plugin {
  obox_index_fs = compress:maybe-<algorithm>:<level>:<...>

All of the object storage backends should be set up to compress index bundle objects. This commonly shrinks the indexes down to 20-30% of the original size with gzip -6 compression. It’s possible to use also other compression algorithms.

See the fs-compress plugin fs-compress and zlib plugin plugin documentation for supported algorithms and the meaning of the level parameter.


Compression auto-detection for index bundles became available (via maybe-<algorithm>) as of v2.2.34+.

Email object (a/k/a message blob data) compression has generally been done with the zlib plugin instead of via the compress fs wrapper.


# NOTE: Using this has some trade-offs with obox installations, see below.
mail_plugins = $mail_plugins zlib
plugin {
  zlib_save = <algorithm>
  zlib_save_level = <level>

However, the problem with this with obox is that the mail files are written compressed to fscache. On one hand this increases the fscache’s file, but on the other hand it requres Dovecot to always uncompress the files before accessing them.

This decompression uses a temporary file that is written to mail_temp_dir. By using the compress fs wrapper after fscache in obox_fs line the mails are stored uncompressed in fscache, and reading the mails from there doesn’t require writing to mail_temp_dir.

Compression status of email object data is auto-detected. Therefore, zlib_save may safely be added to a currently existing system; existing non-compressed mail objects will be identified correctly.