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
- Maildir and filesystems
- Directory Structure
- Issues with the specification
- Procmail Problems
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.
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>:
,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.
,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
General comparisons of Maildir on different filesystems
http://www.htiweb.inf.br/benchmark/fsbench.htm (including some graphs)
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 was built to be fast with lots of small files, so it works well with maildir.
XFS performance seems to depend on a lot of factors, also on the system and the file system parameters.
There are early reports on the dovecot mailing list which suggest that XFS seems quite a lot slower than ext3 or ReiserFS: http://www.dovecot.org/list/dovecot/2007-January/018994.html
But then again others recommend XFS for the use with Maildir and dovecot: http://www.dovecot.org/list/dovecot/2006-May/013216.html
This 2007 Linux.conf.au talk about "Choosing and Tuning Linux File Systems" (Slides as PDF) also recommends XFS for Maildir (alternatively ext3 with small blocks and high inodetofile ratio)
- Someone else wrote here in the wiki: 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.
- Comparisons which suggest XFS as being best choice:
Mounting XFS with logbufs=8 option might increase the speed.
Create the XFS with options -b size=1024 -d su=16k,sw=3 -l logdev=<some_other_device> (Source: http://www.thesmbexchange.com/eng/qmail_fs_benchmark.html)
Use mkfs.xfs -f -l size=32768b,version=2 and mount.xfs -o noatime,logbufs=8,logbsize=131072 (Source: http://www.htiweb.inf.br/benchmark/fsbench.htm)
Dovecot uses Maildir++ 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.
subscriptions file contains IMAP's mailbox subscriptions. (Note difference with Mbox.)
Issues with the specification
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.
Qmail's how a message is delivered page suggests to deliver the mail like this:
- Create a unique filename (only "time.pid.host" here, later Maildir spec has been updated to allow more uniqueness identifiers)
Do stat(tmp/<filename>). If the stat() found a file, wait 2 seconds and go back to step 1.
Create and write the message to the tmp/<filename>.
- 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.
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.