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Re: Subversion's use of Berkeley DB [#11511]

From: Philip Martin <philip_at_codematters.co.uk>
Date: 2004-12-09 23:40:22 CET

Branko Œibej <brane@xbc.nu> writes:

> C. Michael Pilato wrote:
>
>>Keith Bostic <bostic@abyssinian.sleepycat.com> writes:
>>
>>> As a springboard for that discussion, I propose we find a
>>> serialization point for all threads of control using a
>>> Subversion repository so we can determine if a thread of
>>> control is the first thread of control entering the database
>>> environment after a possible application or system failure.
>>
>>My suggestion is that libsvn_fs_base grows the same serialization that
>>mod_db4 uses, which is based around the use of a shared memory segment
>>with a reference count in it.
>>
> There is a pretty fundamental problem with using a reference count
> like that. If a process that is accessing BDB crashes after having
> incremented the refcount, the refcount is gets out if sync and is
> useless. Also, other processes that are already running might wedge on
> a lock owned by the crashed process. I see no way to resolve this.

We have the libsvn_repos lock, which is the current mechanism to
ensures that recovery gets exclusive access. Normal read/write
repository access, such as svn_repos_open(), takes a non-exclusive
lock, while "special" access, such as svn_repos_recover(), takes an
exclusive lock.

Suppose svn_repos_open() were first to make a non-blocking attempt to
take an exclusive lock, if that fails it just carries on as at present
and tries to take a non-exclusive lock. If however svn_repos_open()
manages to take an exclusive lock then it can do whatever it wants
(perhaps run recovery?) and then it can drop the exclusive lock, take
a non-exclusive one and continue as at present.

This would mean that recovery would get run whenever anything accessed
a repository that was not otherwise being accessed, is that the
desired behaviour? Should be simple to implement, but I don't know
what sort of performance effect it would have. Do we really want to
run recovery that often?

-- 
Philip Martin
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Received on Fri Dec 10 03:25:29 2004

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