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Re: Block-level CRC checks

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Richard Huxton <dev(at)archonet(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Bruce Momjian <bruce(at)momjian(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2009-12-01 23:11:36
Message-ID: 24541.1259709096@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Richard Huxton <dev(at)archonet(dot)com> writes:
> So what is the cost of calculating the hint-bits for a whole block of
> tuples in one go vs reading that block from actual spinning disk?

Potentially a couple of hundred times worse, if you're unlucky and each
XID on the page requires visiting a different block of clog that's also
not in memory.  The average case probably isn't that bad, but I think
we'd likely be talking at least a factor of two penalty --- you'd be
hopelessly optimistic to assume you didn't need at least one clog visit
per page.

Also, if you want to assume that you're lucky and the XIDs mostly fall
within a fairly recent range of clog pages, you're still not out of the
woods.  In that situation what you are talking about is a spectacular
increase in the hit rate for cached clog pages --- which are already a
known contention bottleneck in many scenarios.

> While on that topic - I'm assuming freezing requires substantially more
> effort than updating hint bits?

It's a WAL-logged page change, so at minimum double the cost.

			regards, tom lane

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