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 [http://cr.yp.to/proto/maildir.html]
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.txt>, $Spam,
$<NonSpam.txt> 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 [http://cr.yp.to/proto/maildir.html]. The <non-standard
fields> isn't used by Dovecot for anything currently.

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

 * ',S=<size>': <size> contains the file size. Getting the size from the
   filename avoids doing a stat(), which may improve the performance. This is
   especially useful with<Maildir++ quota> [Quota.Maildir.txt].
 * ',W=<vsize>': <vsize> contains the file's RFC822.SIZE, ie. the file size
   with linefeeds being CR+LF characters. If the message was stored with CR+LF
   linefeeds,<size> and <vsize> are the same. Setting this may give a small
   speedup because now Dovecot doesn't need to calculate the size itself.

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

Maildir and 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 appears to be quite a lot slower than ext3 or ReiserFS. See
http://dovecot.org/list/dovecot/2007-January/018994.html

Mounting XFS with logbufs=8 option might increase the speed.

The above results are system dependant - XFS on TSL 3.0.5 works almost twice as
fast as our prior EXT3 installation of which is significant in size. ReiserFS
is also a good option.

Directory Structure
-------------------

Dovecot uses Maildir++
[http://www.inter7.com/courierimap/README.maildirquota.html] directory layout
for organizing mailbox directories. This means that all the folders are
directly inside '~/Maildir' directory:

 * '~/Maildir/new', '~/Maildir/cur' and '~/Maildir/tmp' directories contain the
   messages for INBOX. The 'tmp' directory is used during delivery, new
   messages arrive in 'new' and read shall be moved to 'cur' by the clients.
 * '~/Maildir/.folder/' is a mailbox folder
 * '~/Maildir/.folder.subfolder/' is a subfolder of a folder (ie.
   "folder/subfolder")

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> [MailLocation.txt].

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
[http://www.qmail.org/man/man5/maildir.html] 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
----------

 * Official Maildir format page [http://cr.yp.to/proto/maildir.html]
 * Qmail's how to deliver to Maildir man page
   [http://www.qmail.org/man/man5/maildir.html]
 * Maildir++ [http://www.inter7.com/courierimap/README.maildirquota.html]
 * Wikipedia [http://en.wikipedia.org/wiki/Maildir] 

(This file was created from the wiki on 2009-01-05 04:42)
