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diff --git a/proto/QSHAPE_README.html b/proto/QSHAPE_README.html new file mode 100644 index 0000000..14209fe --- /dev/null +++ b/proto/QSHAPE_README.html @@ -0,0 +1,938 @@ +<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN" + "http://www.w3.org/TR/html4/loose.dtd"> + +<html> + +<head> + +<title>Postfix Bottleneck Analysis</title> + +<meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> + +</head> + +<body> + +<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Bottleneck Analysis</h1> + +<hr> + +<h2>Purpose of this document </h2> + +<p> This document is an introduction to Postfix queue congestion analysis. +It explains how the qshape(1) program can help to track down the +reason for queue congestion. qshape(1) is bundled with Postfix +2.1 and later source code, under the "auxiliary" directory. This +document describes qshape(1) as bundled with Postfix 2.4. </p> + +<p> This document covers the following topics: </p> + +<ul> + +<li><a href="#qshape">Introducing the qshape tool</a> + +<li><a href="#trouble_shooting">Trouble shooting with qshape</a> + +<li><a href="#healthy">Example 1: Healthy queue</a> + +<li><a href="#dictionary_bounce">Example 2: Deferred queue full of +dictionary attack bounces</a></li> + +<li><a href="#active_congestion">Example 3: Congestion in the active +queue</a></li> + +<li><a href="#backlog">Example 4: High volume destination backlog</a> + +<li><a href="#queues">Postfix queue directories</a> + +<ul> + +<li> <a href="#maildrop_queue"> The "maildrop" queue </a> + +<li> <a href="#hold_queue"> The "hold" queue </a> + +<li> <a href="#incoming_queue"> The "incoming" queue </a> + +<li> <a href="#active_queue"> The "active" queue </a> + +<li> <a href="#deferred_queue"> The "deferred" queue </a> + +</ul> + +<li><a href="#credits">Credits</a> + +</ul> + +<h2><a name="qshape">Introducing the qshape tool</a></h2> + +<p> When mail is draining slowly or the queue is unexpectedly large, +run qshape(1) as the super-user (root) to help zero in on the problem. +The qshape(1) program displays a tabular view of the Postfix queue +contents. </p> + +<ul> + +<li> <p> On the horizontal axis, it displays the queue age with +fine granularity for recent messages and (geometrically) less fine +granularity for older messages. </p> + +<li> <p> The vertical axis displays the destination (or with the +"-s" switch the sender) domain. Domains with the most messages are +listed first. </p> + +</ul> + +<p> For example, in the output below we see the top 10 lines of +the (mostly forged) sender domain distribution for captured spam +in the "hold" queue: </p> + +<blockquote> +<pre> +$ qshape -s hold | head + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 486 0 0 1 0 0 2 4 20 40 419 + yahoo.com 14 0 0 1 0 0 0 0 1 0 12 + extremepricecuts.net 13 0 0 0 0 0 0 0 2 0 11 + ms35.hinet.net 12 0 0 0 0 0 0 0 0 1 11 + winnersdaily.net 12 0 0 0 0 0 0 0 2 0 10 + hotmail.com 11 0 0 0 0 0 0 0 0 1 10 + worldnet.fr 6 0 0 0 0 0 0 0 0 0 6 + ms41.hinet.net 6 0 0 0 0 0 0 0 0 0 6 + osn.de 5 0 0 0 0 0 1 0 0 0 4 +</pre> +</blockquote> + +<ul> + +<li> <p> The "T" column shows the total (in this case sender) count +for each domain. The columns with numbers above them, show counts +for messages aged fewer than that many minutes, but not younger +than the age limit for the previous column. The row labeled "TOTAL" +shows the total count for all domains. </p> + +<li> <p> In this example, there are 14 messages allegedly from +yahoo.com, 1 between 10 and 20 minutes old, 1 between 320 and 640 +minutes old and 12 older than 1280 minutes (1440 minutes in a day). +</p> + +</ul> + +<p> When the output is a terminal intermediate results showing the top 20 +domains (-n option) are displayed after every 1000 messages (-N option) +and the final output also shows only the top 20 domains. This makes +qshape useful even when the deferred queue is very large and it may +otherwise take prohibitively long to read the entire deferred queue. </p> + +<p> By default, qshape shows statistics for the union of both the +incoming and active queues which are the most relevant queues to +look at when analyzing performance. </p> + +<p> One can request an alternate list of queues: </p> + +<blockquote> +<pre> +$ qshape deferred +$ qshape incoming active deferred +</pre> +</blockquote> + +<p> this will show the age distribution of the deferred queue or +the union of the incoming active and deferred queues. </p> + +<p> Command line options control the number of display "buckets", +the age limit for the smallest bucket, display of parent domain +counts and so on. The "-h" option outputs a summary of the available +switches. </p> + +<h2><a name="trouble_shooting">Trouble shooting with qshape</a> +</h2> + +<p> Large numbers in the qshape output