From: | Greg Smith <greg(at)2ndQuadrant(dot)com> |
---|---|
To: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: 16-bit page checksums for 9.2 |
Date: | 2012-01-10 05:04:40 |
Message-ID: | 4F0BC6E8.3070905@2ndQuadrant.com |
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Lists: | pgsql-hackers |
On 12/30/11 9:44 AM, Aidan Van Dyk wrote:
> So moving to this new double-write-area bandwagon, we move from a "WAL
> FPW synced at the commit, collect as many other writes, then final
> sync" type system to a system where *EVERY* write requires syncs of 2
> separate 8K writes at buffer write-out time.
It's not quite that bad. The double-write area is going to be a small
chunk of re-used sequential I/O, like the current WAL. And if this
approach shifts some of the full-page writes out of the WAL and toward
the new area instead, that's not a real doubling either. Could probably
put both on the same disk, and in situations where you don't have a
battery-backed write cache it's possible to get a write to both per
rotation.
This idea has been tested pretty extensively as part of MySQL's Innodb
engine. Results there suggest the overhead is in the 5% to 30% range;
some examples mentioning both extremes of that:
http://www.mysqlperformanceblog.com/2006/08/04/innodb-double-write/
http://www.bigdbahead.com/?p=392
Makes me wish I knew off the top of my head how expensive WAL logging
hint bits would be, for comparison sake.
--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
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