dbox Mailbox Format

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.

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, so the indexes don't have to be "synchronized". Dovecot trusts that they're always up-to-date (unless it sees that something is clearly broken).

DANGER

You must not lose the dbox index files, as they can't be regenerated without data loss!

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

dbox storage is extensible. Single instance attachment storage was already implemented as such extension.

dbox Modes

single-dbox (sdbox)

One message per file, similar to Maildir.

For backwards compatibility, dbox is an alias to sdbox in mail_location.

multi-dbox (mdbox)

Multiple messages per file,but unlike mbox stores multiple files per mailbox.

Layout

By default, the dbox filesystem layout is as follows.

Data which isn't the actual message content is stored in a layout common to both sdbox and mdbox.

In these tables <root> is shorthand for the mail location root directory on the filesystem.

Index files can be stored in a different location by using the INDEX parameter in the mail location specification. If the INDEX parameter is specified, it will override the mail location root for index files and the "map index" file (mdbox only).

Location	Description
<root>/mailboxes/INBOX/dbox-Mails/dovecot.index*	Index files for INBOX
<root>/mailboxes/foo/dbox-Mails/dovecot.index*	Index files for mailbox "foo"
<root>/mailboxes/foo/bar/dbox-Mails/dovecot.index*	Index files for mailbox "foo/bar"
<root>/dovecot.mailbox.log*	Mailbox changelog
<root>/subscriptions	Subscribed mailboxes list
<root>/dovecot-uidvalidity*	IMAP UID validity

Note that with dbox the Index files contain significant data which is held nowhere else. Index files for both sdbox and mdbox contain message flags and keywords.

For mdbox, the index file also contains the map_uids which link (via the "map index") to the actual message data. This data cannot be automatically recreated, so it is important that Index files are treated with the same care as message data files.

Actual message content is stored differently depending on whether it is sdbox or mdbox.

Message Storage: sdbox

Location	Description
<root>/mailboxes/INBOX/dbox-Mails/u.*	Numbered files (u.1, u.2, ...) each containing one message of INBOX
<root>/mailboxes/foo/dbox-Mails/u.*	Files each containing one message for mailbox "foo"
<root>/mailboxes/foo/bar/dbox-Mails/u.*	Files each containing one message for mailbox "foo/bar"

Message Storage: mdbox

Location	Description
<root>/storage/dovecot.map.index*	"Map index" containing a record for each message stored
<root>/storage/m.*	Numbered files (u.1, u.2, ...) each containing one or multiple messages

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

Location	Description
~/mdbox/storage/	The mail data for all mailboxes
~/mdbox/mailboxes/	Directories for mailboxes and their index files

The storage directory has files:

File	Description
dovecot.map.index*	The "map index"
m.*	Mail data. Each m.* file contains one or more messages. mdbox_rotate_size can be used to configure how large the files can grow.

The "map index" contains a record for each message:

Key	Description
map_uid	Unique growing 32 bit number for the message.
refcount	16 bit reference counter for this message. Each time the message is copied the refcount is increased.
file_id	File number containing the message. For example if file_id=5, the message is in file m.5.
offset	Offset to message within the file.
size	Space used by the message in the file, including all metadata.

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:

• Each message has a 128 bit globally unique identifier (GUID). The GUID is saved to message metadata in m.* files and also to mailbox indexes. This allows Dovecot to find messages even if map index gets corrupted.
• Whenever index file is rewritten, the old index is renamed to dovecot.index.backup. If the main index becomes corrupted, this backup index is used to restore flags and figure out what messages belong to the mailbox.
• Initial mailbox where message was saved to is stored in the message metadata in m.* files. So if all indexes get lost, the messages are put to their initial mailboxes. This is better than placing everything into a single mailbox.

Alternate Storage

Unlike Maildir, with dbox 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.

When messages are moved from primary storage to alternate storage, only the actual message data (stored in files u.* under sdbox and m.* under mdbox) 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 mdbox 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.

Configuration

To enable this functionality, use the ALT parameter in mail_location. For example:

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. There's no problem having the same (identical) file in both storages.

Keep the unmounted /altstorage directory permissions such that Dovecot mail processes can't create directories under it (e.g. root:root 0755). This way if the alt storage isn't mounted for some reason, Dovecot won't think that all the messages in alt storage were deleted and lose their flags.

dbox and Mail Header Metadata

Unlike when using mbox, where mail headers (for example Status, X-UID, etc.) are used to determine and store metadata, the mail headers within dbox files are (usually) not used for this purpose by Dovecot; neither when mails are created/moved/etc. via IMAP nor when dboxes are placed (e.g. copied or moved in the filesystem) in a mail location (and then "imported" by Dovecot).

Therefore, it is (usually) not necessary, to strip any such mail headers at the MTA, MDA, or LDA (as it is recommended with mbox).

There is one exception, though, namely when pop3_reuse_xuidl = yes: in this case X-UIDL is used for the POP3 UIDLs. Therefore, in this case, is recommended to strip the X-UIDL mail headers case-insensitively at the mail delivery layer.

Accessing Expunged Mails with mdbox

mdbox_deleted storage can be used to access mdbox's all mails that are completely deleted (reference count = 0). The mdbox_deleted parameters should otherwise be exactly the same as mdbox's. Then you can use e.g. doveadm fetch or doveadm import commands to access the mails.

For example, if you have mail_location=mdbox:~/mdbox:INDEX=/var/index/%u: doveadm import mdbox_deleted:~/mdbox:INDEX=/var/index/%u "" subject oops.

This finds a deleted mail with subject "oops" and imports it into INBOX.

Mail Delivery

Some MTA configurations have the MTA directly dropping mail into Maildirs or mboxes. Since most MTAs don't understand the dbox format, this option is not available. Instead, the MTA should use LMTP Server or LDA.

dbox Configuration

Settings

mdbox_preallocate_space

Default	no
Value	boolean
See Also	mdbox_rotate_size mdbox

mdbox only: If enabled, preallocate space for newly created files.

In creation of new mdbox files, their size is immediately preallocated as mdbox_rotate_size.

This setting currently works only in Linux with certain filesystems (ext4 and xfs).

mdbox_rotate_interval

Default	[None]
Value	size
See Also	mdbox

mdbox only: The maximum age the dbox file may reach before it's rotated.

0 means there is no age-based rotation.

mdbox_rotate_size

Default	10M
Value	size
See Also	mdbox

mdbox only: The maximum size the dbox file may reach before it is rotated.

Mail Location

sdbox

To use single-dbox, use the tag sdbox in mail_location:

# single-dbox
mail_location = sdbox:~/dbox

For backwards compatibility, dbox is an alias to sdbox in the mail location. (This usage is deprecated.)

mdbox

To use multi-dbox, use the tag mdbox in mail_location:

# multi-dbox
mail_location = mdbox:~/mdbox

Default mail_location Keys

For dbox, the default mail_location keys: are:

Key	Default Value
MAILDIRBOX	mailboxes/
FULLDIRNAME	dbox-Mails/