provide enough huge pages of the desired size. + To determine the number of huge pages needed, use the + <command>postgres</command> command to see the value of + <xref linkend="guc-shared-memory-size-in-huge-pages"/>. Note that the + server must be shut down to view this runtime-computed parameter. + This might look like: +<programlisting> +$ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput> +3170 +$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput> +Hugepagesize: 2048 kB +$ <userinput>ls /sys/kernel/mm/hugepages</userinput> +hugepages-1048576kB hugepages-2048kB +</programlisting> + + In this example the default is 2MB, but you can also explicitly request + either 2MB or 1GB with <xref linkend="guc-huge-page-size"/> to adapt + the number of pages calculated by + <varname>shared_memory_size_in_huge_pages</varname>. + + While we need at least <literal>3170</literal> huge pages in this example, + a larger setting would be appropriate if other programs on the machine + also need huge pages. + We can set this with: +<programlisting> +# <userinput>sysctl -w vm.nr_hugepages=3170</userinput> +</programlisting> + Don't forget to add this setting to <filename>/etc/sysctl.conf</filename> + so that it is reapplied after reboots. For non-default huge page sizes, + we can instead use: +<programlisting> +# <userinput>echo 3170 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages</userinput> +</programlisting> + It is also possible to provide these settings at boot time using + kernel parameters such as <literal>hugepagesz=2M hugepages=3170</literal>. + </para> + + <para> + Sometimes the kernel is not able to allocate the desired number of huge + pages immediately due to fragmentation, so it might be necessary + to repeat the command or to reboot. (Immediately after a reboot, most of + the machine's memory should be available to convert into huge pages.) + To verify the huge page allocation situation for a given size, use: +<programlisting> +$ <userinput>cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages</userinput> +</programlisting> + </para> + + <para> + It may also be necessary to give the database server's operating system + user permission to use huge pages by setting + <varname>vm.hugetlb_shm_group</varname> via <application>sysctl</application>, and/or + give permission to lock memory with <command>ulimit -l</command>. + </para> + + <para> + The default behavior for huge pages in + <productname>PostgreSQL</productname> is to use them when possible, with + the system's default huge page size, and + to fall back to normal pages on failure. To enforce the use of huge + pages, you can set <xref linkend="guc-huge-pages"/> + to <literal>on</literal> in <filename>postgresql.conf</filename>. + Note that with this setting <productname>PostgreSQL</productname> will fail to + start if not enough huge pages are available. + </para> + + <para> + For a detailed description of the <productname>Linux</productname> huge + pages feature have a look + at <ulink url="https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt"></ulink>. + </para> + + </sect2> + </sect1> + + + <sect1 id="server-shutdown"> + <title>Shutting Down the Server</title> + + <indexterm zone="server-shutdown"> + <primary>shutdown</primary> + </indexterm> + + <para> + There are several ways to shut down the database server. + Under the hood, they all reduce to sending a signal to the supervisor + <command>postgres</command> process. + </para> + + <para> + If you are using a pre-packaged version + of <productname>PostgreSQL</productname>, and you used its provisions + for starting the server, then you should also use its provisions for + stopping the server. Consult the package-level documentation for + details. + </para> + + <para> + When managing the server directly, you can control the type of shutdown + by sending different signals to the <command>postgres</command> + process: + + <variablelist> + <varlistentry> + <term><systemitem>SIGTERM</systemitem><indexterm><primary>SIGTERM</primary></indexterm></term> + <listitem> + <para> + This is the <firstterm>Smart Shutdown</firstterm> mode. + After receiving <systemitem>SIGTERM</systemitem>, the server + disallows new connections, but lets existing sessions end their + work normally. It shuts down only after all of the sessions terminate. + If the server is in recovery when a smart + shutdown is requested, recovery and streaming replication will be + stopped only after all regular sessions have terminated. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><systemitem>SIGINT</systemitem><indexterm><primary>SIGINT</primary></indexterm></term> + <listitem> + <para> + This is the <firstterm>Fast Shutdown</firstterm> mode. + The server disallows new connections and sends all existing + server processes <systemitem>SIGTERM</systemitem>, which will cause them + to abort their current transactions and exit promptly. It then + waits for all server processes to exit and finally shuts down. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><systemitem>SIGQUIT</systemitem><indexterm><primary>SIGQUIT</primary></indexterm></term> + <listitem> + <para> + This is the <firstterm>Immediate Shutdown</firstterm> mode. + The server will send <systemitem>SIGQUIT</systemitem> to all child + processes and wait for them to terminate. If any do not terminate + within 5 seconds, they will be sent <systemitem>SIGKILL</systemitem>. + The supervisor server process exits as soon as all child processes have + exited, without doing normal database shutdown processing. + This will lead to recovery (by + replaying the WAL log) upon next start-up. This is recommended + only in emergencies. + </para> + </listitem> + </varlistentry> + </variablelist> + </para> + + <para> + The <xref linkend="app-pg-ctl"/> program provides a convenient + interface for sending these signals to shut down the server. + Alternatively, you can send the signal directly using <command>kill</command> + on non-Windows systems. + The <acronym>PID</acronym> of the <command>postgres</command> process can be + found using the <command>ps</command> program, or from the file + <filename>postmaster.pid</filename> in the data directory. For + example, to do a fast shutdown: +<screen> +$ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput> +</screen> + </para> + + <important> + <para> + It