mu-index - Man Page

index e-mail messages stored in Maildirs


mu index [options]


mu index is the mu command for scanning the contents of Maildir directories and storing the results in a Xapian database. The data can then be queried using mu-find(1).

Before the first time you run mu index, you must run mu init to initialize the database.

index understands Maildirs as defined by Daniel Bernstein for qmail(7). In addition, it understands recursive Maildirs (Maildirs within Maildirs), Maildir++. It can also deal with VFAT-based Maildirs which use '!' or ';' as the separators instead of ':'.

E-mail messages which are not stored in something resembling a maildir leaf-directory (cur and new) are ignored, as are the cache directories for notmuch and gnus, and any dot-directory.

Starting with mu 1.5.x, symlinks are followed, and can be spread over multiple filesystems; however note that moving files around is much faster when multiple filesystems are not involved.

If there is a file called .noindex in a directory, the contents of that directory and all of its subdirectories will be ignored. This can be useful to exclude certain directories from the indexing process, for example directories with spam-messages.

If there is a file called .noupdate in a directory, the contents of that directory and all of its subdirectories will be ignored, unless we do a full rebuild (with mu init). This can be useful to speed up things you have some maildirs that never change. Note that you can still search for these messages, this only affects updating the database. .noupdate is ignored when you start indexing with an empty database (such as directly after mu init.

There also the --lazy-check which can greatly speed up indexing; see below for details.

The first run of mu index may take a few minutes if you have a lot of mail (tens of thousands of messages). Fortunately, such a full scan needs to be done only once; after that it suffices to index the changes, which goes much faster. See the 'Note on performance (i,ii,iii)' below for more information.

The optional 'phase two' of the indexing-process is the removal of messages from the database for which there is no longer a corresponding file in the Maildir. If you do not want this, you can use -n, --nocleanup.

When mu index catches one of the signals SIGINT, SIGHUP or SIGTERM (e.g., when you press Ctrl-C during the indexing process), it tries to shutdown gracefully; it tries to save and commit data, and close the database etc. If it receives another signal (e.g., when pressing Ctrl-C once more), mu index will terminate immediately.


Some of the general options are described in the mu(1) man-page and not here, as they apply to multiple mu commands.


in lazy-check mode, mu does not consider messages for which the time-stamp (ctime) of the directory they reside in has not changed since the previous indexing run. This is much faster than the non-lazy check, but won't update messages that have change (rather than having been added or removed), since merely editing a message does not update the directory time-stamp. Of course, you can run mu-index occasionally without --lazy-check, to pick up such messages.


disables the database cleanup that mu does by default after indexing.

A note on performance (i)

As a non-scientific benchmark, a simple test on the author's machine (a Thinkpad X61s laptop using Linux 2.6.35 and an ext3 file system) with no existing database, and a maildir with 27273 messages:

 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
 $ time mu index --quiet
 66,65s user 6,05s system 27% cpu 4:24,20 total

(about 103 messages per second)

A second run, which is the more typical use case when there is a database already, goes much faster:

 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
 $ time mu index --quiet
 0,48s user 0,76s system 10% cpu 11,796 total

(more than 56818 messages per second)

Note that each test flushes the caches first; a more common use case might be to run mu index when new mail has arrived; the cache may stay quite 'warm' in that case:

 $ time mu index --quiet
 0,33s user 0,40s system 80% cpu 0,905 total

which is more than 30000 messages per second.

A note on performance (ii)

As per June 2012, we did the same non-scientific benchmark, this time with an Intel i5-2500 CPU @ 3.30GHz, an ext4 file system and a maildir with 22589 messages. We start without an existing database.

 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
 $ time mu index --quiet
 27,79s user 2,17s system 48% cpu 1:01,47 total

(about 813 messages per second)

A second run, which is the more typical use case when there is a database already, goes much faster:

 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
 $ time mu index --quiet
 0,13s user 0,30s system 19% cpu 2,162 total

(more than 173000 messages per second)

A note on performance (iii)

As per July 2016, we did the same non-scientific benchmark, again with the Intel i5-2500 CPU @ 3.30GHz, an ext4 file system. This time, the maildir contains 72525 messages.

 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
 $ time mu index --quiet
 40,34s user 2,56s system 64% cpu 1:06,17 total

(about 1099 messages per second).

A note on performance (iv)

A few years later and its June 2022. There's a lot more happening during indexing, but indexing became multi-threaded and machines are faster; e.g. this is with an AMD Ryzen Threadripper 1950X (32) @ 3.399GHz.

The instructions are a little different since we have a proper repeatable benchmark now. After building,

 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
% THREAD_NUM=4 build/lib/tests/bench-indexer -m perf
# random seed: R02Sf5c50e4851ec51adaf301e0e054bd52b
# Start of bench tests
# Start of indexer tests
indexed 5000 messages in 20 maildirs in 3763ms; 752 μs/message; 1328 messages/s (4 thread(s))
ok 1 /bench/indexer/4-cores
# End of indexer tests
# End of bench tests

Things are again a little faster, even though the index does a lot more now (text-normalizatian, and pre-generating message-sexps). A faster machine helps, too!

Return Value

mu index return 0 upon successful completion; any other number signals an error.


Please report bugs if you find any:


Dirk-Jan C. Binnema <>

See Also

maildir(5), mu(1), mu-init(1), mu-find(1), mu-cfind(1)

Referenced By

mu(1), mu-add(1), mu-cfind(1), mu-easy(1), mu-find(1), mu-help(1), mu-info(1), mu-init(1), mu-remove(1).

June 2022 User Manuals