represent a large number of +messages that are destined to (or alleged to come from) a particular +domain. It should be possible to tell at a glance which domains +dominate the queue sender or recipient counts, approximately when +a burst of mail started, and when it stopped. </p> + +<p> The problem destinations or sender domains appear near the top +left corner of the output table. Remember that the active queue +can accommodate up to 20000 ($qmgr_message_active_limit) messages. +To check whether this limit has been reached, use: </p> + +<blockquote> +<pre> +$ qshape -s active <i>(show sender statistics)</i> +</pre> +</blockquote> + +<p> If the total sender count is below 20000 the active queue is +not yet saturated, any high volume sender domains show near the +top of the output. + +<p> With oqmgr(8) the active queue is also limited to at most 20000 +recipient addresses ($qmgr_message_recipient_limit). To check for +exhaustion of this limit use: </p> + +<blockquote> +<pre> +$ qshape active <i>(show recipient statistics)</i> +</pre> +</blockquote> + +<p> Having found the high volume domains, it is often useful to +search the logs for recent messages pertaining to the domains in +question. </p> + +<blockquote> +<pre> +# Find deliveries to example.com +# +$ tail -10000 /var/log/maillog | + egrep -i ': to=<.*@example\.com>,' | + less + +# Find messages from example.com +# +$ tail -10000 /var/log/maillog | + egrep -i ': from=<.*@example\.com>,' | + less +</pre> +</blockquote> + +<p> You may want to drill in on some specific queue ids: </p> + +<blockquote> +<pre> +# Find all messages for a specific queue id. +# +$ tail -10000 /var/log/maillog | egrep ': 2B2173FF68: ' +</pre> +</blockquote> + +<p> Also look for queue manager warning messages in the log. These +warnings can suggest strategies to reduce congestion. </p> + +<blockquote> +<pre> +$ egrep 'qmgr.*(panic|fatal|error|warning):' /var/log/maillog +</pre> +</blockquote> + +<p> When all else fails try the Postfix mailing list for help, but +please don't forget to include the top 10 or 20 lines of qshape(1) +output. </p> + +<h2><a name="healthy">Example 1: Healthy queue</a></h2> + +<p> When looking at just the incoming and active queues, under +normal conditions (no congestion) the incoming and active queues +are nearly empty. Mail leaves the system almost as quickly as it +comes in or is deferred without congestion in the active queue. +</p> + +<blockquote> +<pre> +$ qshape <i>(show incoming and active queue status)</i> + + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 5 0 0 0 1 0 0 0 1 1 2 + meri.uwasa.fi 5 0 0 0 1 0 0 0 1 1 2 +</pre> +</blockquote> + +<p> If one looks at the two queues separately, the incoming queue +is empty or perhaps briefly has one or two messages, while the +active queue holds more messages and for a somewhat longer time: +</p> + +<blockquote> +<pre> +$ qshape incoming + + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 0 0 0 0 0 0 0 0 0 0 0 + +$ qshape active + + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 5 0 0 0 1 0 0 0 1 1 2 + meri.uwasa.fi 5 0 0 0 1 0 0 0 1 1 2 +</pre> +</blockquote> + +<h2><a name="dictionary_bounce">Example 2: Deferred queue full of +dictionary attack bounces</a></h2> + +<p> This is from a server where recipient validation is not yet +available for some of the hosted domains. Dictionary attacks on +the unvalidated domains result in bounce backscatter. The bounces +dominate the queue, but with proper tuning they do not saturate the +incoming or active queues. The high volume of deferred mail is not +a direct cause for alarm. </p> + +<blockquote> +<pre> +$ qshape deferred | head + + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 2234 4 2 5 9 31 57 108 201 464 1353 + heyhihellothere.com 207 0 0 1 1 6 6 8 25 68 92 + pleazerzoneprod.com 105 0 0 0 0 0 0 0 5 44 56 + groups.msn.com 63 2 1 2 4 4 14 14 14 8 0 + orion.toppoint.de 49 0 0 0 1 0 2 4 3 16 23 + kali.com.cn 46 0 0 0 0 1 0 2 6 12 25 + meri.uwasa.fi 44 0 0 0 0 1 0 2 8 11 22 + gjr.paknet.com.pk 43 1 0 0 1 1 3 3 6 12 16 + aristotle.algonet.se 41 0 0 0 0 0 1 2 11 12 15 +</pre> +</blockquote> + +<p> The domains shown are mostly bulk-mailers and all the volume +is the tail end of the time distribution, showing that short term +arrival rates are moderate. Larger numbers and lower message ages +are more indicative of current trouble. Old mail still going nowhere +is largely harmless so long as the active and incoming queues are +short. We can also see that the groups.msn.com undeliverables are +low rate steady stream rather than a concentrated dictionary attack +that is now over. </p> + +<blockquote> +<pre> +$ qshape -s deferred | head + + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 2193 4 4 5 8 33 56 104 205 465 1309 + MAILER-DAEMON 1709 4 4 5 8 33 55 101 198 