278 lines
7.7 KiB
HTML
278 lines
7.7 KiB
HTML
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<html>
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<head>
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<title> Postfix Configuration - Resource Controls</title>
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</head>
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<body>
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<h1><a href="big-picture.html"><img src="small-picture.gif" width="115" height="45"></a> Postfix Configuration - Resource Controls</h1>
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<hr>
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<a href="index.html">Up one level</a> | <a href="basic.html">Basic
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Configuration</a> | <a href="uce.html">UCE Controls</a> | <a
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href="rate.html">Rate Controls</a> | Resource Controls | <a
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href="rewrite.html">Address Manipulation</a>
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<h2> Introduction</h2>
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The Postfix system is designed to run within a finite memory budget.
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To this end, there are configurable limits on the <i>size</i> of
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in-memory objects such as text line fragments, on the <i>number of
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instances</i> of such objects, and on the <i>time</i> an operation
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may take. In addition, strategies are in place for dealing with
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resource exhaustion. The idea is to keep running under conditions
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of stress, without making the problem worse.
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<p>
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<ul>
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<li> <a href="#size">Object size limits</a>
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<p>
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<li> <a href="#count">Object count limits</a>
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<p>
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<li> <a href="#time">Time limits</a>
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<p>
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<li> <a href="#lock">Acquiring exclusive file locks</a>
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<p>
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<li> <a href="#fan">Error recovery</a>
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</ul>
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<a name="size"><h2> Object size limits</h2> </a>
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The first step towards a fixed memory resource budget is to limit
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the size of each in-memory object. Once the size of in-memory
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objects is limited, total memory consumption is limited by limiting
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the number of object instances. Simple, no?
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<p>
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<dl>
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<dt> <b>line_length_limit</b> (default: 2048 bytes)
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<dd> How long a line of text can be before it is broken up into
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pieces. All Postfix perimeter programs (<a href="smtpd.8.html">SMTP
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server</a>, <a href="smtp.8.html">SMTP client</a>, <a
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href="pickup.8.html">local pickup</a> and <a href="local.8.html">local
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delivery</a>) enforce this line length limit when reading data from
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an untrusted source. Long lines are reconstructed upon delivery.
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<p>
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<dt> <b>header_size_limit</b> (default: 102400 bytes)
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<dd> How much text may be carried in a multi-line message header.
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Header text that does not fit in <b>$header_size_limit</b> bytes
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overflows into the message body. This limit is enforced by the <a
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href="cleanup.8.html"> cleanup</a> header rewriting code.
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</dl>
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<p>
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The following parameters restrict the use of file system storage:
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<dl>
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<dt> <b>message_size_limit</b> (default: 10240000 bytes)
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<dd> The maximal size of a Postfix queue file for inbound mail,
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including envelope information (sender, recipient, etc.).
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<p>
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<dt> <b>queue_minfree</b> (default: no restriction)
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<dd> How many bytes of free space are needed in the queue file
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system. The <a href="smtpd.8.html">SMTP server</a> declines inbound
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mail delivery requests when there is insufficient space (the mail
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will be accepted once enough space becomes available). There is
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no default limit; however, it seems like a good idea to require at
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least several times <b>$message_size_limit</b> so that the mail
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system won't get stuck on a single large message.
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<p>
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<dt> <b>bounce_size_limit</b> (default: 50000 bytes)
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<dd> How much of an undelivered message is sent back to the sender.
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</dl>
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<a name="count"><h2> Object count limits</h2> </a>
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Once the sizes of memory objects have been limited, the next step
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to implement Postfix's finite memory budget is to limit the number
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of in-memory object instances.
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<dl>
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<dt> <b>qmgr_message_recipient_limit</b> (default: 10000)
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<dd> An upper bound on the number of <a href="qmgr.8.html">queue
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manager</a> in-memory recipient address data structures. This
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parameter also controls the number of instances of other in-memory
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data structures. See, for example, the <a
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href="rate.html#backoff">delivery rate control</a> documentation.
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<p>
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<dt> <b>qmgr_message_active_limit</b> (default: 1000)
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<dd> An upper limit on the number of messages in the <b>active</b>
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queue. For an introduction to the Postfix queue organization see
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the <a href="queuing.html">Postfix overview</a> documentation.