is best not to use <systemitem>SIGKILL</systemitem> to shut down the + server. Doing so will prevent the server from releasing shared memory and + semaphores. Furthermore, <systemitem>SIGKILL</systemitem> kills + the <command>postgres</command> process without letting it relay the + signal to its subprocesses, so it might be necessary to kill the + individual subprocesses by hand as well. + </para> + </important> + + <para> + To terminate an individual session while allowing other sessions to + continue, use <function>pg_terminate_backend()</function> (see <xref + linkend="functions-admin-signal-table"/>) or send a + <systemitem>SIGTERM</systemitem> signal to the child process associated with + the session. + </para> + </sect1> + + <sect1 id="upgrading"> + <title>Upgrading a <productname>PostgreSQL</productname> Cluster</title> + + <indexterm zone="upgrading"> + <primary>upgrading</primary> + </indexterm> + + <indexterm zone="upgrading"> + <primary>version</primary> + <secondary>compatibility</secondary> + </indexterm> + + <para> + This section discusses how to upgrade your database data from one + <productname>PostgreSQL</productname> release to a newer one. + </para> + + <para> + Current <productname>PostgreSQL</productname> version numbers consist of a + major and a minor version number. For example, in the version number 10.1, + the 10 is the major version number and the 1 is the minor version number, + meaning this would be the first minor release of the major release 10. For + releases before <productname>PostgreSQL</productname> version 10.0, version + numbers consist of three numbers, for example, 9.5.3. In those cases, the + major version consists of the first two digit groups of the version number, + e.g., 9.5, and the minor version is the third number, e.g., 3, meaning this + would be the third minor release of the major release 9.5. + </para> + + <para> + Minor releases never change the internal storage format and are always + compatible with earlier and later minor releases of the same major version + number. For example, version 10.1 is compatible with version 10.0 and + version 10.6. Similarly, for example, 9.5.3 is compatible with 9.5.0, + 9.5.1, and 9.5.6. To update between compatible versions, you simply + replace the executables while the server is down and restart the server. + The data directory remains unchanged — minor upgrades are that + simple. + </para> + + <para> + For <emphasis>major</emphasis> releases of <productname>PostgreSQL</productname>, the + internal data storage format is subject to change, thus complicating + upgrades. The traditional method for moving data to a new major version + is to dump and restore the database, though this can be slow. A + faster method is <xref linkend="pgupgrade"/>. Replication methods are + also available, as discussed below. + (If you are using a pre-packaged version + of <productname>PostgreSQL</productname>, it may provide scripts to + assist with major version upgrades. Consult the package-level + documentation for details.) + </para> + + <para> + New major versions also typically introduce some user-visible + incompatibilities, so application programming changes might be required. + All user-visible changes are listed in the release notes (<xref + linkend="release"/>); pay particular attention to the section + labeled "Migration". Though you can upgrade from one major version + to another without upgrading to intervening versions, you should read + the major release notes of all intervening versions. + </para> + + <para> + Cautious users will want to test their client applications on the new + version before switching over fully; therefore, it's often a good idea to + set up concurrent installations of old and new versions. When + testing a <productname>PostgreSQL</productname> major upgrade, consider the + following categories of possible changes: + </para> + + <variablelist> + + <varlistentry> + <term>Administration</term> + <listitem> + <para> + The capabilities available for administrators to monitor and control + the server often change and improve in each major release. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>SQL</term> + <listitem> + <para> + Typically this includes new SQL command capabilities and not changes + in behavior, unless specifically mentioned in the release notes. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Library API</term> + <listitem> + <para> + Typically libraries like <application>libpq</application> only add new + functionality, again unless mentioned in the release notes. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>System Catalogs</term> + <listitem> + <para> + System catalog changes usually only affect database management tools. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Server C-language API</term> + <listitem> + <para> + This involves changes in the backend function API, which is written + in the C programming language. Such changes affect code that + references backend functions deep inside the server. + </para> + </listitem> + </varlistentry> + + </variablelist> + + <sect2 id="upgrading-via-pgdumpall"> + <title>Upgrading Data via <application>pg_dumpall</application></title> + + <para> + One upgrade method is to dump data from one major version of + <productname>PostgreSQL</productname> and restore it in another — to do + this, you must use a <emphasis>logical</emphasis> backup tool like + <application>pg_dumpall</application>; file system + level backup methods will not work. (There are checks in place that prevent + you from using a data directory with an incompatible version of + <productname>PostgreSQL</productname>, so no great harm can be done by + trying to start the wrong server version on a data directory.) + </para> + + <para> + It is recommended that you use the <application>pg_dump</application> and + <application>pg_dumpall</application> programs from the <emphasis>newer</emphasis> + version of + <productname>PostgreSQL</productname>, to take advantage of enhancements + that might have been made in these programs. Current releases of the + dump programs can read data from any server version back to 9.2. + </para> + + <para> + These instructions assume that your existing installation is under the + <filename>/usr/local/pgsql</filename> directory, and