NAME

       bup-margin - figure out your deduplication safety margin

SYNOPSIS

       bup margin [options...]

DESCRIPTION

       bup margin  iterates  through  all objects in your bup repository, calculating the largest
       number of prefix bits shared between any two entries.   This  number,  n,  identifies  the
       longest  subset of SHA-1 you could use and still encounter a collision between your object
       ids.

       For example, one system that was tested had a collection of 11 million  objects  (70  GB),
       and  bup margin  returned  45.   That means a 46-bit hash would be sufficient to avoid all
       collisions among that set of objects; each object in that  repository  could  be  uniquely
       identified by its first 46 bits.

       The  number  of  bits  needed  seems to increase by about 1 or 2 for every doubling of the
       number of objects.  Since SHA-1 hashes have 160 bits, that leaves 115 bits of margin.   Of
       course,  because  SHA-1  hashes are essentially random, it's theoretically possible to use
       many more bits with far fewer objects.

       If you're paranoid about the  possibility  of  SHA-1  collisions,  you  can  monitor  your
       repository  by  running bup margin occasionally to see if you're getting dangerously close
       to 160 bits.

OPTIONS

       --predict
              Guess the offset into each index file where a particular object  will  appear,  and
              report  the  maximum  deviation  of  the  correct  answer  from the guess.  This is
              potentially useful for tuning an interpolation search algorithm.

       --ignore-midx
              don't use .midx files, use only .idx files.  This is only really useful  when  used
              with --predict.

EXAMPLE

              $ bup margin
              Reading indexes: 100.00% (1612581/1612581), done.
              40
              40 matching prefix bits
              1.94 bits per doubling
              120 bits (61.86 doublings) remaining
              4.19338e+18 times larger is possible

              Everyone on earth could have 625878182 data sets
              like yours, all in one repository, and we would
              expect 1 object collision.

              $ bup margin --predict
              PackIdxList: using 1 index.
              Reading indexes: 100.00% (1612581/1612581), done.
              915 of 1612581 (0.057%)

SEE ALSO

       bup-midx(1), bup-save(1)

BUP

       Part of the bup(1) suite.

AUTHORS

       Avery Pennarun <apenwarr@gmail.com>.