Postfix Performance Tuning

Purpose of Postfix performance tuning

The hints and tips in this document help you improve the performance of Postfix systems that already work. If your Postfix system is unable to receive or deliver mail, then you need to solve those problems first, using the DEBUG_README document as guidance.

For tuning external content filter performance, first read the respective information in the FILTER_README and SMTPD_PROXY_README documents. Then make sure to avoid latency in the content filter code. As much as possible avoid performing queries against external data sources with a high or highly variable delay. Your content filter will run with a small concurrency to avoid CPU/memory starvation, and if any latency creeps in, content filter throughput will suffer. High volume environments should avoid RBL lookups, complex database queries and so on.

Topics on mail receiving performance:

Topics on mail delivery performance:

Other Postfix performance tuning topics:

The following tools can be used to measure mail system performance under artificial loads. They are normally not installed with Postfix.

General mail receiving performance tips

  • Read and understand the maildrop queue, incoming queue, and active queue discussions in the QSHAPE_README document.

  • Run a local name server to reduce slow-down due to DNS lookups. If you run multiple Postfix systems, point each local name server to a shared forwarding server to reduce the number of lookups across the upstream network link.

  • Eliminate unnecessary LDAP lookups, by specifying a domain filter. This eliminates lookups for addresses in remote domains, and eliminates lookups of partial addresses. See ldap_table(5) for details.

When Postfix responds slowly to SMTP clients:

  • Look for obvious signs of trouble as described in the DEBUG_README document, and eliminate those problems first.

  • Turn off your header_checks and body_checks patterns and see if the problem goes away.

  • Turn off chroot operation as described in the DEBUG_README document and see if the problem goes away.

  • If Postfix logs the SMTP client as "unknown" then you have a name service problem: the name server is bad, or the resolv.conf file contains bad information, or some packet filter is blocking the DNS requests or replies.

  • If the number of smtpd(8) processes has reached the process limit as specified in, new SMTP clients must wait until a process becomes available. Increase the number of processes if memory permits. See the instructions given under "Tuning the number of Postfix processes".

Doing more work with your SMTP server processes

With Postfix versions 2.0 and earlier, the smtpd(8) server pauses before reporting an error to an SMTP client. The idea is called tar pitting. However, these delays also slow down Postfix. When the smtpd(8) server replies slowly, sessions take more time, so that more smtpd(8) server processes are needed to handle the load. When your Postfix smtpd(8) server process limit is reached, new clients must wait until a server process becomes available. This means that all clients experience poor performance.

You can speed up the handling of smtpd(8) server error replies by turning off the delay:

    # Not needed with Postfix 2.1
    smtpd_error_sleep_time = 0

With the above setting, Postfix 2.0 and earlier can serve more SMTP clients with the same number SMTP server processes. The next section describes how Postfix deals with clients that make a large number of errors.

Slowing down SMTP clients that make many errors

The Postfix smtpd(8) server maintains a per-session error count. The error count is reset when a message is transferred successfully, and is incremented when a client request is unrecognized or unimplemented, when a client request violates access restrictions, or when some other error happens.

As the per-session error count increases, the smtpd(8) server changes behavior and begins to insert delays into the responses. The idea is to slow down a run-away client in order to limit resource usage. The behavior is Postfix version dependent.

IMPORTANT: These delays slow down Postfix, too. When too much delay is configured, the number of simultaneous SMTP sessions will increase until it reaches the smtpd(8) server process limit, and new SMTP clients must wait until an smtpd(8) server process becomes available.

Postfix version 2.1 and later:

Postfix version 2.0 and earlier:

Measures against clients that make too many connections

Note: this feature is not included with Postfix version 2.1.

The Postfix smtpd(8) server can limit the number of simultaneous connections from the same SMTP client, as well as the number of connections that a client is allowed to make per unit time. These statistics are maintained by the anvil(8) server (translation: if anvil(8) breaks, then connection limits stop working).

IMPORTANT: These limits are designed to protect the smtpd(8) server against flagrant abuse. Do not use these limits to regulate legitimate traffic: mail will suffer grotesque delays if you do so.

General mail delivery performance tips

  • Read and understand the maildrop queue, incoming queue, active queue and deferred queue discussions in the QSHAPE_README document.

  • In case of slow delivery, run the qshape tool as described in the QSHAPE_README document.

  • Submit multiple recipients per message instead of submitting messages with only a few recipients.

  • Submit mail via SMTP instead of /usr/sbin/sendmail. You may have to adjust the smtpd_recipient_limit parameter setting.

  • Don't overwhelm the disk with mail submissions. Optimize the mail submission rate by tuning the number of parallel submissions and/or by tuning the Postfix in_flow_delay parameter setting.

  • Run a local name server to reduce slow-down due to DNS lookups. If you run multiple Postfix systems, point each local name server to a shared forwarding server to reduce the number of lookups across the upstream network link.

