This documentation is for Dovecot v1.x, see wiki2 for v2.x documentation.

Maildir

This format debuted with the qmail server in the mid-1990s. Each mailbox folder is a directory and each message a file. This improves efficiency because individual emails can be modified, deleted and added without affecting the mailbox or other emails, and makes it safer to use on networked file systems such as NFS.

Dovecot extensions

Since the standard maildir specification doesn't provide everything needed to fully support the IMAP protocol, Dovecot had to create some of its own non-standard extensions. The extensions still keep the maildir standards compliant, so MUAs not supporting the extensions can still safely use it as a normal maildir.

IMAP UID mapping

IMAP requires each message to have a permanent unique ID number. Dovecot uses dovecot-uidlist file to keep UID <-> filename mapping. The file is basically in the same format as Courier IMAP's courierimapuiddb file, except for one difference (see below).

The file begins with a header:

1 1173189136 20221

Where 1 means the file format version number, 1173189136 is the IMAP UIDVALIDITY and 20221 is the UID that will be given to the next added message. The version number is always 1 currently. Dovecot used to have version number 2 also for a while, so if the number is ever increased it needs to become version 3.

After the header comes the list of UID <-> filename mappings:

123 1035478339.27041_118.foo.org
20220 1035478339.27041_118.foo.org:2,S

Because with maildir the filename changes every time the message's flags change, the filename listed in the file doesn't necessarily exist. With Courier IMAP the filenames contained only the maildir file's basename (ie. everything before ":2," string). Dovecot instead writes the file's last known full filename. Usually this allows opening the file without reading the directory's contents to find the file's current file name.

The dovecot-uidlist file doesn't need to be locked for reading. When writing dovecot-uidlist.lock file needs to be created. The dovecot-uidlist file must never be directly modified, it can only be replaced with rename() call.

dovecot-uidlist is updated lazily to optimize for disk I/O. If a message is expunged, it may not be removed from dovecot-uidlist until sometimes later. This means that if you create a new file using the same file name as what already exists in dovecot-uidlist, Dovecot thinks you "unexpunged" message by restoring a message from backup. This causes a warning to be logged and the file to be renamed.

Note that messages must not be modified once they've been delivered. IMAP (and Dovecot) requires that messages are immutable. If you wish to modify them in any way, create a new message instead and expunge the old one.

IMAP keywords

All the non-standard message flags are called keywords in IMAP. Some clients use these automatically for marking spam (eg. $Junk, $NonJunk, $Spam, $NonSpam keywords). Thunderbird uses labels which map to keywords $Label1, $Label2, etc.

Dovecot stores keywords in the maildir filename's flags field using letters a..z. This means that only 26 keywords are possible to store in the maildir. If more are used, they're still stored in Dovecot's index files. The mapping from single letters to keyword names is stored in dovecot-keywords file. The file is in format:

0 $Junk
1 $NonJunk

0 means letter 'a' in the maildir filename, 1 means 'b' and so on. The file doesn't need to be locked for reading, but when writing dovecot-uidlist file must be locked. The file must not be directly modified, it can only be replaced with rename() call.

Maildir filename extensions

The standard filename definition is: "<base filename>:2,<flags>". Dovecot has extended the <flags> field to be "<flags>[,<non-standard fields>]". This means that if Dovecot sees a comma in the <flags> field while updating flags in the filename, it doesn't touch anything after the comma. However other maildir MUAs may mess them up, so it's still not such a good idea to do that. Basic <flags> are described here. The <non-standard fields> isn't used by Dovecot for anything currently.

Dovecot supports reading a few fields from the <base filename>:

A maildir filename with those fields would look something like: 1035478339.27041_118.foo.org,S=1000,W=1030:2,S

Maildir and filesystems

General comparisons of Maildir on different filesystems

Linux ext2 / ext3

The main disadvantage is that searching can be slightly slower, and access to very large mailboxes (thousands of messages) can get slow with filesystems which don't have directory indexes.

