Zlib plugin
===========

Zlib plugin can be used to read compressed mbox, maildir or dbox files. It can
be also used to write(via IMAP, <LDA.txt> and/or <LMTP.txt>) compressed
messages to <dbox> [MailboxFormat.dbox.txt] or Maildir mailboxes.   Zlib plugin
supports compression using zlib/gzip, bzlib/bzip2, liblzma/xz (v2.2.9+) and
liblz4/lz4 (v2.2.11+).

Configuration:

---%<-------------------------------------------------------------------------
# Enable zlib plugin globally for reading/writing:
mail_plugins = $mail_plugins zlib

# Enable these only if you want compression while saving:
plugin {
  zlib_save_level = 6 # 1..9; default is 6
  zlib_save = gz # or bz2, xz or lz4
}
---%<-------------------------------------------------------------------------

mbox
----

Compressed mbox files can be accessed only as read-only. The compression is
detected based on the file name, so your compressed mboxes should end with .gz
or .bz2 extension. There is no support for compression during saving.

dbox
----

Mails can be stored as compressed. Existing uncompressed mails can't currently
be directly compressed (or vice versa). You could, however, use <dsync>
[Tools.Dsync.txt] to copy all mails to another location (which saves them
compressed) and then replace the original location with the new compressed
location. You can do this by treating the operation the same as if you were
migrating from one mailbox format to another (see the dsync page examples).

Maildir
-------

When this plugin is loaded Dovecot can read both compressed and uncompressed
files from Maildir. If you've enabled both gzip and bzip2 support you can have
files compressed with either one of them in the Maildir. The compression is
detected by reading the first few bytes from the file and figuring out if it's
a valid gzip or bzip2 header. The file name doesn't matter. This means that an
IMAP client could also try to exploit security holes in zlib/bzlib by writing
specially crafted mails using IMAP's APPEND command. This is prevented by
Dovecot not allowing clients to save mails that are detected as compressed.

All mails must have ',S=<size>' in their filename where <size> contains the
original uncompressed mail size, otherwise there will be problems with quota
calculation as well as other potential random failures. Note that if the
filename doesn't contain the ',S=<size>' before compression, adding it
afterwards changes the base filename and thus the message UID. The safest thing
to do is simply to not compress such files.

You should also preserve the file's mtime so INTERNALDATE doesn't change.

If you want to use dsync to convert to a compressed Maildir you may need -o
maildir_copy_with_hardlinks=no (this is set to yes by default and will prevent
compression).

Compression
-----------

You'll probably want to use some cronjob to compress old mails. However note
that to avoid seeing duplicate mails in rare race conditions you'll have to use
the included maildirlock utility. The idea is to:

 1. Find the mails you want to compress in a single maildir.
     * Skip files that don't have ',S=<size>' in the filename.
 2. Compress the mails to 'tmp/'
     * Update the compressed files' mtimes to be the same as they were in the
       original files (e.g. touch command)
 3. Run 'maildirlock <path> <timeout>'. It writes PID to stdout, save it.
     * <path> is path to the directory containing Maildir's dovecot-uidlist
       (the control directory, if it's separate)
     * <timeout> specifies how long to wait for the lock before failing.
 4. If maildirlock grabbed the lock successfully (exit code 0) you can
    continue.
 5. For each mail you compressed:
     1. Verify that it still exists where you last saw it.
     2. If it doesn't exist, delete the compressed file. Its flags may have
        been changed or it may have been expunged. This happens rarely, so just
        let the next run handle it.
     3. If the file does exist, 'rename()' (mv) the compressed file over the
        original file.
         * Dovecot can now read the file, but to avoid compressing it again on
           the next run, you'll probably want to rename it again to include
           e.g. a "Z" flag in the file name to mark that it was compressed
           (e.g.'1223212411.M907959P17184.host,S=3271:2,SZ'). Remember that the
           Maildir specifications [http://cr.yp.to/proto/maildir.html] require
           that the flags are sorted by their ASCII value, although Dovecot
           itself doesn't care about that.
 6. Unlock the maildir by sending a TERM signal to the maildirlock process
    (killing the PID it wrote to stdout).

(This file was created from the wiki on 2017-10-10 04:42)