Adding upstream version 8.2402.0+dfsg.upstream/8.2402.0+dfsg

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+Understanding rsyslog Queues
+============================
+
+Rsyslog uses queues whenever two activities need to be loosely coupled.
+With a queue, one part of the system "produces" something while another
+part "consumes" this something. The "something" is most often syslog
+messages, but queues may also be used for other purposes.
+
+This document provides a good insight into technical details, operation
+modes and implications. In addition to it, an :doc:`rsyslog queue concepts
+overview <../whitepapers/queues_analogy>` document exists which tries to explain
+queues with the help of some analogies. This may probably be a better
+place to start reading about queues. I assume that once you have
+understood that document, the material here will be much easier to grasp
+and look much more natural.
+
+The most prominent example is the main message queue. Whenever rsyslog
+receives a message (e.g. locally, via UDP, TCP or in whatever else way),
+it places these messages into the main message queue. Later, it is
+dequeued by the rule processor, which then evaluates which actions are
+to be carried out. In front of each action, there is also a queue, which
+potentially de-couples the filter processing from the actual action
+(e.g. writing to file, database or forwarding to another host).
+
+Where are Queues Used?
+----------------------
+
+Currently, queues are used for the main message queue and for the
+actions.
+
+There is a single main message queue inside rsyslog. Each input module
+delivers messages to it. The main message queue worker filters messages
+based on rules specified in rsyslog.conf and dispatches them to the
+individual action queues. Once a message is in an action queue, it is
+deleted from the main message queue.
+
+There are multiple action queues, one for each configured action. By
+default, these queues operate in direct (non-queueing) mode. Action
+queues are fully configurable and thus can be changed to whatever is
+best for the given use case.
+
+Future versions of rsyslog will most probably utilize queues at other
+places, too.
+
+Wherever "*<object>*\ "  is used in the config file statements,
+substitute "*<object>*\ " with either "MainMsg" or "Action". The former
+will set main message queue parameters, the later parameters for the
+next action that will be created. Action queue parameters can not be
+modified once the action has been specified. For example, to tell the
+main message queue to save its content on shutdown, use
+*$MainMsgQueueSaveOnShutdown on*".
+
+If the same parameter is specified multiple times before a queue is
+created, the last one specified takes precedence. The main message queue
+is created after parsing the config file and all of its potential
+includes. An action queue is created each time an action selector is
+specified. Action queue parameters are reset to default after an action
+queue has been created (to provide a clean environment for the next
+action).
+
+Not all queues necessarily support the full set of queue configuration
+parameters, because not all are applicable. For example, disk queues
+always have exactly one worker thread. This cannot be overridden by
+configuration parameters. Tries to do so are ignored.
+
+Queue Modes
+-----------
+
+Rsyslog supports different queue modes, some with submodes. Each of them
+has specific advantages and disadvantages. Selecting the right queue
+mode is quite important when tuning rsyslogd. The queue mode (aka
+"type") is set via the "*$<object>QueueType*\ " config directive.
+
+Direct Queues
+~~~~~~~~~~~~~
+
+Direct queues are **non**-queuing queues. A queue in direct mode does
+neither queue nor buffer any of the queue elements but rather passes the
+element directly (and immediately) from the producer to the consumer.
+This sounds strange, but there is a good reason for this queue type.
+
+Direct mode queues allow to use queues generically, even in places where
+queuing is not always desired. A good example is the queue in front of
+output actions. While it makes perfect sense to buffer forwarding
+actions or database writes, it makes only limited sense to build up a
+queue in front of simple local file writes. Yet, rsyslog still has a
+queue in front of every action. So for file writes, the queue mode can
+simply be set to "direct", in which case no queuing happens.
+
+Please note that a direct queue also is the only queue type that passes
+back the execution return code (success/failure) from the consumer to
+the producer. This, for example, is needed for the backup action logic.
+Consequently, backup actions require the to-be-checked action to use a
+"direct" mode queue.
+
+To create a direct queue, use the "*$<object>QueueType Direct*\ " config
+directive.