that the data area is in + <filename>/usr/local/pgsql/data</filename>. Substitute your paths + appropriately. + </para> + + <procedure> + <step> + <para> + If making a backup, make sure that your database is not being updated. + This does not affect the integrity of the backup, but the changed + data would of course not be included. If necessary, edit the + permissions in the file <filename>/usr/local/pgsql/data/pg_hba.conf</filename> + (or equivalent) to disallow access from everyone except you. + See <xref linkend="client-authentication"/> for additional information on + access control. + </para> + + <para> + <indexterm> + <primary>pg_dumpall</primary> + <secondary>use during upgrade</secondary> + </indexterm> + + To back up your database installation, type: +<screen> +<userinput>pg_dumpall > <replaceable>outputfile</replaceable></userinput> +</screen> + </para> + + <para> + To make the backup, you can use the <application>pg_dumpall</application> + command from the version you are currently running; see <xref + linkend="backup-dump-all"/> for more details. For best + results, however, try to use the <application>pg_dumpall</application> + command from <productname>PostgreSQL</productname> &version;, + since this version contains bug fixes and improvements over older + versions. While this advice might seem idiosyncratic since you + haven't installed the new version yet, it is advisable to follow + it if you plan to install the new version in parallel with the + old version. In that case you can complete the installation + normally and transfer the data later. This will also decrease + the downtime. + </para> + </step> + + <step> + <para> + Shut down the old server: +<screen> +<userinput>pg_ctl stop</userinput> +</screen> + On systems that have <productname>PostgreSQL</productname> started at boot time, + there is probably a start-up file that will accomplish the same thing. For + example, on a <systemitem class="osname">Red Hat Linux</systemitem> system one + might find that this works: +<screen> +<userinput>/etc/rc.d/init.d/postgresql stop</userinput> +</screen> + See <xref linkend="runtime"/> for details about starting and + stopping the server. + </para> + </step> + + <step> + <para> + If restoring from backup, rename or delete the old installation + directory if it is not version-specific. It is a good idea to + rename the directory, rather than + delete it, in case you have trouble and need to revert to it. Keep + in mind the directory might consume significant disk space. To rename + the directory, use a command like this: +<screen> +<userinput>mv /usr/local/pgsql /usr/local/pgsql.old</userinput> +</screen> + (Be sure to move the directory as a single unit so relative paths + remain unchanged.) + </para> + </step> + + <step> + <para> + Install the new version of <productname>PostgreSQL</productname> as + outlined in <xref linkend="install-procedure"/>. + </para> + </step> + + <step> + <para> + Create a new database cluster if needed. Remember that you must + execute these commands while logged in to the special database user + account (which you already have if you are upgrading). +<programlisting> +<userinput>/usr/local/pgsql/bin/initdb -D /usr/local/pgsql/data</userinput> +</programlisting> + </para> + </step> + + <step> + <para> + Restore your previous <filename>pg_hba.conf</filename> and any + <filename>postgresql.conf</filename> modifications. + </para> + </step> + + <step> + <para> + Start the database server, again using the special database user + account: +<programlisting> +<userinput>/usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data</userinput> +</programlisting> + </para> + </step> + + <step> + <para> + Finally, restore your data from backup with: +<screen> +<userinput>/usr/local/pgsql/bin/psql -d postgres -f <replaceable>outputfile</replaceable></userinput> +</screen> + using the <emphasis>new</emphasis> <application>psql</application>. + </para> + </step> + </procedure> + + <para> + The least downtime can be achieved by installing the new server in + a different directory and running both the old and the new servers + in parallel, on different ports. Then you can use something like: + +<programlisting> +pg_dumpall -p 5432 | psql -d postgres -p 5433 +</programlisting> + to transfer your data. + </para> + + </sect2> + + <sect2 id="upgrading-via-pg-upgrade"> + <title>Upgrading Data via <application>pg_upgrade</application></title> + + <para> + The <xref linkend="pgupgrade"/> module allows an installation to + be migrated in-place from one major <productname>PostgreSQL</productname> + version to another. Upgrades can be performed in minutes, + particularly with <option>--link</option> mode. It requires steps similar to + <application>pg_dumpall</application> above, e.g., starting/stopping the server, + running <application>initdb</application>. The <application>pg_upgrade</application> <link + linkend="pgupgrade">documentation</link> outlines the necessary steps. + </para> + + </sect2> + + <sect2 id="upgrading-via-replication"> + <title>Upgrading Data via Replication</title> + + <para> + It is also possible to use logical replication methods to create a standby + server with the updated version of <productname>PostgreSQL</productname>. + This is possible because logical replication supports + replication between different major versions of + <productname>PostgreSQL</productname>. The standby can be on the same computer or + a different computer. Once it has synced up with the primary server + (running the older version of <productname>PostgreSQL</productname>), you can + switch primaries and make the standby the primary and shut down the older + database instance. Such a switch-over results in only several seconds + of downtime for an upgrade. + </para> + + <para> + This method of upgrading can be performed using the built-in logical + replication facilities as well as using external logical replication + systems such as <productname>pglogical</productname>, + <productname>Slony</productname>, <productname>Londiste</productname>, and + <productname>Bucardo</productname>. + </para> + </sect2> + </sect1> + + <sect1 id="preventing-server-spoofing"> + <title>Preventing Server Spoofing</title> + + <indexterm zone="preventing-server-spoofing"> + <primary>server