452 849 + example.com 263 0 0 0 0 0 0 0 0 2 261 + example.org 209 0 0 0 0 0 1 3 6 11 188 + example.net 6 0 0 0 0 0 0 0 0 0 6 + example.edu 3 0 0 0 0 0 0 0 0 0 3 + example.gov 2 0 0 0 0 0 0 0 1 0 1 + example.mil 1 0 0 0 0 0 0 0 0 0 1 +</pre> +</blockquote> + +<p> Looking at the sender distribution, we see that as expected +most of the messages are bounces. </p> + +<h2><a name="active_congestion">Example 3: Congestion in the active +queue</a></h2> + +<p> This example is taken from a Feb 2004 discussion on the Postfix +Users list. Congestion was reported with the active and incoming +queues large and not shrinking despite very large delivery agent +process limits. The thread is archived at: +http://groups.google.com/groups?threadm=c0b7js$2r65$1@FreeBSD.csie.NCTU.edu.tw +and +http://archives.neohapsis.com/archives/postfix/2004-02/thread.html#1371 +</p> + +<p> Using an older version of qshape(1) it was quickly determined +that all the messages were for just a few destinations: </p> + +<blockquote> +<pre> +$ qshape <i>(show incoming and active queue status)</i> + + T A 5 10 20 40 80 160 320 320+ + TOTAL 11775 9996 0 0 1 1 42 94 221 1420 + user.sourceforge.net 7678 7678 0 0 0 0 0 0 0 0 + lists.sourceforge.net 2313 2313 0 0 0 0 0 0 0 0 + gzd.gotdns.com 102 0 0 0 0 0 0 0 2 100 +</pre> +</blockquote> + +<p> The "A" column showed the count of messages in the active queue, +and the numbered columns showed totals for the deferred queue. At +10000 messages (Postfix 1.x active queue size limit) the active +queue is full. The incoming was growing rapidly. </p> + +<p> With the trouble destinations clearly identified, the administrator +quickly found and fixed the problem. It is substantially harder to +glean the same information from the logs. While a careful reading +of mailq(1) output should yield similar results, it is much harder +to gauge the magnitude of the problem by looking at the queue +one message at a time. </p> + +<h2><a name="backlog">Example 4: High volume destination backlog</a></h2> + +<p> When a site you send a lot of email to is down or slow, mail +messages will rapidly build up in the deferred queue, or worse, in +the active queue. The qshape output will show large numbers for +the destination domain in all age buckets that overlap the starting +time of the problem: </p> + +<blockquote> +<pre> +$ qshape deferred | head + + T 5 10 20 40 80 160 320 640 1280 1280+ + TOTAL 5000 200 200 400 800 1600 1000 200 200 200 200 + highvolume.com 4000 160 160 320 640 1280 1440 0 0 0 0 + ... +</pre> +</blockquote> + +<p> Here the "highvolume.com" destination is continuing to accumulate +deferred mail. The incoming and active queues are fine, but the +deferred queue started growing some time between 1 and 2 hours ago +and continues to grow. </p> + +<p> If the high volume destination is not down, but is instead +slow, one might see similar congestion in the active queue. Active +queue congestion is a greater cause for alarm; one might need to +take measures to ensure that the mail is deferred instead or even +add an access(5) rule asking the sender to try again later. </p> + +<p> If a high volume destination exhibits frequent bursts of consecutive +connections refused by all MX hosts or "421 Server busy errors", it +is possible for the queue manager to mark the destination as "dead" +despite the transient nature of the errors. The destination will be +retried again after the expiration of a $minimal_backoff_time timer. +If the error bursts are frequent enough it may be that only a small +quantity of email is delivered before the destination is again marked +"dead". In some cases enabling static (not on demand) connection +caching by listing the appropriate nexthop domain in a table included in +"smtp_connection_cache_destinations" may help to reduce the error rate, +because most messages will re-use existing connections. </p> + +<p> The MTA that has been observed most frequently to exhibit such +bursts of errors is Microsoft Exchange, which refuses connections +under load. Some proxy virus scanners in front of the Exchange +server propagate the refused connection to the client as a "421" +error. </p> + +<p> Note that it is now possible to configure Postfix to exhibit similarly +erratic behavior by misconfiguring the anvil(8) service. Do not use +anvil(8) for steady-state rate limiting, its purpose is (unintentional) +DoS prevention and the rate limits set should be very generous! </p> + +<p> If one finds oneself needing to deliver a high volume of mail to a +destination that exhibits frequent brief bursts of errors and connection +caching does not solve the problem, there is a subtle workaround. </p> + +<ul> + +<li> <p> Postfix version 2.5 and later: </p> + +<ul> + +<li> <p> In master.cf set up a dedicated clone of the "smtp" transport +for the