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Re: Hung Vacuum in 8.3

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-bugs(at)postgresql(dot)org
Subject: Re: Hung Vacuum in 8.3
Date: 2011-02-22 14:27:53
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugs
On Tue, Feb 22, 2011 at 6:26 AM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> Actually it's not waiting for the LockBuffer LWLock. it's waiting
> until your query unpins the buffer it wants. Vacuum tries to get an
> exclusive lock on the buffer, if it gets it then it checks if anyone
> is using that buffer. If someone is then it unlocks the buffer and
> waits until nobody has it pinned.

How bad it would be if we made LockBufferForCleanup() not wait?  If we
can't obtain the buffer cleanup lock immediately, we just skip that
page and continue on.  That would prevent us from updating
relfrozenxid, I guess, but we already can't do that if there are any
bits set in the visibility map.   It could also leave some bloat in
the table, but probably not much (he says hopefully).

Robert Haas
The Enterprise PostgreSQL Company

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