spoofing</primary> + </indexterm> + + <para> + While the server is running, it is not possible for a malicious user + to take the place of the normal database server. However, when the + server is down, it is possible for a local user to spoof the normal + server by starting their own server. The spoof server could read + passwords and queries sent by clients, but could not return any data + because the <varname>PGDATA</varname> directory would still be secure because + of directory permissions. Spoofing is possible because any user can + start a database server; a client cannot identify an invalid server + unless it is specially configured. + </para> + + <para> + One way to prevent spoofing of <literal>local</literal> + connections is to use a Unix domain socket directory (<xref + linkend="guc-unix-socket-directories"/>) that has write permission only + for a trusted local user. This prevents a malicious user from creating + their own socket file in that directory. If you are concerned that + some applications might still reference <filename>/tmp</filename> for the + socket file and hence be vulnerable to spoofing, during operating system + startup create a symbolic link <filename>/tmp/.s.PGSQL.5432</filename> that points + to the relocated socket file. You also might need to modify your + <filename>/tmp</filename> cleanup script to prevent removal of the symbolic link. + </para> + + <para> + Another option for <literal>local</literal> connections is for clients to use + <link linkend="libpq-connect-requirepeer"><literal>requirepeer</literal></link> + to specify the required owner of the server process connected to + the socket. + </para> + + <para> + To prevent spoofing on TCP connections, either use + SSL certificates and make sure that clients check the server's certificate, + or use GSSAPI encryption (or both, if they're on separate connections). + </para> + + <para> + To prevent spoofing with SSL, the server + must be configured to accept only <literal>hostssl</literal> connections (<xref + linkend="auth-pg-hba-conf"/>) and have SSL key and certificate files + (<xref linkend="ssl-tcp"/>). The TCP client must connect using + <literal>sslmode=verify-ca</literal> or + <literal>verify-full</literal> and have the appropriate root certificate + file installed (<xref linkend="libq-ssl-certificates"/>). + </para> + + <para> + To prevent spoofing with GSSAPI, the server must be configured to accept + only <literal>hostgssenc</literal> connections + (<xref linkend="auth-pg-hba-conf"/>) and use <literal>gss</literal> + authentication with them. The TCP client must connect + using <literal>gssencmode=require</literal>. + </para> + </sect1> + + <sect1 id="encryption-options"> + <title>Encryption Options</title> + + <indexterm zone="encryption-options"> + <primary>encryption</primary> + </indexterm> + + <para> + <productname>PostgreSQL</productname> offers encryption at several + levels, and provides flexibility in protecting data from disclosure + due to database server theft, unscrupulous administrators, and + insecure networks. Encryption might also be required to secure + sensitive data such as medical records or financial transactions. + </para> + + <variablelist> + + <varlistentry> + <term>Password Encryption</term> + <listitem> + + <para> + Database user passwords are stored as hashes (determined by the setting + <xref linkend="guc-password-encryption"/>), so the administrator cannot + determine the actual password assigned to the user. If SCRAM or MD5 + encryption is used for client authentication, the unencrypted password is + never even temporarily present on the server because the client encrypts + it before being sent across the network. SCRAM is preferred, because it + is an Internet standard and is more secure than the PostgreSQL-specific + MD5 authentication protocol. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Encryption For Specific Columns</term> + + <listitem> + <para> + The <xref linkend="pgcrypto"/> module allows certain fields to be + stored encrypted. + This is useful if only some of the data is sensitive. + The client supplies the decryption key and the data is decrypted + on the server and then sent to the client. + </para> + + <para> + The decrypted data and the decryption key are present on the + server for a brief time while it is being decrypted and + communicated between the client and server. This presents a brief + moment where the data and keys can be intercepted by someone with + complete access to the database server, such as the system + administrator. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Data Partition Encryption</term> + + <listitem> + <para> + Storage encryption can be performed at the file system level or the + block level. Linux file system encryption options include eCryptfs + and EncFS, while FreeBSD uses PEFS. Block level or full disk + encryption options include dm-crypt + LUKS on Linux and GEOM + modules geli and gbde on FreeBSD. Many other operating systems + support this functionality, including Windows. + </para> + + <para> + This mechanism prevents unencrypted data from being read from the + drives if the drives or the entire computer is stolen. This does + not protect against attacks while the file system is mounted, + because when mounted, the operating system provides an unencrypted + view of the data. However, to mount the file system, you need some + way for the encryption key to be passed to the operating system, + and sometimes the key is stored somewhere on the host that mounts + the disk. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Encrypting Data Across A Network</term> + + <listitem> + <para> + SSL connections encrypt all data sent across the network: the + password, the queries, and the data returned. The + <filename>pg_hba.conf</filename> file allows administrators to specify + which hosts can use non-encrypted connections (<literal>host</literal>) + and which require SSL-encrypted connections + (<literal>hostssl</literal>). Also, clients can specify that they + connect to servers only via SSL. + </para> + + <para> + GSSAPI-encrypted connections encrypt all data sent across the network, + including queries and data returned. (No password is sent across the + network.) The <filename>pg_hba.conf</filename> file allows + administrators to specify which hosts can use non-encrypted connections + (<literal>host</literal>) and which require GSSAPI-encrypted connections + (<literal>hostgssenc</literal>). Also, clients can specify that they + connect to servers only on GSSAPI-encrypted connections + (<literal>gssencmode=require</literal>). + </para> + + <para> + <application>Stunnel</application> or + <application>SSH</application> can also be used to encrypt + transmissions. