xenial (1) bup-margin.1.gz

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.

EXAMPLES

              $ 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>.