  • Reduce the smtp_connect_timeout and smtp_helo_timeout values so that Postfix does not waste lots of time connecting to non-responding smtpd(8) servers.

  • Use a dedicated mail delivery transport for problematic destinations, with reduced timeouts and with adjusted concurrency. See "Tuning the number of simultaneous deliveries" below.

  • Use a fallback_relay host for mail that cannot be delivered upon the first attempt. This "graveyard" machine can use shorter retry times for difficult to reach destinations. See "Tuning the frequency of deferred mail delivery attempts" below.

  • Speed up disk updates with a large (64MB) persistent write cache. This allows disk updates to be sorted for optimal access speed without compromising file system integrity when the system crashes.

  • Use a solid-state disk (a persistent RAM disk). This is an expensive solution that should be used in combination with short SMTP timeouts and a fallback_relay "graveyard" machine that delivers mail for problem destinations.

Tuning the number of simultaneous deliveries

Although Postfix can be configured to run 1000 SMTP client processes at the same time, it is rarely desirable that it makes 1000 simultaneous connections to the same remote system. For this reason, Postfix has safety mechanisms in place to avoid this so-called "thundering herd" problem.

The Postfix queue manager implements the analog of the TCP slow start flow control strategy: when delivering to a site, send a small number of messages first, then increase the concurrency as long as all goes well; reduce concurrency in the face of congestion.

  • The initial_destination_concurrency parameter (default: 5) controls how many messages are initially sent to the same destination before adapting delivery concurrency. Of course, this setting is effective only as long as it does not exceed the process limit and the destination concurrency limit for the specific mail transport channel.

  • The default_destination_concurrency_limit parameter (default: 20) controls how many messages may be sent to the same destination simultaneously. You can override this setting for specific message delivery transports by taking the name of the entry and appending "_destination_concurrency_limit".

Examples of transport specific concurrency limits are:

  • The local_destination_concurrency_limit parameter (default: 2) controls how many messages are delivered simultaneously to the same local recipient. The recommended limit is low because delivery to the same mailbox must happen sequentially, so massive parallelism is not useful. Another good reason to limit delivery concurrency to the same recipient: if the recipient has an expensive shell command in her .forward file, or if the recipient is a mailing list manager, you don't want to run too many instances of those processes the same time.

  • The default smtp_destination_concurrency_limit of 20 seems enough to noticeably load a system without bringing it to its knees. Be careful when changing this to a much larger number.

The above default values of the concurrency limits work well in a broad range of situations. Knee-jerk changes to these parameters in the face of congestion can actually make problems worse. Specifically, large destination concurrencies should never be the default. They should be used only for transports that deliver mail to a small number of high volume domains.

A common situation where high concurrency is called for is on gateways relaying a high volume of mail from between the Internet and an intranet mail environment. Approximately half the mail (assuming equal volumes inbound and outbound) will be destined for the internal mail hubs. Since the internal mail hubs will be receiving all external mail exclusively from the gateway, it is reasonable to configure the gateway to make greater demands on the capacity of the internal SMTP servers.

The tuning of the inbound concurrency limits need not be trial and error. A high volume capable mailhub should be able to easily handle 50 or 100 (rather than the default 20) simultaneous connections, especially if the gateway forwards to multiple MX hosts. When all MX hosts are up and accepting connections in a timely fashion, throughput will be high. If any MX host is down and completely unresponsive, the average connection latency rises to at least 1/N * $smtp_connection_timeout, if there are N MX hosts. This limits throughput to at most the destination concurrency * N / $smtp_connection_timeout.

For example, with a destination concurrency of 100 and 2 MX hosts, each host will handle up to 50 simultaneous connections. If one MX host is down and the default SMTP connection timeout is 30s, the throughput limit is 100 * 2 / 30 ~= 6 messages per second. This suggests that high volume destinations with good connectivity and multiple MX hosts need a lower connection timeout, values as low as 5s or even 1s can be used to prevent congestion when one or more, but not all MX hosts are down.

If necessary, set a higher transport_destination_concurrency_limit (in since this is a queue manager parameter) and a lower smtp_connection_timeout (with a "-o" override in since this parameter has no per-transport name) for the relay transport and any transports dedicated for specific high volume destinations.

Tuning the number of recipients per delivery

The default_destination_recipient_limit parameter (default: 50) controls how many recipients a Postfix delivery agent will send with each copy of an email message. You can override this setting for specific Postfix delivery agents. For example, "uucp_destination_recipient_limit = 100" would limit the number of recipients per UUCP delivery to 100.

If an email message exceeds the recipient limit for some destination, the Postfix queue manager breaks up the list of recipients into smaller lists. Postfix will attempt to send multiple copies of the message in parallel.