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>SSL Host Authentication</term> + + <listitem> + <para> + It is possible for both the client and server to provide SSL + certificates to each other. It takes some extra configuration + on each side, but this provides stronger verification of identity + than the mere use of passwords. It prevents a computer from + pretending to be the server just long enough to read the password + sent by the client. It also helps prevent <quote>man in the middle</quote> + attacks where a computer between the client and server pretends to + be the server and reads and passes all data between the client and + server. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Client-Side Encryption</term> + + <listitem> + <para> + If the system administrator for the server's machine cannot be trusted, + it is necessary + for the client to encrypt the data; this way, unencrypted data + never appears on the database server. Data is encrypted on the + client before being sent to the server, and database results have + to be decrypted on the client before being used. + </para> + </listitem> + </varlistentry> + + </variablelist> + + </sect1> + + <sect1 id="ssl-tcp"> + <title>Secure TCP/IP Connections with SSL</title> + + <indexterm zone="ssl-tcp"> + <primary>SSL</primary> + <secondary>TLS</secondary> + </indexterm> + + <para> + <productname>PostgreSQL</productname> has native support for using + <acronym>SSL</acronym> connections to encrypt client/server communications + for increased security. This requires that + <productname>OpenSSL</productname> is installed on both client and + server systems and that support in <productname>PostgreSQL</productname> is + enabled at build time (see <xref linkend="installation"/>). + </para> + + <para> + The terms <acronym>SSL</acronym> and <acronym>TLS</acronym> are often used + interchangeably to mean a secure encrypted connection using a + <acronym>TLS</acronym> protocol. <acronym>SSL</acronym> protocols are the + precursors to <acronym>TLS</acronym> protocols, and the term + <acronym>SSL</acronym> is still used for encrypted connections even though + <acronym>SSL</acronym> protocols are no longer supported. + <acronym>SSL</acronym> is used interchangeably with <acronym>TLS</acronym> + in <productname>PostgreSQL</productname>. + + </para> + <sect2 id="ssl-setup"> + <title>Basic Setup</title> + + <para> + With <acronym>SSL</acronym> support compiled in, the + <productname>PostgreSQL</productname> server can be started with + support for encrypted connections using <acronym>TLS</acronym> protocols + enabled by setting the parameter + <xref linkend="guc-ssl"/> to <literal>on</literal> in + <filename>postgresql.conf</filename>. The server will listen for both normal + and <acronym>SSL</acronym> connections on the same TCP port, and will negotiate + with any connecting client on whether to use <acronym>SSL</acronym>. By + default, this is at the client's option; see <xref + linkend="auth-pg-hba-conf"/> about how to set up the server to require + use of <acronym>SSL</acronym> for some or all connections. + </para> + + <para> + To start in <acronym>SSL</acronym> mode, files containing the server certificate + and private key must exist. By default, these files are expected to be + named <filename>server.crt</filename> and <filename>server.key</filename>, respectively, in + the server's data directory, but other names and locations can be specified + using the configuration parameters <xref linkend="guc-ssl-cert-file"/> + and <xref linkend="guc-ssl-key-file"/>. + </para> + + <para> + On Unix systems, the permissions on <filename>server.key</filename> must + disallow any access to world or group; achieve this by the command + <command>chmod 0600 server.key</command>. Alternatively, the file can be + owned by root and have group read access (that is, <literal>0640</literal> + permissions). That setup is intended for installations where certificate + and key files are managed by the operating system. The user under which + the <productname>PostgreSQL</productname> server runs should then be made a + member of the group that has access to those certificate and key files. + </para> + + <para> + If the data directory allows group read access then certificate files may + need to be located outside of the data directory in order to conform to the + security requirements outlined above. Generally, group access is enabled + to allow an unprivileged user to backup the database, and in that case the + backup software will not be able to read the certificate files and will + likely error. + </para> + + <para> + If the private key is protected with a passphrase, the + server will prompt for the passphrase and will not start until it has + been entered. + Using a passphrase by default disables the ability to change the server's + SSL configuration without a server restart, but see <xref + linkend="guc-ssl-passphrase-command-supports-reload"/>. + Furthermore, passphrase-protected private keys cannot be used at all + on Windows. + </para> + + <para> + The first certificate in <filename>server.crt</filename> must be the + server's certificate because it must match the server's private key. + The certificates of <quote>intermediate</quote> certificate authorities + can also be appended to the file. Doing this avoids the necessity of + storing intermediate certificates on clients, assuming the root and + intermediate certificates were created with <literal>v3_ca</literal> + extensions. (This sets the certificate's basic constraint of + <literal>CA</literal> to <literal>true</literal>.) + This allows easier expiration of intermediate certificates. + </para> + + <para> + It is not necessary to add the root certificate to + <filename>server.crt</filename>. Instead, clients must have the root + certificate of the server's certificate chain. + </para> + </sect2> + + <sect2 