+
+Disk Queues
+~~~~~~~~~~~
+
+Disk queues use disk drives for buffering. The important fact is that
+they always use the disk and do not buffer anything in memory. Thus, the
+queue is ultra-reliable, but by far the slowest mode. For regular use
+cases, this queue mode is not recommended. It is useful if log data is
+so important that it must not be lost, even in extreme cases.
+
+When a disk queue is written, it is done in chunks. Each chunk receives
+its individual file. Files are named with a prefix (set via the
+"*$<object>QueueFilename*\ " config directive) and followed by a 7-digit
+number (starting at one and incremented for each file). Chunks are 10mb
+by default, a different size can be set via
+the"*$<object>QueueMaxFileSize*\ " config directive. Note that the size
+limit is not a sharp one: rsyslog always writes one complete queue
+entry, even if it violates the size limit. So chunks are actually a
+little bit (usually less than 1k) larger then the configured size. Each
+chunk also has a different size for the same reason. If you observe
+different chunk sizes, you can relax: this is not a problem.
+
+Writing in chunks is used so that processed data can quickly be deleted
+and is free for other uses - while at the same time keeping no
+artificial upper limit on disk space used. If a disk quota is set
+(instructions further below), be sure that the quota/chunk size allows
+at least two chunks to be written. Rsyslog currently does not check that
+and will fail miserably if a single chunk is over the quota.
+
+Creating new chunks costs performance but provides quicker ability to
+free disk space. The 10mb default is considered a good compromise
+between these two. However, it may make sense to adapt these settings to
+local policies. For example, if a disk queue is written on a dedicated
+200gb disk, it may make sense to use a 2gb (or even larger) chunk size.
+
+Please note, however, that the disk queue by default does not update its
+housekeeping structures every time it writes to disk. This is for
+performance reasons. In the event of failure, data will still be lost
+(except when manually is mangled with the file structures). However,
+disk queues can be set to write bookkeeping information on checkpoints
+(every n records), so that this can be made ultra-reliable, too. If the
+checkpoint interval is set to one, no data can be lost, but the queue is
+exceptionally slow.
+
+Each queue can be placed on a different disk for best performance and/or
+isolation. This is currently selected by specifying different
+*$WorkDirectory* config directives before the queue creation statement.
+
+To create a disk queue, use the "*$<object>QueueType Disk*\ " config
+directive. Checkpoint intervals can be specified via
+"*$<object>QueueCheckpointInterval*\ ", with 0 meaning no checkpoints.
+Note that disk-based queues can be made very reliable by issuing a
+(f)sync after each write operation. Starting with version 4.3.2, this
+can be requested via "*<object>QueueSyncQueueFiles on/off* with the
+default being off. Activating this option has a performance penalty, so
+it should not be turned on without reason.
+
+If you happen to lose or otherwise need the housekeeping structures and 
+have all yours queue chunks you can use perl script included in rsyslog
+package to generate it. 
+Usage: recover_qi.pl -w *$WorkDirectory* -f QueueFileName -d 8 > QueueFileName.qi
+
+
+In-Memory Queues
+~~~~~~~~~~~~~~~~
+
+In-memory queue mode is what most people have on their mind when they
+think about computing queues. Here, the enqueued data elements are held
+in memory. Consequently, in-memory queues are very fast. But of course,
+they do not survive any program or operating system abort (what usually
+is tolerable and unlikely). Be sure to use an UPS if you use in-memory
+mode and your log data is important to you. Note that even in-memory
+queues may hold data for an infinite amount of time when e.g. an output
+destination system is down and there is no reason to move the data out
+of memory (lying around in memory for an extended period of time is NOT
+a reason). Pure in-memory queues can't even store queue elements
+anywhere else than in core memory.
+
+There exist two different in-memory queue modes: LinkedList and
+FixedArray. Both are quite similar from the user's point of view, but
+utilize different algorithms.
