NAME

SYNOPSIS

DESCRIPTION

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

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.)

SEE ALSO

BUP

AUTHORS