Updated: 2022/Sep/29

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FLUSH(8)                    System Manager's Manual                   FLUSH(8)

       flush - Postfix fast flush server

       flush [generic Postfix daemon options]

       The flush(8) server maintains a record of deferred mail by destination.
       This information is used to improve the performance of the SMTP ETRN
       request, and of its command-line equivalent, "sendmail -qR" or
       "postqueue -f".  This program expects to be run from the master(8)
       process manager.

       The record is implemented as a per-destination logfile with as contents
       the queue IDs of deferred mail. A logfile is append-only, and is
       truncated when delivery is requested for the corresponding destination.
       A destination is the part on the right-hand side of the right-most @ in
       an email address.

       Per-destination logfiles of deferred mail are maintained only for
       eligible destinations. The list of eligible destinations is specified
       with the fast_flush_domains configuration parameter, which defaults to

       This server implements the following requests:

       add sitename queueid
              Inform the flush(8) server that the message with the specified
              queue ID is queued for the specified destination.

       send_site sitename
              Request delivery of mail that is queued for the specified

       send_file queueid
              Request delivery of the specified deferred message.

              Refresh non-empty per-destination logfiles that were not read in
              $fast_flush_refresh_time hours, by simulating send requests (see
              above) for the corresponding destinations.

              Delete empty per-destination logfiles that were not updated in
              $fast_flush_purge_time days.

              This request completes in the background.

       purge  Do a refresh for all per-destination logfiles.

       The flush(8) server is not security-sensitive. It does not talk to the
       network, and it does not talk to local users.  The fast flush server
       can run chrooted at fixed low privilege.

       Problems and transactions are logged to syslogd(8) or postlogd(8).

       Fast flush logfiles are truncated only after a "send" request, not when
       mail is actually delivered, and therefore can accumulate outdated or
       redundant data. In order to maintain sanity, "refresh" must be executed
       periodically. This can be automated with a suitable wakeup timer
       setting in the master.cf configuration file.

       Upon receipt of a request to deliver mail for an eligible destination,
       the flush(8) server requests delivery of all messages that are listed
       in that destination's logfile, regardless of the recipients of those
       messages. This is not an issue for mail that is sent to a relay_domains
       destination because such mail typically only has recipients in one

       Changes to main.cf are picked up automatically as flush(8) processes
       run for only a limited amount of time. Use the command "postfix reload"
       to speed up a change.

       The text below provides only a parameter summary. See postconf(5) for
       more details including examples.

       config_directory (see 'postconf -d' output)
              The default location of the Postfix main.cf and master.cf
              configuration files.

       daemon_timeout (18000s)
              How much time a Postfix daemon process may take to handle a
              request before it is terminated by a built-in watchdog timer.

       fast_flush_domains ($relay_domains)
              Optional list of destinations that are eligible for
              per-destination logfiles with mail that is queued to those

       fast_flush_refresh_time (12h)
              The time after which a non-empty but unread per-destination
              "fast flush" logfile needs to be refreshed.

       fast_flush_purge_time (7d)
              The time after which an empty per-destination "fast flush"
              logfile is deleted.

       ipc_timeout (3600s)
              The time limit for sending or receiving information over an
              internal communication channel.

       max_idle (100s)
              The maximum amount of time that an idle Postfix daemon process
              waits for an incoming connection before terminating voluntarily.

       max_use (100)
              The maximal number of incoming connections that a Postfix daemon
              process will service before terminating voluntarily.

       parent_domain_matches_subdomains (see 'postconf -d' output)
              A list of Postfix features where the pattern "example.com" also
              matches subdomains of example.com, instead of requiring an
              explicit ".example.com" pattern.

       process_id (read-only)
              The process ID of a Postfix command or daemon process.

       process_name (read-only)
              The process name of a Postfix command or daemon process.

       queue_directory (see 'postconf -d' output)
              The location of the Postfix top-level queue directory.

       syslog_facility (mail)
              The syslog facility of Postfix logging.

       syslog_name (see 'postconf -d' output)
              A prefix that is prepended to the process name in syslog
              records, so that, for example, "smtpd" becomes "prefix/smtpd".

       Available in Postfix 3.3 and later:

       service_name (read-only)
              The master.cf service name of a Postfix daemon process.

       /var/spool/postfix/flush, "fast flush" logfiles.

       smtpd(8), SMTP server
       qmgr(8), queue manager
       postconf(5), configuration parameters
       master(5), generic daemon options
       master(8), process manager
       postlogd(8), Postfix logging
       syslogd(8), system logging

       Use "postconf readme_directory" or "postconf html_directory" to locate
       this information.
       ETRN_README, Postfix ETRN howto

       The Secure Mailer license must be distributed with this software.

       This service was introduced with Postfix version 1.0.

       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA

       Wietse Venema
       Google, Inc.
       111 8th Avenue
       New York, NY 10011, USA