destination in question. In the example below we will call +it "fragile". </p> + +<li> <p> In master.cf configure a reasonable process limit for the +cloned smtp transport (a number in the 10-20 range is typical). </p> + +<li> <p> IMPORTANT!!! In main.cf configure a large per-destination +pseudo-cohort failure limit for the cloned smtp transport. </p> + +<pre> +/etc/postfix/main.cf: + transport_maps = hash:/etc/postfix/transport + fragile_destination_concurrency_failed_cohort_limit = 100 + fragile_destination_concurrency_limit = 20 + +/etc/postfix/transport: + example.com fragile: + +/etc/postfix/master.cf: + # service type private unpriv chroot wakeup maxproc command + fragile unix - - n - 20 smtp +</pre> + +<p> See also the documentation for +default_destination_concurrency_failed_cohort_limit and +default_destination_concurrency_limit. </p> + +</ul> + +<li> <p> Earlier Postfix versions: </p> + +<ul> + +<li> <p> In master.cf set up a dedicated clone of the "smtp" +transport for the destination in question. In the example below +we will call it "fragile". </p> + +<li> <p> In master.cf configure a reasonable process limit for the +transport (a number in the 10-20 range is typical). </p> + +<li> <p> IMPORTANT!!! In main.cf configure a very large initial +and destination concurrency limit for this transport (say 2000). </p> + +<pre> +/etc/postfix/main.cf: + transport_maps = hash:/etc/postfix/transport + initial_destination_concurrency = 2000 + fragile_destination_concurrency_limit = 2000 + +/etc/postfix/transport: + example.com fragile: + +/etc/postfix/master.cf: + # service type private unpriv chroot wakeup maxproc command + fragile unix - - n - 20 smtp +</pre> + +<p> See also the documentation for default_destination_concurrency_limit. +</p> + +</ul> + +</ul> + +<p> The effect of this configuration is that up to 2000 +consecutive errors are tolerated without marking the destination +dead, while the total concurrency remains reasonable (10-20 +processes). This trick is only for a very specialized situation: +high volume delivery into a channel with multi-error bursts +that is capable of high throughput, but is repeatedly throttled by +the bursts of errors. </p> + +<p> When a destination is unable to handle the load even after the +Postfix process limit is reduced to 1, a desperate measure is to +insert brief delays between delivery attempts. </p> + +<ul> + +<li> <p> Postfix version 2.5 and later: </p> + +<ul> + +<li> <p> In master.cf set up a dedicated clone of the "smtp" transport +for the problem destination. In the example below we call it "slow". +</p> + +<li> <p> In main.cf configure a short delay between deliveries to +the same destination. </p> + +<pre> +/etc/postfix/main.cf: + transport_maps = hash:/etc/postfix/transport + slow_destination_rate_delay = 1 + slow_destination_concurrency_failed_cohort_limit = 100 + +/etc/postfix/transport: + example.com slow: + +/etc/postfix/master.cf: + # service type private unpriv chroot wakeup maxproc command + slow unix - - n - - smtp +</pre> + +</ul> + +<p> See also the documentation for default_destination_rate_delay. </p> + +<p> This solution forces the Postfix smtp(8) client to wait for +$slow_destination_rate_delay seconds between deliveries to the same +destination. </p> + +<p> IMPORTANT!! The large slow_destination_concurrency_failed_cohort_limit +value is needed. This prevents Postfix from deferring all mail for +the same destination after only one connection or handshake error +(the reason for this is that non-zero slow_destination_rate_delay +forces a per-destination concurrency of 1). </p> + +<li> <p> Earlier Postfix versions: </p> + +<ul> + +<li> <p> In the transport map entry for the problem destination, +specify a dead host as the primary nexthop. </p> + +<li> <p> In the master.cf entry for the transport specify the +problem destination as the fallback_relay and specify a small +smtp_connect_timeout value. </p> + +<pre> +/etc/postfix/main.cf: + transport_maps = hash:/etc/postfix/transport + +/etc/postfix/transport: + example.com slow:[dead.host] + +/etc/postfix/master.cf: + # service type private unpriv chroot wakeup maxproc command + slow unix - - n - 1 smtp + -o fallback_relay=problem.example.com + -o smtp_connect_timeout=1 + -o smtp_connection_cache_on_demand=no +</pre> + +</ul> + +<p> This solution forces the Postfix smtp(8) client to wait for +$smtp_connect_timeout seconds between deliveries. The connection +caching feature is disabled to prevent the client from skipping +over the dead host. </p> + +</ul> + +<h2><a name="queues">Postfix queue directories</a></h2> + +<p> The following sections describe Postfix queues: their purpose, +what normal behavior looks like, and how to diagnose abnormal +behavior. </p> + +<h3> <a name="maildrop_queue"> The "maildrop" queue </a> </h3> + +<p> Messages