IMPORTANT: Be careful when increasing the recipient limit per message delivery; some smtpd(8) servers abort the connection when they run out of memory or when a hard recipient limit is reached, so that the message will never be delivered.

The smtpd_recipient_limit parameter (default: 1000) controls how many recipients the Postfix smtpd(8) server will take per delivery. The default limit is more than any reasonable SMTP client would send. The limit exists to protect the local mail system against a run-away client.

Tuning the frequency of deferred mail delivery attempts

When a Postfix delivery agent (smtp(8), local(8), etc.) is unable to deliver a message it may blame the message itself, or it may blame the receiving party.

  • When the delivery agent blames the message, the queue manager gives the queue file a time stamp into the future, so it won't be looked at for a while. By default, the amount of time to cool down is the amount of time that has passed since the message arrived. This results in so-called exponential backoff behavior.

  • When the delivery agent blames the receiving party (for example a local recipient user, or a remote host), the queue manager not only advances the queue file time stamp, but also puts the receiving party on a "dead" list so that it will be skipped for some amount of time.

This process is governed by a bunch of little parameters.

queue_run_delay (default: 1000 seconds)
How often the queue manager scans the queue for deferred mail.
minimal_backoff_time (default: 1000 seconds)
The minimal amount of time a message won't be looked at, and the minimal amount of time to stay away from a "dead" destination.
maximal_backoff_time (default: 4000 seconds)
The maximal amount of time a message won't be looked at after a delivery failure.
maximal_queue_lifetime (default: 5 days)
How long a message stays in the queue before it is sent back as undeliverable. Specify 0 for mail that should be returned immediately after the first unsuccessful delivery attempt.
bounce_queue_lifetime (default: 5 days, available with Postfix version 2.1 and later)
How long a MAILER-DAEMON message stays in the queue before it is considered undeliverable. Specify 0 for mail that should be tried only once.
qmgr_message_recipient_limit (default: 20000)
The size of many in-memory queue manager data structures. Among others, this parameter limits the size of the short-term, in-memory list of "dead" destinations. Destinations that don't fit the list are not added.

IMPORTANT: If you increase the frequency of deferred mail delivery attempts, or if you flush the deferred mail queue frequently, then you may find that Postfix mail delivery performance actually becomes worse. The symptoms are as follows:

  • The active queue becomes saturated with mail that has delivery problems. New mail enters the active queue only when an old message is deferred. This is a slow process that usually requires timing out one or more SMTP connections.

  • All available Postfix delivery agents become occupied trying to connect to unreachable sites etc. New mail has to wait until a delivery agent becomes available. This is a slow process that usually requires timing out one or more SMTP connections.

When mail is being deferred frequently, fixing the problem is always better than increasing the frequency of delivery attempts. However, if you can control only the delivery attempt frequency, consider using a dedicated fallback_relay "graveyard" machine for bad destinations so that they do not ruin the performance of normal mail deliveries.

Tuning the number of Postfix processes

The default_process_limit configuration parameter gives direct control over how many daemon processes Postfix will run. As of Postfix 2.0 the default limit is 100 smtp client processes, 100 smtp server processes, and so on. This may overwhelm systems with little memory, as well as networks with low bandwidth.

You can change the global process limit by specifying a non-default default_process_limit in the file. For example, to run up to 10 smtp client processes, 10 smtp server processes, and so on:

    default_process_limit = 10

You need to execute "postfix reload" to make the change effective. The limits are enforced by the Postfix master(8) daemon which does not automatically read when it changes.

You can override the process limit for specific Postfix daemons by editing the file. For example, if you do not wish to receive 100 SMTP messages at the same time, but do not want to change the process limits for local mail deliveries, you could specify:

    # ====================================================================
    # service type  private unpriv  chroot  wakeup  maxproc command + args
    #               (yes)   (yes)   (yes)   (never) (100)
    # ====================================================================
    . . .
    smtp      inet  n       -       -       -       10      smtpd
    . . .

Tuning the number of open files or sockets

When Postfix opens too many files or sockets, processes will abort with fatal errors, and the system may log "file table full" errors.

  • Reduce the number of processes as described under "Tuning the number of Postfix processes" above. Fewer processes need fewer open files and sockets.

  • Configure the kernel for more open files and sockets. The details are extremely system dependent and change with the operating system version. Be sure to verify the following information with your system tuning guide:

    • Some FreeBSD kernel parameters can be specified in /boot/loader.conf, and some can be changed with sysctl commands. Which is which depends on the version.

    • Linux kernel parameters can be specified in /etc/sysctl.conf and can also be changed with sysctl commands:

    • Solaris kernel parameters can be specified in /etc/system, as described in the Solaris FAQ entry titled "How can I increase the number of file descriptors per process?"

      * set hard limit on file descriptors
      set rlim_fd_max = 4096
      * set soft limit on file descriptors
      set rlim_fd_cur = 1024