This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

dbox

dbox is Dovecot's own high-performance mailbox format. The original version was introduced in v1.0 alpha4, but since then it has been completely redesigned in v1.1 series and improved even further in v2.0.

dbox can be used in two ways:

  1. sdbox: One message per file (single-dbox), similar to Maildir. For backwards compatibility, "dbox" is an alias to "sdbox" in mail_location.

  2. mdbox: Multiple messages per file (multi-dbox), but unlike mbox multiple files per mailbox.

One of the main reasons for dbox's high performance is that it uses Dovecot's index files as the only storage for message flags and keywords. This means that indexes don't have to be "synchronized". Dovecot trusts that they're always up-to-date (unless it sees that something is clearly broken).

dbox has a feature for transparently moving message data to an alternate storage area.

dbox storage is extensible, so in future there will be other extensions. Some things that are planned:

Alternate storage

Unlike Maildir the message file names don't change. This makes it possible to support storing files in multiple directories or mount points. dbox supports looking up files from "altpath" if they're not found from the primary path. This means that it's possible to move older mails that are rarely accessed to cheaper (slower) storage.

To enable this functionality, use the ALT parameter in the mail location. For example, specifying the mail location as:

mail_location = mdbox:/var/vmail/%d/%n:ALT=/altstorage/vmail/%d/%n

will make Dovecot look for message data first under /var/vmail/%d/%n ("primary storage"), and if it is not found there it will look under /altstorage/vmail/%d/%n ("alternate storage") instead.

When messages are moved from primary storage to alternate storage, only the actual message data (stored in files u.* under single-dbox and m.* under multi-dbox) is moved to alternate storage; everything else remains in the primary storage.

Message data can be moved from primary storage to alternate storage using doveadm altmove.

The granularity at which data is moved to alternate storage is individual messages. This is true even for multi-dbox when multiple messages are stored in a single m.* storage file. If individual messages from an m.* storage file need to be moved to alternate storage, the message data is written out to a different m.* storage file (either new or existing) in the alternate storage area and the "map index" updated accordingly.

Alternate storage is completely transparent at the IMAP/POP level. Users accessing mail through IMAP or POP cannot normally tell if any given message is stored in primary storage or alternate storage. Conceivably users might be able to measure a performance difference; the point is that there is no IMAP/POP command which could be used to expose this information. It is entirely possible to have a mail folder which contains a mix of messages stored in primary storage and alternate storage.

An upshot of the way alternate storage works is that any given storage file (mailboxes/<folder>/dbox-Mails/u.* (sdbox) or storage/m.* (mdbox)) can only appear either in the primary storage area or the alternate storage area but not both — if the corresponding file appears in both areas then there is an inconsistency.

Multi-dbox

You can enable multi-dbox with:

mail_location = mdbox:~/mdbox

The directory layout (under ~/mdbox/) is:

The storage directory has files:

Each m.* file contains one or more messages. mdbox_rotate_size setting can be used to configure how large the files can grow.

The map index contains a record for each message:

Mailbox indexes refer to messages only using map_uids. This allows messages to be moved to different files by updating only the map index. Copying is done simply by appending a new record to mailbox index containing the existing map_uid and increasing its refcount. If refcount grows over 32768, currently Dovecot gives an error message. It's unlikely anyone really wants to copy the same message that many times.

Expunging a message only decreases the message's refcount. The space is later freed in "purge" step. This is typically done in a nightly cronjob when there's less disk I/O activity. The purging first finds all files that have refcount=0 mails. Then it goes through each file and copies the refcount>0 mails to other mdbox files (to the same files as where newly saved messages would also go), updates the map index and finally deletes the original file. So there is never any overwriting or file truncation.

The purging can be invoked explicitly running doveadm purge.

There are several safety features built into dbox to avoid losing messages or their state if map index or mailbox index gets corrupted: