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From Evgeny Kotkov <>
Subject Re: [PATCH] Introduce per-instance filesystem UUIDs
Date Mon, 08 Sep 2014 14:19:22 GMT
Evgeny Kotkov <> writes:

> Ben Reser <> writes:
>>> I think part of the problem here has been we (as in WANdisco folks) have
>>> discussed the idea of an instance ID for repositories in the past to solve
>>> the range of replacing the repository without clearing the cache issues.
>>> But this change is being added for a very different reason.
>>> Evgeny has implemented the instance ID for the purpose of solving the
>>> problem of two different repositories not being able to be locked if they
>>> happen to have the same UUID.  This happens because we use a mutex to handle
>>> locking between threads and that mutex can't distinguish between different
>>> repositories with identical UUIDs.
>>> Currently the code on trunk adds the instance ID to the cache keys.  I'm not
>>> sure we should be doing that (though both brane and stefan2 requested that
>>> be done).  As per the discussion today at the SHF hackathon the instance ID
>>> can't resolve the failure to clear the cache issues.  The best it can do is
>>> narrow the window for these issues to exist.  That would seem like a good
>>> thing but I think it creates a huge false sense of security.  We will
>>> ultimately have someone that comes along with a corrupted repository, we're
>>> going to say you replaced the repo while the server was running and the
>>> user is going to say "But I've been doing this for years without any
>>> problem."
>>> Without the instance ID in the cache keys users are unlikely to actually
>>> corrupt their repository (just like they would be with them, it's a pretty
>>> hard race to hit).  But they are likely to get errors related to the cache
>>> being stale.  This gives them a giant hint that what they're doing is wrong
>>> and gives us an opportunity to educate them.
>> Evgeny do you have any thoughts on this?  I think it's probably best to
>> remove the instance id from the cache keys.  I believe brane and stefan2
>> were convinced of this at the hackathon.
> Sounds good to me.
> However, prior to (re-)narrowing the scope of this change, I would love to
> spend a bit of time and actually reproduce the repository corruption race
> (with or without instance IDs) — just in order to obtain a bit of information,
> which I currently lack.  I plan to undo the corresponding part of the
> changeset right after that.

Done in r1623402.

Evgeny Kotkov

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