that have been submitted via the Postfix sendmail(1) +command, but not yet brought into the main Postfix queue by the +pickup(8) service, await processing in the "maildrop" queue. Messages +can be added to the "maildrop" queue even when the Postfix system +is not running. They will begin to be processed once Postfix is +started. </p> + +<p> The "maildrop" queue is drained by the single threaded pickup(8) +service scanning the queue directory periodically or when notified +of new message arrival by the postdrop(1) program. The postdrop(1) +program is a setgid helper that allows the unprivileged Postfix +sendmail(1) program to inject mail into the "maildrop" queue and +to notify the pickup(8) service of its arrival. </p> + +<p> All mail that enters the main Postfix queue does so via the +cleanup(8) service. The cleanup service is responsible for envelope +and header rewriting, header and body regular expression checks, +automatic bcc recipient processing, milter content processing, and +reliable insertion of the message into the Postfix "incoming" queue. </p> + +<p> In the absence of excessive CPU consumption in cleanup(8) header +or body regular expression checks or other software consuming all +available CPU resources, Postfix performance is disk I/O bound. +The rate at which the pickup(8) service can inject messages into +the queue is largely determined by disk access times, since the +cleanup(8) service must commit the message to stable storage before +returning success. The same is true of the postdrop(1) program +writing the message to the "maildrop" directory. </p> + +<p> As the pickup service is single threaded, it can only deliver +one message at a time at a rate that does not exceed the reciprocal +disk I/O latency (+ CPU if not negligible) of the cleanup service. +</p> + +<p> Congestion in this queue is indicative of an excessive local message +submission rate or perhaps excessive CPU consumption in the cleanup(8) +service due to excessive body_checks, or (Postfix ≥ 2.3) high latency +milters. </p> + +<p> Note, that once the active queue is full, the cleanup service +will attempt to slow down message injection by pausing $in_flow_delay +for each message. In this case "maildrop" queue congestion may be +a consequence of congestion downstream, rather than a problem in +its own right. </p> + +<p> Note, you should not attempt to deliver large volumes of mail via +the pickup(8) service. High volume sites should avoid using "simple" +content filters that re-inject scanned mail via Postfix sendmail(1) +and postdrop(1). </p> + +<p> A high arrival rate of locally submitted mail may be an indication +of an uncaught forwarding loop, or a run-away notification program. +Try to keep the volume of local mail injection to a moderate level. +</p> + +<p> The "postsuper -r" command can place selected messages into +the "maildrop" queue for reprocessing. This is most useful for +resetting any stale content_filter settings. Requeuing a large number +of messages using "postsuper -r" can clearly cause a spike in the +size of the "maildrop" queue. </p> + +<h3> <a name="hold_queue"> The "hold" queue </a> </h3> + +<p> The administrator can define "smtpd" access(5) policies, or +cleanup(8) header/body checks that cause messages to be automatically +diverted from normal processing and placed indefinitely in the +"hold" queue. Messages placed in the "hold" queue stay there until +the administrator intervenes. No periodic delivery attempts are +made for messages in the "hold" queue. The postsuper(1) command +can be used to manually release messages into the "deferred" queue. +</p> + +<p> Messages can potentially stay in the "hold" queue longer than +$maximal_queue_lifetime. If such "old" messages need to be released from +the "hold" queue, they should typically be moved into the "maildrop" +queue using "postsuper -r", so that the message gets a new timestamp and +is given more than one opportunity to be delivered. Messages that are +"young" can be moved directly into the "deferred" queue using +"postsuper -H". </p> + +<p> The "hold" queue plays little role in Postfix performance, and +monitoring of the "hold" queue is typically more closely motivated +by tracking spam and malware, than by performance issues. </p> + +<h3> <a name="incoming_queue"> The "incoming" queue </a> </h3> + +<p> All new mail entering the Postfix queue is written by the +cleanup(8) service into the "incoming" queue. New queue files are +created owned by the "postfix" user with an access bitmask (or +mode) of 0600. Once a queue file is ready for further processing +the cleanup(8) service changes the queue file mode to 0700 and +notifies the queue manager of new mail arrival. The queue manager +ignores incomplete queue files whose mode is 0600, as these are +still being