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       bup-midx - create a multi-index (.midx) file from several .idx files


       bup midx [-o outfile] <-a|-f|idxnames...>


       bup midx  creates  a multi-index (.midx) file from one or more git pack
       index (.idx) files.

       You should run this command occasionally to  ensure  your  backups  run
       quickly and without requiring too much RAM.


              use  the  given  output filename for the .midx file.  Default is

              automatically generate new .midx files for any .idx files  where
              it would be appropriate.

              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.


             $ bup midx -a
             Merging 21 indexes (2278559 objects).
             Table size: 524288 (17 bits)
             Reading indexes: 100.00% (2278559/2278559), done.


       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

       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.)

       With large repositories, you should  be  sure  to  run  bup midx -a  or
       bup midx -f  every  now  and  then so that creating backups will remain


       bup-save(1), bup-margin(1), bup-memtest(1)


       Part of the bup(1) suite.


       Avery Pennarun <>.