Performance tuning

Disk I/O optimization

Usually heavily loaded IMAP and POP3 servers don’t use much CPU, but they use all the disk I/O they can get. So reducing disk I/O is probably the most useful optimization you can do.

• See Mbox Mailbox Format for mbox-specific optimizations.

• See Maildir Mailbox Format for Maildir-specific optimizations.

• See dbox Mailbox Format for Dovecot’s own high-performance mailbox format. It usually gives much better performance than mbox/Maildir.

• See full text search indexes for optimizing IMAP SEARCH command.

• See Dovecot as a POP3 server for POP3 optimizations, especially pop3_no_flag_updates=yes

• mailbox_list_index=yes can help a lot by replying to IMAP STATUS (and similar) lookups from a single index without having to open each mailbox index separately. This is the default in v2.3+.

  • Also mailbox_list_index_very_dirty_syncs=yes makes Dovecot assume that the list index is up-to-date.

• mail_prefetch_count setting may be helpful with some mailbox formats

• mail_location = ...:VOLATILEDIR=/tmp/dovecot-volatile/%2.256Nu/%u moves e.g. lock files to the volatile directory. This is helpful especially if mail_location otherwise points to a remote filesystem like NFS. (v2.2.32+)

• If the acl plugin is used, but only global ACLs are needed, set acl_globals_only=yes (v2.2.31+)

CPU usage optimization

• See Login processes for optimizing CPU usage caused by logins

• See auth_cache_size setting for caching passdb and userdb lookups

  • To distribute password hash calculations to multiple CPU cores (via auth-worker processes), set auth_cache_verify_password_with_worker=yes. (v2.2.34+)

• Services having client_limit>1 and process_limit>1, set process_min_avail to the number of CPU cores.

• To reduce forks by reusing existing processes for new requests increase service { service_count } from 1 to higher (e.g. 100) for imap and pop3 services. It’s better not to set it too high or unlimited (0), because different users use different amounts of memory, and it’s wasteful when a lot of processes end up having a lot of “free” memory.

Memory usage optimization

There aren’t many settings which affect Dovecot’s memory usage. In general Dovecot uses as much memory as it needs, which is usually quite little.

• auth_cache_size controls maximum memory size for caching passdb/userdb lookups

• High-performance mode for login processes.

• imap_hibernate_timeout controls when to move IDLEing IMAP connections to wait for changes in a shared imap-hibernate process. This frees up the imap process.

Note that these settings do not directly affect the memory usage:

• service { vsz_limit }: These are simply safe guards against potential memory leaks. If the process’s virtual size reaches the limit, the process is killed by the kernel.

• service { process_limit } and service { client_limit }: These are mostly to avoid DoS attacks using up all your memory.