Re: [Monotone-devel] Hash collisions resiliency

[J C Lawrence]

On Wed, 13 Apr 2005 15:51:46 +0200
Jon Bright <address@hidden> wrote:
> address@hidden wrote:

<Re-ordering the quotes slightly>

>> Please understand that I do realise the improbability of a collision.

> If you do, then you hopefully also realise that there's not very much
> point in considering the answers to these questions...

No, I don't understand that there's not much point.  Just because
something is improbable does not mean it will not happen.  I rather like
the uses of hashes in Monotone, but I'm concerned that in the case of a
hash collision a repository may be silently borked, and may remain
borked for some time without big two meter high red neon signs lighting
up in front of all the repository users.  That worries me from a risk
management perspective.  A very low risk of fatality is often worse than
a moderate risk of severe injury.  The second can be managed.  The first
can't.

Very large chunks of the historical source bases for HP-UX and IRIX are
no longer recoverable due to silent NFS corruption of RCS ,v files.  It
wasn't detected due to RCS' use of reverse diffs.  (Queue SCCS forward
diff evangelism) I'd like an SCM system which tells me in unequivocal
terms when something critical goes really bad, even if it is vanishingly
improbable.

>> Should there be a hash collision:

...

>> -- If I did care, how would I know that there had been a collision?

> Things would start to break.  Incorrect files or revisions might be
> checked out, files or revisions might not be checked in or
> synchronised. Depends where the collision is.

To be specific: I or someone on the team would have to notice these
facts through simple manual observation of some sort of unexpected
behaviour, or would there be the equivalent of Bubba the Neanderthal
whacking me upside the head with a clue-by-four and yelling, "HEY BOZO,
YOU HAVE A HASH COLLISION!"?

--
J C Lawrence
---------(*)                Satan, oscillate my metallic sonatas.
address@hidden               He lived as a devil, eh?
http://www.kanga.nu/~claw/  Evil is a name of a foeman, as I live.