+
+A FixedArray queue uses a fixed, pre-allocated array that holds pointers
+to queue elements. The majority of space is taken up by the actual user
+data elements, to which the pointers in the array point. The pointer
+array itself is comparatively small. However, it has a certain memory
+footprint even if the queue is empty. As there is no need to dynamically
+allocate any housekeeping structures, FixedArray offers the best run
+time performance (uses the least CPU cycle). FixedArray is best if there
+is a relatively low number of queue elements expected and performance is
+desired. It is the default mode for the main message queue (with a limit
+of 10,000 elements).
+
+A LinkedList queue is quite the opposite. All housekeeping structures
+are dynamically allocated (in a linked list, as its name implies). This
+requires somewhat more runtime processing overhead, but ensures that
+memory is only allocated in cases where it is needed. LinkedList queues
+are especially well-suited for queues where only occasionally a
+than-high number of elements need to be queued. A use case may be
+occasional message burst. Memory permitting, it could be limited to e.g.
+200,000 elements which would take up only memory if in use. A FixedArray
+queue may have a too large static memory footprint in such cases.
+
+**In general, it is advised to use LinkedList mode if in doubt**. The
+processing overhead compared to FixedArray is low and may be outweighed by
+the reduction in memory use. Paging in most-often-unused pointer array
+pages can be much slower than dynamically allocating them.
+
+To create an in-memory queue, use the "*$<object>QueueType
+LinkedList*\ " or  "*$<object>QueueType FixedArray*\ " config directive.
+
+Disk-Assisted Memory Queues
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If a disk queue name is defined for in-memory queues (via
+*$<object>QueueFileName*), they automatically become "disk-assisted"
+(DA). In that mode, data is written to disk (and read back) on an
+as-needed basis.
+
+Actually, the regular memory queue (called the "primary queue") and a
+disk queue (called the "DA queue") work in tandem in this mode. Most
+importantly, the disk queue is activated if the primary queue is full or
+needs to be persisted on shutdown. Disk-assisted queues combine the
+advantages of pure memory queues with those of  pure disk queues. Under
+normal operations, they are very fast and messages will never touch the
+disk. But if there is need to, an unlimited amount of messages can be
+buffered (actually limited by free disk space only) and data can be
+persisted between rsyslogd runs.
+
+With a DA-queue, both disk-specific and in-memory specific configuration
+parameters can be set. From the user's point of view, think of a DA
+queue like a "super-queue" which does all within a single queue [from
+the code perspective, there is some specific handling for this case, so
+it is actually much like a single object].
+
+DA queues are typically used to de-couple potentially long-running and
+unreliable actions (to make them reliable). For example, it is
+recommended to use a disk-assisted linked list in-memory queue in front
+of each database and "send via tcp" action. Doing so makes these actions
+reliable and de-couples their potential low execution speed from the
+rest of your rules (e.g. the local file writes). There is a howto on
+`massive database inserts <rsyslog_high_database_rate.html>`_ which
+nicely describes this use case. It may even be a good read if you do not
+intend to use databases.
+
+With DA queues, we do not simply write out everything to disk and then
+run as a disk queue once the in-memory queue is full. A much smarter
+algorithm is used, which involves a "high watermark" and a "low
+watermark". Both specify numbers of queued items. If the queue size
+reaches high watermark elements, the queue begins to write data elements
+to disk. It does so until it reaches the low water mark elements. At
+this point, it stops writing until either high water mark is reached
+again or the on-disk queue becomes empty, in which case the queue
+reverts back to in-memory mode, only. While holding at the low
+watermark, new elements are actually enqueued in memory. They are
+eventually written to disk, but only if the high water mark is ever
+reached again. If it isn't, these items never touch the disk. So even
+when a queue runs disk-assisted, there is in-memory data present (this
+is a big difference to pure disk queues!).
