Tino writes:

> Well, we've already got MD4 checksums of file blocks. And if I
> understand everything correctly, we DO GET collisions, therefore the
> hash chains.

These collisions are because the BackupPC digest is only computed
over the first part of the file.

> Of course, this if for 256k blocks, IIRC. And "only" 128 bit hashes.
> But I don't like the idea of relying on probabilities. I've got enough
> uncertainties by flaky hardware, bugs etc.

Lessfs uses 192 bit checksums on each file block.  The chance of
a collision is vanishingly small (and vanishingly smaller than a
128 bit checksum).  I don't know if it compares the actual data
(as BackupPC does).  Perhaps that could be added as an additional
option for the paranoid.

> I won't trust such a file system for backup data.

Most commercial systems use these techniques.

Craig

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