
NAMEbupmargin  figure out your deduplication safety marginSYNOPSISbup margin [options...]DESCRIPTIONbup 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 SHA1 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 46bit 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 SHA1 hashes have 160 bits, that leaves 115 bits of margin. Of course, because SHA1 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 SHA1 collisions, you can monitor your repository by running bup margin occasionally to see if you’re getting dangerously close to 160 bits. OPTIONS
EXAMPLES
SEE ALSObupmidx(1), bupsave(1)BUPPart of the bup(1) suite.AUTHORSAvery Pennarun <apenwarr@gmail.com>.
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