written by cleanup. </p> + +<p> The queue manager scans the incoming queue bringing any new +mail into the "active" queue if the active queue resource limits +have not been exceeded. By default, the active queue accommodates +at most 20000 messages. Once the active queue message limit is +reached, the queue manager stops scanning the incoming (and deferred, +see below) queue. </p> + +<p> Under normal conditions the incoming queue is nearly empty (has +only mode 0600 files), with the queue manager able to import new +messages into the active queue as soon as they become available. +</p> + +<p> The incoming queue grows when the message input rate spikes +above the rate at which the queue manager can import messages into +the active queue. The main factors slowing down the queue manager +are disk I/O and lookup queries to the trivial-rewrite service. If the queue +manager is routinely not keeping up, consider not using "slow" +lookup services (MySQL, LDAP, ...) for transport lookups or speeding +up the hosts that provide the lookup service. If the problem is I/O +starvation, consider striping the queue over more disks, faster controllers +with a battery write cache, or other hardware improvements. At the very +least, make sure that the queue directory is mounted with the "noatime" +option if applicable to the underlying filesystem. </p> + +<p> The in_flow_delay parameter is used to clamp the input rate +when the queue manager starts to fall behind. The cleanup(8) service +will pause for $in_flow_delay seconds before creating a new queue +file if it cannot obtain a "token" from the queue manager. </p> + +<p> Since the number of cleanup(8) processes is limited in most +cases by the SMTP server concurrency, the input rate can exceed +the output rate by at most "SMTP connection count" / $in_flow_delay +messages per second. </p> + +<p> With a default process limit of 100, and an in_flow_delay of +1s, the coupling is strong enough to limit a single run-away injector +to 1 message per second, but is not strong enough to deflect an +excessive input rate from many sources at the same time. </p> + +<p> If a server is being hammered from multiple directions, consider +raising the in_flow_delay to 10 seconds, but only if the incoming +queue is growing even while the active queue is not full and the +trivial-rewrite service is using a fast transport lookup mechanism. +</p> + +<h3> <a name="active_queue"> The "active" queue </a> </h3> + +<p> The queue manager is a delivery agent scheduler; it works to +ensure fast and fair delivery of mail to all destinations within +designated resource limits. </p> + +<p> The active queue is somewhat analogous to an operating system's +process run queue. Messages in the active queue are ready to be +sent (runnable), but are not necessarily in the process of being +sent (running). </p> + +<p> While most Postfix administrators think of the "active" queue +as a directory on disk, the real "active" queue is a set of data +structures in the memory of the queue manager process. </p> + +<p> Messages in the "maildrop", "hold", "incoming" and "deferred" +queues (see below) do not occupy memory; they are safely stored on +disk waiting for their turn to be processed. The envelope information +for messages in the "active" queue is managed in memory, allowing +the queue manager to do global scheduling, allocating available +delivery agent processes to an appropriate message in the active +queue. </p> + +<p> Within the active queue, (multi-recipient) messages are broken +up into groups of recipients that share the same transport/nexthop +combination; the group size is capped by the transport's recipient +concurrency limit. </p> + +<p> Multiple recipient groups (from one or more messages) are queued +for delivery grouped by transport/nexthop combination. The +<b>destination</b> concurrency limit for the transports caps the number +of simultaneous delivery attempts for each nexthop. Transports with +a <b>recipient</b> concurrency limit of 1 are special: these are grouped +by the actual recipient address rather than the nexthop, yielding +per-recipient concurrency limits rather than per-domain +concurrency limits. Per-recipient limits are appropriate when +performing final delivery to mailboxes rather than when relaying +to a remote server. </p> + +<p> Congestion occurs in the active queue when one or more destinations +drain slower than the corresponding message input rate. </p> + +<p> Input into the active queue comes both from new mail in the "incoming" +queue, and retries of mail in the "deferred" queue. Should the "deferred" +queue get really large, retries of old mail can dominate the arrival +rate of new mail. Systems with more CPU, faster disks and more network +bandwidth can deal with larger deferred queues, but as a rule of