id="ssl-openssl-config"> + <title>OpenSSL Configuration</title> + + <para> + <productname>PostgreSQL</productname> reads the system-wide + <productname>OpenSSL</productname> configuration file. By default, this + file is named <filename>openssl.cnf</filename> and is located in the + directory reported by <literal>openssl version -d</literal>. + This default can be overridden by setting environment variable + <envar>OPENSSL_CONF</envar> to the name of the desired configuration file. + </para> + + <para> + <productname>OpenSSL</productname> supports a wide range of ciphers + and authentication algorithms, of varying strength. While a list of + ciphers can be specified in the <productname>OpenSSL</productname> + configuration file, you can specify ciphers specifically for use by + the database server by modifying <xref linkend="guc-ssl-ciphers"/> in + <filename>postgresql.conf</filename>. + </para> + + <note> + <para> + It is possible to have authentication without encryption overhead by + using <literal>NULL-SHA</literal> or <literal>NULL-MD5</literal> ciphers. However, + a man-in-the-middle could read and pass communications between client + and server. Also, encryption overhead is minimal compared to the + overhead of authentication. For these reasons NULL ciphers are not + recommended. + </para> + </note> + </sect2> + + <sect2 id="ssl-client-certificates"> + <title>Using Client Certificates</title> + + <para> + To require the client to supply a trusted certificate, + place certificates of the root certificate authorities + (<acronym>CA</acronym>s) you trust in a file in the data + directory, set the parameter <xref linkend="guc-ssl-ca-file"/> in + <filename>postgresql.conf</filename> to the new file name, and add the + authentication option <literal>clientcert=verify-ca</literal> or + <literal>clientcert=verify-full</literal> to the appropriate + <literal>hostssl</literal> line(s) in <filename>pg_hba.conf</filename>. + A certificate will then be requested from the client during SSL + connection startup. (See <xref linkend="libpq-ssl"/> for a description + of how to set up certificates on the client.) + </para> + + <para> + For a <literal>hostssl</literal> entry with + <literal>clientcert=verify-ca</literal>, the server will verify + that the client's certificate is signed by one of the trusted + certificate authorities. If <literal>clientcert=verify-full</literal> + is specified, the server will not only verify the certificate + chain, but it will also check whether the username or its mapping + matches the <literal>cn</literal> (Common Name) of the provided certificate. + Note that certificate chain validation is always ensured when the + <literal>cert</literal> authentication method is used + (see <xref linkend="auth-cert"/>). + </para> + + <para> + Intermediate certificates that chain up to existing root certificates + can also appear in the <xref linkend="guc-ssl-ca-file"/> file if + you wish to avoid storing them on clients (assuming the root and + intermediate certificates were created with <literal>v3_ca</literal> + extensions). Certificate Revocation List (CRL) entries are also + checked if the parameter <xref linkend="guc-ssl-crl-file"/> or + <xref linkend="guc-ssl-crl-dir"/> is set. + </para> + + <para> + The <literal>clientcert</literal> authentication option is available for + all authentication methods, but only in <filename>pg_hba.conf</filename> lines + specified as <literal>hostssl</literal>. When <literal>clientcert</literal> is + not specified, the server verifies the client certificate against its CA + file only if a client certificate is presented and the CA is configured. + </para> + + <para> + There are two approaches to enforce that users provide a certificate during login. + </para> + + <para> + The first approach makes use of the <literal>cert</literal> authentication + method for <literal>hostssl</literal> entries in <filename>pg_hba.conf</filename>, + such that the certificate itself is used for authentication while also + providing ssl connection security. See <xref linkend="auth-cert"/> for details. + (It is not necessary to specify any <literal>clientcert</literal> options + explicitly when using the <literal>cert</literal> authentication method.) + In this case, the <literal>cn</literal> (Common Name) provided in + the certificate is checked against the user name or an applicable mapping. + </para> + + <para> + The second approach combines any authentication method for <literal>hostssl</literal> + entries with the verification of client certificates by setting the + <literal>clientcert</literal> authentication option to <literal>verify-ca</literal> + or <literal>verify-full</literal>. The former option only enforces that + the certificate is valid, while the latter also ensures that the + <literal>cn</literal> (Common Name) in the certificate matches + the user name or an applicable mapping. + </para> + </sect2> + + <sect2 id="ssl-server-files"> + <title>SSL Server File Usage</title> + + <para> + <xref linkend="ssl-file-usage"/> summarizes the files that are + relevant to the SSL setup on the server. (The shown file names are default + names. The locally configured names could be different.) + </para> + + <table id="ssl-file-usage"> + <title>SSL Server File Usage</title> + <tgroup cols="3"> + <thead> + <row> + <entry>File</entry> + <entry>Contents</entry> + <entry>Effect</entry> + </row> + </thead> + + <tbody> + + <row> + <entry><xref linkend="guc-ssl-cert-file"/> (<filename>$PGDATA/server.crt</filename>)</entry> + <entry>server certificate</entry> + <entry>sent to client to indicate server's identity</entry> + </row> + + <row> + <entry><xref linkend="guc-ssl-key-file"/> (<filename>$PGDATA/server.key</filename>)</entry> + <entry>server private key</entry> + <entry>proves server certificate was sent by the owner; does not indicate + certificate owner is trustworthy</entry> + </row> + + <row> + <entry><xref linkend="guc-ssl-ca-file"/></entry> + <entry>trusted certificate authorities</entry> + <entry>checks that client certificate is + signed by a trusted certificate authority</entry> + </row> + + <row> + <entry><xref linkend="guc-ssl-crl-file"/></entry> + <entry>certificates revoked by certificate authorities</entry> + <entry>client certificate must not be on this