On 22.12.2010 15:59, Simon Riggs wrote:
On Wed, 2010-12-22 at 15:30 +0200, Heikki Linnakangas wrote:
My gut feeling is that a reasonable compromise is to set hint bits like
we do today, but don't mark the page as dirty when only hint bits are
set. That way you get the benefit of hint bits for tuples that are
frequently accessed and stay in buffer cache. But you don't spend any
extra I/O to set them. I'd really like to see a worst-case scenario
benchmark of a patch that does that.
That sounds great, but still prevents block checksums and that is a very
valuable feature for robustness.
It does? The problem with block checksums is that if you modify a page
and don't have a corresponding WAL record for it, like a hint bit
update, you can have a torn page so that the checksum doesn't match.
Refraining from dirtying the page when a hint bit is updated avoids the
problem. With that change, we only ever write pages to disk that have a
WAL record associated with it, with full-page images as necessary to
avoid torn pages.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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