mu/man/mu-index.1

.TH MU-INDEX 1 "June 2022" "User Manuals"

.SH NAME

mu index \- index e-mail messages stored in Maildirs

.SH SYNOPSIS

.B mu index [options]

.SH DESCRIPTION

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

Before the first time you run \fBmu index\fR, you must run \fBmu init\fR to
initialize the database.

\fBindex\fR understands Maildirs as defined by Daniel Bernstein for
\fBqmail\fR(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 (\fIcur\fR and \fInew\fR) are ignored, as are the cache
directories for \fInotmuch\fR and \fIgnus\fR, 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 \fI.noindex\fR 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 \fI.noupdate\fR in a directory, the contents of that
directory and all of its subdirectories will be ignored, unless we do a full
rebuild (with \fBmu init\fR). 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. \fI.noupdate\fR is ignored
when you start indexing with an empty database (such as directly after \fImu
init\fR.

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

The first run of \fBmu index\fR 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 \fB\-n\fR, \fB\-\-nocleanup\fR.

When \fBmu index\fR catches one of the signals \fBSIGINT\fR, \fBSIGHUP\fR or
\fBSIGTERM\fR (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), \fBmu index\fR will terminate immediately.

.SH OPTIONS

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

.TP
\fB\-\-lazy-check\fR
in lazy-check mode, \fBmu\fR 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 \fBmu-index\fR
occasionally without \fB\-\-lazy-check\fR, to pick up such messages.

.TP
\fB\-\-nocleanup\fR
disables the database cleanup that \fBmu\fR does by default after indexing.

.SS 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:

.nf
 $ 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
.fi
(about 103 messages per second)

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

.nf
 $ 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
.fi
(more than 56818 messages per second)

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

.nf
 $ time mu index --quiet
 0,33s user 0,40s system 80% cpu 0,905 total
.fi
which is more than 30000 messages per second.


.SS 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.

.nf
 $ 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
.fi
(about 813 messages per second)

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

.nf
 $ 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
.fi
(more than 173000 messages per second)


.SS 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.

.nf
 $ 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
.fi
(about 1099 messages per second).

.SS 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,

.nf
 $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
% THREAD_NUM=4 build/lib/tests/bench-indexer -m perf
# random seed: R02Sf5c50e4851ec51adaf301e0e054bd52b
1..1
# 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
.fi

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!

.SH RETURN VALUE

\fBmu index\fR return 0 upon successful completion; any other number signals an
error.

.SH BUGS

Please report bugs if you find any:
.BR https://github.com/djcb/mu/issues

.SH AUTHOR

Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>

.SH "SEE ALSO"

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