list</entry> + </row> + + </tbody> + </tgroup> + </table> + + <para> + The server reads these files at server start and whenever the server + configuration is reloaded. On <systemitem class="osname">Windows</systemitem> + systems, they are also re-read whenever a new backend process is spawned + for a new client connection. + </para> + + <para> + If an error in these files is detected at server start, the server will + refuse to start. But if an error is detected during a configuration + reload, the files are ignored and the old SSL configuration continues to + be used. On <systemitem class="osname">Windows</systemitem> systems, if an error in + these files is detected at backend start, that backend will be unable to + establish an SSL connection. In all these cases, the error condition is + reported in the server log. + </para> + </sect2> + + <sect2 id="ssl-certificate-creation"> + <title>Creating Certificates</title> + + <para> + To create a simple self-signed certificate for the server, valid for 365 + days, use the following <productname>OpenSSL</productname> command, + replacing <replaceable>dbhost.yourdomain.com</replaceable> with the + server's host name: +<programlisting> +openssl req -new -x509 -days 365 -nodes -text -out server.crt \ + -keyout server.key -subj "/CN=<replaceable>dbhost.yourdomain.com</replaceable>" +</programlisting> + Then do: +<programlisting> +chmod og-rwx server.key +</programlisting> + because the server will reject the file if its permissions are more + liberal than this. + For more details on how to create your server private key and + certificate, refer to the <productname>OpenSSL</productname> documentation. + </para> + + <para> + While a self-signed certificate can be used for testing, a certificate + signed by a certificate authority (<acronym>CA</acronym>) (usually an + enterprise-wide root <acronym>CA</acronym>) should be used in production. + </para> + + <para> + To create a server certificate whose identity can be validated + by clients, first create a certificate signing request + (<acronym>CSR</acronym>) and a public/private key file: +<programlisting> +openssl req -new -nodes -text -out root.csr \ + -keyout root.key -subj "/CN=<replaceable>root.yourdomain.com</replaceable>" +chmod og-rwx root.key +</programlisting> + Then, sign the request with the key to create a root certificate + authority (using the default <productname>OpenSSL</productname> + configuration file location on <productname>Linux</productname>): +<programlisting> +openssl x509 -req -in root.csr -text -days 3650 \ + -extfile /etc/ssl/openssl.cnf -extensions v3_ca \ + -signkey root.key -out root.crt +</programlisting> + Finally, create a server certificate signed by the new root certificate + authority: +<programlisting> +openssl req -new -nodes -text -out server.csr \ + -keyout server.key -subj "/CN=<replaceable>dbhost.yourdomain.com</replaceable>" +chmod og-rwx server.key + +openssl x509 -req -in server.csr -text -days 365 \ + -CA root.crt -CAkey root.key -CAcreateserial \ + -out server.crt +</programlisting> + <filename>server.crt</filename> and <filename>server.key</filename> + should be stored on the server, and <filename>root.crt</filename> should + be stored on the client so the client can verify that the server's leaf + certificate was signed by its trusted root certificate. + <filename>root.key</filename> should be stored offline for use in + creating future certificates. + </para> + + <para> + It is also possible to create a chain of trust that includes + intermediate certificates: +<programlisting> +# root +openssl req -new -nodes -text -out root.csr \ + -keyout root.key -subj "/CN=<replaceable>root.yourdomain.com</replaceable>" +chmod og-rwx root.key +openssl x509 -req -in root.csr -text -days 3650 \ + -extfile /etc/ssl/openssl.cnf -extensions v3_ca \ + -signkey root.key -out root.crt + +# intermediate +openssl req -new -nodes -text -out intermediate.csr \ + -keyout intermediate.key -subj "/CN=<replaceable>intermediate.yourdomain.com</replaceable>" +chmod og-rwx intermediate.key +openssl x509 -req -in intermediate.csr -text -days 1825 \ + -extfile /etc/ssl/openssl.cnf -extensions v3_ca \ + -CA root.crt -CAkey root.key -CAcreateserial \ + -out intermediate.crt + +# leaf +openssl req -new -nodes -text -out server.csr \ + -keyout server.key -subj "/CN=<replaceable>dbhost.yourdomain.com</replaceable>" +chmod og-rwx server.key +openssl x509 -req -in server.csr -text -days 365 \ + -CA intermediate.crt -CAkey intermediate.key -CAcreateserial \ + -out server.crt +</programlisting> + <filename>server.crt</filename> and + <filename>intermediate.crt</filename> should be concatenated + into a certificate file bundle and stored on the server. + <filename>server.key</filename> should also be stored on the server. + <filename>root.crt</filename> should be stored on the client so + the client can verify that the server's leaf certificate was signed + by a chain of certificates linked to its trusted root certificate. + <filename>root.key</filename> and <filename>intermediate.key</filename> + should be stored offline for use in creating future certificates. + </para> + </sect2> + + </sect1> + + <sect1 id="gssapi-enc"> + <title>Secure TCP/IP Connections with GSSAPI Encryption</title> + + <indexterm zone="gssapi-enc"> + <primary>gssapi</primary> + </indexterm> + + <para> + <productname>PostgreSQL</productname> also has native support for + using <acronym>GSSAPI</acronym> to encrypt client/server communications for + increased security. Support requires that a <acronym>GSSAPI</acronym> + implementation (such as MIT Kerberos) is installed on both client and server + systems, and that support in <productname>PostgreSQL</productname> is + enabled at build time (see <xref linkend="installation"/>). + </para> + + <sect2 id="gssapi-setup"> + <title>Basic Setup</title> + + <para> + The <productname>PostgreSQL</productname> server will listen for both + normal and <acronym>GSSAPI</acronym>-encrypted connections on the same TCP + port, and will negotiate with any connecting client whether to + use <acronym>GSSAPI</acronym> for encryption (and for authentication). By + default, this decision is up to the client (which means it can be + downgraded by an attacker); see <xref linkend="auth-pg-hba-conf"/> about + setting up the server to require