Old versions of ext2 and ext3 on Linux don't support directory indexing (to speed up access), but newer versions of ext3 do, although you may have to manually enable it. You can check if the indexing is already enabled with tune2fs:

tune2fs -l /dev/hda3 | grep features

If you see dir_index, you're all set. If dir_index is missing, add it using:

umount /dev/hda3
tune2fs -O dir_index /dev/hda3
e2fsck -fD /dev/hda3
mount /dev/hda3

ReiserFS

ReiserFS was built to be fast with lots of small files, so it works well with maildir.

XFS

XFS performance seems to depend on a lot of factors, also on the system and the file system parameters.

Various tips

Directory Structure

Dovecot uses Maildir++ directory layout for organizing mailbox directories. This means that all the folders are directly inside ~/Maildir directory:

Most importantly this means that if your maildir folders exist in eg. ~/Maildir/folder and ~/Maildir/folder/subfolder, Dovecot won't see them unless you rename them to Maildir++ layout. v1.1 supports them by adding :LAYOUT=fs to mail_location.

Issues with the specification

Locking

Although maildir was designed to be lockless, Dovecot locks the maildir while doing modifications to it or while looking for new messages in it. This is required because otherwise Dovecot might temporarily see mails incorrectly deleted, which would cause trouble. Basically the problem is that if one process modifies the maildir (eg. a rename() to change a message's flag), another process in the middle of listing files at the same time could skip a file. The skipping happens because readdir() system call doesn't guarantee that all the files are returned if the directory is modified between the calls to it. This problem exists with all the commonly used filesystems.

Because Dovecot uses its own non-standard locking (dovecot-uidlist.lock dotlock file), other MUAs accessing the maildir don't support it. This means that if another MUA is updating messages' flags or expunging messages, Dovecot might temporarily lose some message. After the next sync when it finds it again, an error message may be written to log and the message will receive a new UID.

Delivering mails to new/ directory doesn't have any problems, so there's no need for LDAs to support any type of locking.

Mail delivery

Qmail's how a message is delivered page suggests to deliver the mail like this:

  1. Create a unique filename (only "time.pid.host" here, later Maildir spec has been updated to allow more uniqueness identifiers)
  2. Do stat(tmp/<filename>). If the stat() found a file, wait 2 seconds and go back to step 1.

  3. Create and write the message to the tmp/<filename>.

  4. link() it into new/ directory. Although not mentioned here, the link() could again fail if the mail existed in new/ dir. In that case you should probably go back to step 1.

All this trouble is rather pointless. Only the first step is what really guarantees that the mails won't get overwritten, the rest just sounds nice. Even though they might catch a problem once in a while, they give no guaranteed protection and will just as easily pass duplicate filenames through and overwrite existing mails.

Step 2 is pointless because there's a race condition between steps 2 and 3. PID/host combination by itself should already guarantee that it never finds such a file. If it does, something's broken and the stat() check won't help since another process might be doing the same thing at the same time, and you end up writing to the same file in tmp/, causing the mail to get corrupted.

In step 4 the link() would fail if an identical file already existed in the maildir, right? Wrong. The file may already have been moved to cur/ directory, and since it may contain any number of flags by then you can't check with a simple stat() anymore if it exists or not.

Step 2 was pointed out to be useful if clock had moved backwards. However again this doesn't give any actual safety guarantees, because an identical base filename could already exist in cur/. Besides if the system was just rebooted, the file in tmp/ could probably be even overwritten safely (assuming it wasn't already link()ed to new/).

So really, all that's important in not getting mails overwritten in your maildir is the step 1: Always create filenames that are guaranteed to be unique. Forget about the 2 second waits and such that the Qmail's man page talks about.

Procmail Problems

Maildir format is somewhat compatible with MH format. This is sometimes a problem when people configure their procmail to deliver mails to Maildir/new. This makes procmail create the messages in MH format, which basically means that the file is called msg.inode_number. While this appears to work first, after expunging messages from the maildir the inodes are freed and will be reused later. This means that another file with the same name may come to the maildir, which makes Dovecot think that an expunged file reappeared into the mailbox and an error is logged.

The proper way to configure procmail to deliver to a Maildir is to use Maildir/ as the destination.

References

MailboxFormat/Maildir (last edited 2010-11-15 07:05:38 by ppp121-45-248-9)