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<p>
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<dt> <b>duplicate_filter_limit</b> (default: 1000)
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<dd> How many recipient addresses the <a href="local.8.html">local
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delivery</a> agent and <a href="cleanup.8.html">address cleanup</a>
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daemon remember when delivering a message. A recipient address is
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ignored when it is found in the remembered list.
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</dl>
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<a name="time"><h2> Time limits</h2> </a>
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External commands are given a finite time for completion. Such
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commands are run by the <a href="local.8.html">local</a> delivery
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agent when it finds a "|<i>command</i>" destination in an <a
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href="aliases.5.html">alias</a> database, <a
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href="aliases.5.html">:include:</a> file or <a
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href="aliases.5.html">.forward</a> file. The <a
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href="pipe.8.html">pipe</a> mailer implements an alternative way
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to pipe mail into external commands.
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<dl>
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<dt> <b>command_time_limit</b> (default: 1000 seconds)
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<dd> How long the <a href="local.8.html">local</a> delivery agent
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will wait before aborting an external command.
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<p>
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<dt> <i>service_name</i><b>_time_limit</b> (default:
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<b>$command_time_limit</b>)
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<dd> The time limit for delivery to external commands via the
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<b>pipe</b> mailer. For <i>service_name</i>, substitute the service
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name (the first field in the <b>master.cf </b> file).
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</dl>
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<a name="lock"><h2> Acquiring exclusive file locks</h2> </a>
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Internally, the Postfix programs cooperate in a very disciplined
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manner and rarely need to fight for exclusive file access. However,
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access conflicts may happen on the outside, for example, when mail
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has to be delivered while a user is accessing her mailbox. Postfix
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supports two types of file locks:
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<ul>
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<li>Internal locks, implemented with the <b>fcntl()</b> or
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<b>flock()</b> system primitives.
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<p>
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<li>External locks, implemented as files named <i>file</i><b>.lock</b>.
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</ul>
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Depending on the host system, Postfix uses one method or both.
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The following configuration parameters control how Postfix deals
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with file locks:
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<dl>
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<dt> <b>deliver_lock_attempts</b> (default: 5)
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<dd> The number of times to try locking a file before giving up.
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<p>
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<dt> <b>deliver_lock_delay</b> (default: 1 second)
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<dd> How long to wait between attempts to lock a file.
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<p>
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<dt> <b>stale_lock_time</b> (default: 500)
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<dd> How old an external lock file may be before it is forcibly
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removed.
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</dl>
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<a name="fan"><h2> Error recovery</h2> </a>
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Under conditions of severe stress, available system resources may
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be insufficient to accommodate Postfix's needs. The world may also
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seem to fall apart when a Postfix configuration file is broken, or
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when a Postfix program is defective.
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<p>
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The general approach taken in the face of disaster is to terminate
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with a fatal run-time error (or with a panic in case of software
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problems), and to try again after some time (the <a
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href="master.8.html">master</a> daemon will restart processes after
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some delay). Each failed attempt is logged; hopefully, someone will
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notice the problem and fix it.
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<p>
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Some recovery strategies were implemented very early during Postfix
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development, and haven't been made configurable yet. What follows
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is the beginning of a growing list of recovery control parameters:
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<dl>
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<dt> <b>fork_attempts</b> (default: 5 times)
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<dd> The number of times to attempt to create a new process before
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giving up.
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<p>
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<dt> <b>fork_delay</b> (default: 1 second)
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<dd> The delay between attempts to create a new process.
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<p>
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<dt> <b>transport_retry_time</b> (default: 60 seconds)
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<dd> The amount of time between queue manager attempts to contact
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an apparently defunct Postfix delivery service.
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</dl>
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<hr>
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<a href="index.html">Up one level</a> | <a href="basic.html">Basic
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Configuration</a> | <a href="uce.html">UCE Controls</a> | <a
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href="rate.html">Rate Controls</a> | Resource Controls | <a
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href="rewrite.html">Address Manipulation</a>
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</body>
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</html>
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