the use of <acronym>GSSAPI</acronym> for + some or all connections. + </para> + + <para> + When using <acronym>GSSAPI</acronym> for encryption, it is common to + use <acronym>GSSAPI</acronym> for authentication as well, since the + underlying mechanism will determine both client and server identities + (according to the <acronym>GSSAPI</acronym> implementation) in any + case. But this is not required; + another <productname>PostgreSQL</productname> authentication method + can be chosen to perform additional verification. + </para> + + <para> + Other than configuration of the negotiation + behavior, <acronym>GSSAPI</acronym> encryption requires no setup beyond + that which is necessary for GSSAPI authentication. (For more information + on configuring that, see <xref linkend="gssapi-auth"/>.) + </para> + </sect2> + </sect1> + + <sect1 id="ssh-tunnels"> + <title>Secure TCP/IP Connections with <application>SSH</application> Tunnels</title> + + <indexterm zone="ssh-tunnels"> + <primary>ssh</primary> + </indexterm> + + <para> + It is possible to use <application>SSH</application> to encrypt the network + connection between clients and a + <productname>PostgreSQL</productname> server. Done properly, this + provides an adequately secure network connection, even for non-SSL-capable + clients. + </para> + + <para> + First make sure that an <application>SSH</application> server is + running properly on the same machine as the + <productname>PostgreSQL</productname> server and that you can log in using + <command>ssh</command> as some user; you then can establish a + secure tunnel to the remote server. A secure tunnel listens on a + local port and forwards all traffic to a port on the remote machine. + Traffic sent to the remote port can arrive on its + <literal>localhost</literal> address, or different bind + address if desired; it does not appear as coming from your + local machine. This command creates a secure tunnel from the client + machine to the remote machine <literal>foo.com</literal>: +<programlisting> +ssh -L 63333:localhost:5432 joe@foo.com +</programlisting> + The first number in the <option>-L</option> argument, 63333, is the + local port number of the tunnel; it can be any unused port. (IANA + reserves ports 49152 through 65535 for private use.) The name or IP + address after this is the remote bind address you are connecting to, + i.e., <literal>localhost</literal>, which is the default. The second + number, 5432, is the remote end of the tunnel, e.g., the port number + your database server is using. In order to connect to the database + server using this tunnel, you connect to port 63333 on the local + machine: +<programlisting> +psql -h localhost -p 63333 postgres +</programlisting> + To the database server it will then look as though you are + user <literal>joe</literal> on host <literal>foo.com</literal> + connecting to the <literal>localhost</literal> bind address, and it + will use whatever authentication procedure was configured for + connections by that user to that bind address. Note that the server will not + think the connection is SSL-encrypted, since in fact it is not + encrypted between the + <application>SSH</application> server and the + <productname>PostgreSQL</productname> server. This should not pose any + extra security risk because they are on the same machine. + </para> + + <para> + In order for the + tunnel setup to succeed you must be allowed to connect via + <command>ssh</command> as <literal>joe@foo.com</literal>, just + as if you had attempted to use <command>ssh</command> to create a + terminal session. + </para> + + <para> + You could also have set up port forwarding as +<programlisting> +ssh -L 63333:foo.com:5432 joe@foo.com +</programlisting> + but then the database server will see the connection as coming in + on its <literal>foo.com</literal> bind address, which is not opened by + the default setting <literal>listen_addresses = + 'localhost'</literal>. This is usually not what you want. + </para> + + <para> + If you have to <quote>hop</quote> to the database server via some + login host, one possible setup could look like this: +<programlisting> +ssh -L 63333:db.foo.com:5432 joe@shell.foo.com +</programlisting> + Note that this way the connection + from <literal>shell.foo.com</literal> + to <literal>db.foo.com</literal> will not be encrypted by the SSH + tunnel. + SSH offers quite a few configuration possibilities when the network + is restricted in various ways. Please refer to the SSH + documentation for details. + </para> + + <tip> + <para> + Several other applications exist that can provide secure tunnels using + a procedure similar in concept to the one just described. + </para> + </tip> + + </sect1> + + <sect1 id="event-log-registration"> + <title>Registering <application>Event Log</application> on <systemitem + class="osname">Windows</systemitem></title> + + <indexterm zone="event-log-registration"> + <primary>event log</primary> + <secondary>event log</secondary> + </indexterm> + + <para> + To register a <systemitem class="osname">Windows</systemitem> + <application>event log</application> library with the operating system, + issue this command: +<screen> +<userinput>regsvr32 <replaceable>pgsql_library_directory</replaceable>/pgevent.dll</userinput> +</screen> + This creates registry entries used by the event viewer, under the default + event source named <literal>PostgreSQL</literal>. + </para> + + <para> + To specify a different event source name (see + <xref linkend="guc-event-source"/>), use the <literal>/n</literal> + and <literal>/i</literal> options: +<screen> +<userinput>regsvr32 /n /i:<replaceable>event_source_name</replaceable> <replaceable>pgsql_library_directory</replaceable>/pgevent.dll</userinput> +</screen> + </para> + + <para> + To unregister the <application>event log</application> library from + the operating system, issue this command: +<screen> +<userinput>regsvr32 /u [/i:<replaceable>event_source_name</replaceable>] <replaceable>pgsql_library_directory</replaceable>/pgevent.dll</userinput> +</screen> + </para> + + <note> + <para> + To enable event logging in the database server, modify + <xref linkend="guc-log-destination"/> to include + <literal>eventlog</literal> in <filename>postgresql.conf</filename>. + </para> + </note> + </sect1> + +</chapter> |