thumb +the deferred queue scales to somewhere between 100,000 and 1,000,000 +messages with good performance unlikely above that "limit". Systems with +queues this large should typically stop accepting new mail, or put the +backlog "on hold" until the underlying issue is fixed (provided that +there is enough capacity to handle just the new mail). </p> + +<p> When a destination is down for some time, the queue manager will +mark it dead, and immediately defer all mail for the destination without +trying to assign it to a delivery agent. In this case the messages +will quickly leave the active queue and end up in the deferred queue +(with Postfix < 2.4, this is done directly by the queue manager, +with Postfix ≥ 2.4 this is done via the "retry" delivery agent). </p> + +<p> When the destination is instead simply slow, or there is a problem +causing an excessive arrival rate the active queue will grow and will +become dominated by mail to the congested destination. </p> + +<p> The only way to reduce congestion is to either reduce the input +rate or increase the throughput. Increasing the throughput requires +either increasing the concurrency or reducing the latency of +deliveries. </p> + +<p> For high volume sites a key tuning parameter is the number of +"smtp" delivery agents allocated to the "smtp" and "relay" transports. +High volume sites tend to send to many different destinations, many +of which may be down or slow, so a good fraction of the available +delivery agents will be blocked waiting for slow sites. Also mail +destined across the globe will incur large SMTP command-response +latencies, so high message throughput can only be achieved with +more concurrent delivery agents. </p> + +<p> The default "smtp" process limit of 100 is good enough for most +sites, and may even need to be lowered for sites with low bandwidth +connections (no use increasing concurrency once the network pipe +is full). When one finds that the queue is growing on an "idle" +system (CPU, disk I/O and network not exhausted) the remaining +reason for congestion is insufficient concurrency in the face of +a high average latency. If the number of outbound SMTP connections +(either ESTABLISHED or SYN_SENT) reaches the process limit, mail +is draining slowly and the system and network are not loaded, raise +the "smtp" and/or "relay" process limits! </p> + +<p> When a high volume destination is served by multiple MX hosts with +typically low delivery latency, performance can suffer dramatically when +one of the MX hosts is unresponsive and SMTP connections to that host +timeout. For example, if there are 2 equal weight MX hosts, the SMTP +connection timeout is 30 seconds and one of the MX hosts is down, the +average SMTP connection will take approximately 15 seconds to complete. +With a default per-destination concurrency limit of 20 connections, +throughput falls to just over 1 message per second. </p> + +<p> The best way to avoid bottlenecks when one or more MX hosts is +non-responsive is to use connection caching. Connection caching was +introduced with Postfix 2.2 and is by default enabled on demand for +destinations with a backlog of mail in the active queue. When connection +caching is in effect for a particular destination, established connections +are re-used to send additional messages, this reduces the number of +connections made per message delivery and maintains good throughput even +in the face of partial unavailability of the destination's MX hosts. </p> + +<p> If connection caching is not available (Postfix < 2.2) or does +not provide a sufficient latency reduction, especially for the "relay" +transport used to forward mail to "your own" domains, consider setting +lower than default SMTP connection timeouts (1-5 seconds) and higher +than default destination concurrency limits. This will further reduce +latency and provide more concurrency to maintain throughput should +latency rise. </p> + +<p> Setting high concurrency limits to domains that are not your own may +be viewed as hostile by the receiving system, and steps may be taken +to prevent you from monopolizing the destination system's resources. +The defensive measures may substantially reduce your throughput or block +access entirely. Do not set aggressive concurrency limits to remote +domains without coordinating with the administrators of the target +domain. </p> + +<p> If necessary, dedicate and tune custom transports for selected high +volume destinations. The "relay" transport is provided for forwarding mail +to domains for which your server is a primary or backup MX host. These can +make up a substantial fraction of your email traffic. Use the "relay" and +not the "smtp" transport to send email to these domains. Using the "relay" +transport allocates a separate delivery agent pool to these