+
+This algorithm prevents unnecessary disk writes, but also leaves some
+additional buffer space for message bursts. Remember that creating disk
+files and writing to them is a lengthy operation. It is too lengthy to
+e.g. block receiving UDP messages. Doing so would result in message
+loss. Thus, the queue initiates DA mode, but still is able to receive
+messages and enqueue them - as long as the maximum queue size is not
+reached. The number of elements between the high water mark and the
+maximum queue size serves as this "emergency buffer". Size it according
+to your needs, if traffic is very bursty you will probably need a large
+buffer here. Keep in mind, though, that under normal operations these
+queue elements will probably never be used. Setting the high water mark
+too low will cause disk-assistance to be turned on more often than
+actually needed.
+
+The water marks can be set via the "*$<object>QueueHighWatermark*\ "
+and  "*$<object>QueueLowWatermark*\ " configuration file directives.
+Note that these are actual numbers, not percentages. Be sure they make
+sense (also in respect to "*$<object>QueueSize*\ "). Rsyslogd does
+perform some checks on the numbers provided, and issues warning when
+numbers are "suspicious".
+
+Limiting the Queue Size
+-----------------------
+
+All queues, including disk queues, have a limit of the number of
+elements they can enqueue. This is set via the "*$<object>QueueSize*\ "
+config parameter. Note that the size is specified in number of enqueued
+elements, not their actual memory size. Memory size limits can not be
+set. A conservative assumption is that a single syslog messages takes up
+512 bytes on average (in-memory, NOT on the wire, this \*is\* a
+difference).
+
+Disk assisted queues are special in that they do **not** have any size
+limit. The enqueue an unlimited amount of elements. To prevent running
+out of space, disk and disk-assisted queues can be size-limited via the
+"*$<object>QueueMaxDiskSpace*\ " configuration parameter. If it is not
+set, the limit is only available free space (and reaching this limit is
+currently not very gracefully handled, so avoid running into it!). If a
+limit is set, the queue can not grow larger than it. Note, however, that
+the limit is approximate. The engine always writes complete records. As
+such, it is possible that slightly more than the set limit is used
+(usually less than 1k, given the average message size). Keeping strictly
+on the limit would be a performance hurt, and thus the design decision
+was to favour performance. If you don't like that policy, simply specify
+a slightly lower limit (e.g. 999,999K instead of 1G).
+
+In general, it is a good idea to limit the physical disk space even if
+you dedicate a whole disk to rsyslog. That way, you prevent it from
+running out of space (future version will have an auto-size-limit logic,
+that then kicks in in such situations).
+
+Worker Thread Pools
+-------------------
+
+Each queue (except in "direct" mode) has an associated pool of worker
+threads. Worker threads carry out the action to be performed on the data
+elements enqueued. As an actual sample, the main message queue's worker
+task is to apply filter logic to each incoming message and enqueue them
+to the relevant output queues (actions).
+
+Worker threads are started and stopped on an as-needed basis. On a
+system without activity, there may be no worker at all running. One is
+automatically started when a message comes in. Similarly, additional
+workers are started if the queue grows above a specific size. The
+"*$<object>QueueWorkerThreadMinimumMessages*\ "  config parameter
+controls worker startup. If it is set to the minimum number of elements
+that must be enqueued in order to justify a new worker startup. For
+example, let's assume it is set to 100. As long as no more than 100
+messages are in the queue, a single worker will be used. When more than
+100 messages arrive, a new worker thread is automatically started.
+Similarly, a third worker will be started when there are at least 300
+messages, a forth when reaching 400 and so on.
+
+It, however, does not make sense to have too many worker threads running
+in parallel. Thus, the upper limit can be set via
+"*$<object>QueueWorkerThreads*\ ". If it, for example, is set to four,
+no more than four workers will ever be started, no matter how many
+elements are enqueued.
+
+Worker threads that have been started are kept running until an
+inactivity timeout happens. The timeout can be set via
+"*$<object>QueueWorkerTimeoutThreadShutdown*\ " and is specified in
+milliseconds. If you do not like to keep the workers running, simply set
+it to 0, which means immediate timeout and thus immediate shutdown. But
+consider that creating threads involves some overhead, and this is why
+we keep them running. If you would like to never shutdown any worker
+threads, specify -1 for this parameter.
