From: | Heikki Linnakangas <heikki(at)enterprisedb(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Greg Smith <gsmith(at)gregsmith(dot)com>, Patches <pgsql-patches(at)postgresql(dot)org> |
Subject: | Re: Load Distributed Checkpoints, take 3 |
Date: | 2007-06-26 14:42:57 |
Message-ID: | 468125F1.4040908@enterprisedb.com |
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Lists: | pgsql-patches |
Tom Lane wrote:
> Anyway, if there are no XLOG records since the last checkpoint, there's
> probably nothing in shared buffers that needs flushing. There might be
> some dirty hint-bits, but the only reason to push those out is to make
> some free buffers available, and doing that is not checkpoint's job (nor
> the all-buffers scan's job); that's what the LRU scan is for.
Yeah, except the LRU scan is not doing a very good job at that. It will
ignore buffers with usage_count > 0, and it only scans
bgwriter_lru_percent buffers ahead of the clock hand.
One pathological case is a COPY of a table slightly smaller than
shared_buffers. That will fill the buffer cache. If you then have a
checkpoint, and after that a SELECT COUNT(*), or a VACUUM, the buffer
cache will be full of pages with just hint-bit-updates, but no WAL
activity since last checkpoint.
But let's fix the LRU scan, rather work around it's deficiencies.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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