bup-margin - figure out your deduplication safety margin
bup margin
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.
$ bup margin
Reading indexes: 100.00% (11188299/11188299), done.
45
bup-midx
(1), bup-save
(1)
Part of the bup
(1) suite.