bup-midx(1) bup-midx(1)
NAME
bup-midx - create a multi-index (.midx) file from several .idx files
SYNOPSIS
bup midx [-o outfile] <-a|-f|idxnames...>
DESCRIPTION
bup midx creates a multi-index (.midx) file from one or more git pack
index (.idx) files.
Note: you should no longer need to run this command by hand. It gets
run automatically by bup-save(1) and similar commands.
OPTIONS
-o, --output=filename.midx
use the given output filename for the .midx file. Default is
auto-generated.
-a, --auto
automatically generate new .midx files for any .idx files where
it would be appropriate.
-f, --force
force generation of a single new .midx file containing all your
.idx files, even if other .midx files already exist. This will
result in the fastest backup performance, but may take a long
time to run.
--dir=packdir
specify the directory containing the .idx/.midx files to work
with. The default is $BUP_DIR/objects/pack.
--max-files
maximum number of .idx files to open at a time. You can use this
if you have an especially small number of file descriptors avail-
able, so that midx can complete (though possibly non-optimally)
even if it can’t open all your .idx files at once. The default
value of this option should be fine for most people.
--check
validate a .midx file by ensuring that all objects in its con-
tained .idx files exist inside the .midx. May be useful for de-
bugging.
EXAMPLES
$ bup midx -a
Merging 21 indexes (2278559 objects).
Table size: 524288 (17 bits)
Reading indexes: 100.00% (2278559/2278559), done.
midx-b66d7c9afc4396187218f2936a87b865cf342672.midx
DISCUSSION
By default, bup uses git-formatted pack files, which consist of a pack
file (containing objects) and an idx file (containing a sorted list of
object names and their offsets in the .pack file).
Normal idx files are convenient because it means you can use git(1) to
access your backup datasets. However, idx files can get slow when you
have a lot of very large packs (which git typically doesn’t have, but
bup often does).
bup .midx files consist of a single sorted list of all the objects con-
tained in all the .pack files it references. This list can be binary
searched in about log2(m) steps, where m is the total number of objects.
To further speed up the search, midx files also have a variable-sized
fanout table that reduces the first n steps of the binary search. With
the help of this fanout table, bup can narrow down which page of the
midx file a given object id would be in (if it exists) with a single
lookup. Thus, typical searches will only need to swap in two pages: one
for the fanout table, and one for the object id.
midx files are most useful when creating new backups, since searching
for a nonexistent object in the repository necessarily requires search-
ing through all the index files to ensure that it does not exist.
(Searching for objects that do exist can be optimized; for example, con-
secutive objects are often stored in the same pack, so we can search
that one first using an MRU algorithm.)
SEE ALSO
bup-save(1), bup-margin(1), bup-memtest(1)
BUP
Part of the bup(1) suite.
AUTHORS
Avery Pennarun ]8;;mailto:apenwarr@gmail.com\apenwarr@gmail.com]8;;\
Bup 0.33.7 2025-01-11 bup-midx(1)
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