+
+Discarding Messages
+~~~~~~~~~~~~~~~~~~~
+
+If the queue reaches the so called "discard watermark" (a number of
+queued elements), less important messages can automatically be
+discarded. This is in an effort to save queue space for more important
+messages, which you even less like to lose. Please note that whenever
+there are more than "discard watermark" messages, both newly incoming as
+well as already enqueued low-priority messages are discarded. The
+algorithm discards messages newly coming in and those at the front of
+the queue.
+
+The discard watermark is a last resort setting. It should be set
+sufficiently high, but low enough to allow for large message burst.
+Please note that it take effect immediately and thus shows effect
+promptly - but that doesn't help if the burst mainly consist of
+high-priority messages...
+
+The discard watermark is set via the "*$<object>QueueDiscardMark*\ "
+directive. The priority of messages to be discarded is set via
+"*$<object>QueueDiscardSeverity*\ ". This directive accepts both the
+usual textual severity as well as a numerical one. To understand it, you
+must be aware of the numerical severity values. They are defined in RFC
+3164:
+
+        ==== ========
+        Code Severity
+        ==== ========
+        0    Emergency: system is unusable
+        1    Alert: action must be taken immediately
+        2    Critical: critical conditions
+        3    Error: error conditions
+        4    Warning: warning conditions
+        5    Notice: normal but significant condition
+        6    Informational: informational messages
+        7    Debug: debug-level messages
+        ==== ========
+
+Anything of the specified severity and (numerically) above it is
+discarded. To turn message discarding off, simply specify the discard
+watermark to be higher than the queue size. An alternative is to specify
+the numerical value 8 as DiscardSeverity. This is also the default
+setting to prevent unintentional message loss. So if you would like to
+use message discarding, you need to set"
+*$<object>QueueDiscardSeverity*" to an actual value.
+
+An interesting application is with disk-assisted queues: if the discard
+watermark is set lower than the high watermark, message discarding will
+start before the queue becomes disk-assisted. This may be a good thing
+if you would like to switch to disk-assisted mode only in cases where it
+is absolutely unavoidable and you prefer to discard less important
+messages first.
+
+Filled-Up Queues
+----------------
+
+If the queue has either reached its configured maximum number of entries
+or disk space, it is finally full. If so, rsyslogd throttles the data
+element submitter. If that, for example, is a reliable input (TCP, local
+log socket), that will slow down the message originator which is a good
+resolution for this scenario.
+
+During throttling, a disk-assisted queue continues to write to disk and
+messages are also discarded based on severity as well as regular
+dequeuing and processing continues. So chances are good the situation
+will be resolved by simply throttling. Note, though, that throttling is
+highly undesirable for unreliable sources, like UDP message reception.
+So it is not a good thing to run into throttling mode at all.
+
+We can not hold processing infinitely, not even when throttling. For
+example, throttling the local log socket too long would cause the
+system at whole come to a standstill. To prevent this, rsyslogd times
+out after a configured period ("*$<object>QueueTimeoutEnqueue*\ ",
+specified in milliseconds) if no space becomes available. As a last
+resort, it then discards the newly arrived message.
+
+If you do not like throttling, set the timeout to 0 - the message will
+then immediately be discarded. If you use a high timeout, be sure you
+know what you do. If a high main message queue enqueue timeout is set,
+it can lead to something like a complete hang of the system. The same
+problem does not apply to action queues.
+
+Rate Limiting
+~~~~~~~~~~~~~
+
+Rate limiting provides a way to prevent rsyslogd from processing things
+too fast. It can, for example, prevent overrunning a receiver system.
+
+Currently, there are only limited rate-limiting features available. The
+"*$<object>QueueDequeueSlowdown*\ "  directive allows to specify how
+long (in microseconds) dequeueing should be delayed. While simple, it
+still is powerful. For example, using a DequeueSlowdown delay of 1,000
+microseconds on a UDP send action ensures that no more than 1,000
+messages can be sent within a second (actually less, as there is also
+some time needed for the processing itself).
