bup-midx - Online in the Cloud

PROGRAM:

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 and $BUP_DIR/indexcache/*.

--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 available, 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 contained .idx files
exist inside the .midx. May be useful for debugging.

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 contained 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 searching through all the index files to
ensure that it does not exist. (Searching for objects that do exist can be optimized; for
example, consecutive objects are often stored in the same pack, so we can search that one
first using an MRU algorithm.)

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