From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
Cc: | pgsql-hackers(at)postgreSQL(dot)org |
Subject: | Re: [COMMITTERS] pgsql: Avoid marking buffer dirty when VACUUM has no work to do. |
Date: | 2011-11-22 02:32:44 |
Message-ID: | 29804.1321929164@sss.pgh.pa.us |
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Lists: | pgsql-committers pgsql-hackers |
I wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> Avoid marking buffer dirty when VACUUM has no work to do.
>> When wal_level = 'hot_standby' we touched the last page of the
>> relation during a VACUUM, even if nothing else had happened.
>> That would alter the LSN of the last block and set the mtime
>> of the relation file unnecessarily. Noted by Thom Brown.
> This doesn't look right to me --- you have not accounted for the
> possibility that btpo_cycleid or BTP_HAS_GARBAGE is changed.
> Also, I'm confused about the business of not setting the LSN. Thom
> claimed that he was seeing the page not change at all (or at least
> md5sum of the file didn't change) despite mtime changing. If we'd
> been plastering a new LSN on the page each time, then that should
> certainly not have been possible. So I now think maybe we've
> mis-analyzed what was happening in his example.
> I think this requires more careful analysis.
Ping? If you don't respond, I'm going to take it on my own authority to
revert this patch, because it's definitely broken as-is, and I don't
think the consequences of not updating the page LSN have been thought
through either.
regards, tom lane
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