destinations +and allows separate tuning of timeouts and concurrency limits. </p> + +<p> Another common cause of congestion is unwarranted flushing of the +entire deferred queue. The deferred queue holds messages that are likely +to fail to be delivered and are also likely to be slow to fail delivery +(time out). As a result the most common reaction to a large deferred queue +(flush it!) is more than likely counter-productive, and typically makes +the congestion worse. Do not flush the deferred queue unless you expect +that most of its content has recently become deliverable (e.g. relayhost +back up after an outage)! </p> + +<p> Note that whenever the queue manager is restarted, there may +already be messages in the active queue directory, but the "real" +active queue in memory is empty. In order to recover the in-memory +state, the queue manager moves all the active queue messages +back into the incoming queue, and then uses its normal incoming +queue scan to refill the active queue. The process of moving all +the messages back and forth, redoing transport table (trivial-rewrite(8) +resolve service) lookups, and re-importing the messages back into +memory is expensive. At all costs, avoid frequent restarts of the +queue manager (e.g. via frequent execution of "postfix reload"). </p> + +<h3> <a name="deferred_queue"> The "deferred" queue </a> </h3> + +<p> When all the deliverable recipients for a message are delivered, +and for some recipients delivery failed for a transient reason (it +might succeed later), the message is placed in the deferred queue. +</p> + +<p> The queue manager scans the deferred queue periodically. The scan +interval is controlled by the queue_run_delay parameter. While a deferred +queue scan is in progress, if an incoming queue scan is also in progress +(ideally these are brief since the incoming queue should be short), the +queue manager alternates between looking for messages in the "incoming" +queue and in the "deferred" queue. This "round-robin" strategy prevents +starvation of either the incoming or the deferred queues. </p> + +<p> Each deferred queue scan only brings a fraction of the deferred +queue back into the active queue for a retry. This is because each +message in the deferred queue is assigned a "cool-off" time when +it is deferred. This is done by time-warping the modification +time of the queue file into the future. The queue file is not +eligible for a retry if its modification time is not yet reached. +</p> + +<p> The "cool-off" time is at least $minimal_backoff_time and at +most $maximal_backoff_time. The next retry time is set by doubling +the message's age in the queue, and adjusting up or down to lie +within the limits. This means that young messages are initially +retried more often than old messages. </p> + +<p> If a high volume site routinely has large deferred queues, it +may be useful to adjust the queue_run_delay, minimal_backoff_time and +maximal_backoff_time to provide short enough delays on first failure +(Postfix ≥ 2.4 has a sensibly low minimal backoff time by default), +with perhaps longer delays after multiple failures, to reduce the +retransmission rate of old messages and thereby reduce the quantity +of previously deferred mail in the active queue. If you want a really +low minimal_backoff_time, you may also want to lower queue_run_delay, +but understand that more frequent scans will increase the demand for +disk I/O. </p> + +<p> One common cause of large deferred queues is failure to validate +recipients at the SMTP input stage. Since spammers routinely launch +dictionary attacks from unrepliable sender addresses, the bounces +for invalid recipient addresses clog the deferred queue (and at high +volumes proportionally clog the active queue). Recipient validation +is strongly recommended through use of the local_recipient_maps and +relay_recipient_maps parameters. Even when bounces drain quickly they +inundate innocent victims of forgery with unwanted email. To avoid +this, do not accept mail for invalid recipients. </p> + +<p> When a host with lots of deferred mail is down for some time, +it is possible for the entire deferred queue to reach its retry +time simultaneously. This can lead to a very full active queue once +the host comes back up. The phenomenon can repeat approximately +every maximal_backoff_time seconds if the messages are again deferred +after a brief burst of congestion. Perhaps, a future Postfix release +will add a random offset to the retry time (or use a combination +of strategies) to reduce the odds of repeated complete deferred +queue flushes. </p> + +<h2><a name="credits">Credits</a></h2> + +<p> The qshape(1) program was developed by Victor Duchovni of Morgan +Stanley, who also wrote the initial version of this document. </p> + +</body> + +</html> |