Re: [9fans] venti

[Sape Mullender <sape@plan9.bell-labs.com>]

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Yes, but even if one venti always leaves the store in a consistent
state, running two may still cause them to wite differemt things to
the same place on disk and this is likely to result in havoc.

	Sape

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From: Richard Miller <9fans@hamnavoe.com>
To: 9fans@9fans.net
Subject: Re: [9fans] venti
Date: Fri, 9 Jan 2009 12:18:35 +0000
Message-ID: <b146f7b71d1aa58ff06d8d63c3b331b4@hamnavoe.com>

> Hmm.  Running two ventis on the same data is, of course, bad.  It's also
> something I haven't seen happen before.

I think it may have happened to me.  Recently I had a "missing score" error
when I was looking for something in my dump fs.  To assess the damage I did
a 'venti/copy -r' to a spare server, which told me 13 pointers had to be
rewritten.

I do sometimes get errors like this from my fossil server:
	could not write super block; waiting 10 seconds
	blistAlloc: called on clean block
but I've been assuming they are benign.  Am I wrong?

My understanding of fossil is that venti writes are ordered such that
the store should not be left in an inconsistent state even after a
sudden shutdown.  If that's the case, I'm guessing the most likely
cause of corruption was a careless experiment with venti when there
was already one running.