0.33.3
bup-margin - figure out your deduplication safety margin
bup margin [options…]
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.
.midx
files, use only .idx
files.
This is only really useful when used with --predict
.
$ 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%)
bup-midx
(1), bup-save
(1)
Part of the bup
(1) suite.