+
+Processing Timeframes
+~~~~~~~~~~~~~~~~~~~~~
+
+Queues can be set to dequeue (process) messages only during certain
+timeframes. This is useful if you, for example, would like to transfer
+the bulk of messages only during off-peak hours, e.g. when you have only
+limited bandwidth on the network path to the central server.
+
+Currently, only a single timeframe is supported and, even worse, it can
+only be specified by the hour. It is not hard to extend rsyslog's
+capabilities in this regard - it was just not requested so far. So if
+you need more fine-grained control, let us know and we'll probably
+implement it. There are two configuration directives, both should be
+used together or results are unpredictable:"
+*$<object>QueueDequeueTimeBegin <hour>*"
+and "*$<object>QueueDequeueTimeEnd <hour>*\ ". The hour parameter must
+be specified in 24-hour format (so 10pm is 22). A use case for this
+parameter can be found in the `rsyslog
+wiki <http://wiki.rsyslog.com/index.php/OffPeakHours>`_.
+
+Performance
+~~~~~~~~~~~
+
+The locking involved with maintaining the queue has a potentially large
+performance impact. How large this is, and if it exists at all, depends
+much on the configuration and actual use case. However, the queue is
+able to work on so-called "batches" when dequeueing data elements. With
+batches, multiple data elements are dequeued at once (with a single
+locking call). The queue dequeues all available elements up to a
+configured upper limit (*<object>DequeueBatchSize <number>*). It is
+important to note that the actual upper limit is dictated by
+availability. The queue engine will never wait for a batch to fill. So
+even if a high upper limit is configured, batches may consist of fewer
+elements, even just one, if there are no more elements waiting in the
+queue.
+
+Batching can improve performance considerably. Note, however, that it
+affects the order in which messages are passed to the queue worker
+threads, as each worker now receive as batch of messages. Also, the
+larger the batch size and the higher the maximum number of permitted
+worker threads, the more main memory is needed. For a busy server, large
+batch sizes (around 1,000 or even more elements) may be useful. Please
+note that with batching, the main memory must hold BatchSize \*
+NumOfWorkers objects in memory (worst-case scenario), even if running in
+disk-only mode. So if you use the default 5 workers at the main message
+queue and set the batch size to 1,000, you need to be prepared that the
+main message queue holds up to 5,000 messages in main memory **in
+addition** to the configured queue size limits!
+
+The queue object's default maximum batch size is eight, but there exists
+different defaults for the actual parts of rsyslog processing that
+utilize queues. So you need to check these object's defaults.
+
+Terminating Queues
+~~~~~~~~~~~~~~~~~~
+
+Terminating a process sounds easy, but can be complex. Terminating a
+running queue is in fact the most complex operation a queue object can
+perform. You don't see that from a user's point of view, but its quite
+hard work for the developer to do everything in the right order.
+
+The complexity arises when the queue has still data enqueued when it
+finishes. Rsyslog tries to preserve as much of it as possible. As a
+first measure, there is a regular queue time out
+("*$<object>QueueTimeoutShutdown*\ ", specified in milliseconds): the
+queue workers are given that time period to finish processing the queue.
+
+If after that period there is still data in the queue, workers are
+instructed to finish the current data element and then terminate. This
+essentially means any other data is lost. There is another timeout
+("*$<object>QueueTimeoutActionCompletion*\ ", also specified in
+milliseconds) that specifies how long the workers have to finish the
+current element. If that timeout expires, any remaining workers are
+cancelled and the queue is brought down.
+
+If you do not like to lose data on shutdown, the
+"*$<object>QueueSaveOnShutdown*\ " parameter can be set to "on". This
+requires either a disk or disk-assisted queue. If set, rsyslogd ensures
+that any queue elements are saved to disk before it terminates. This
+includes data elements there were begun being processed by workers that
+needed to be cancelled due to too-long processing. For a large queue,
+this operation may be lengthy. No timeout applies to a required shutdown
+save.