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 g...@2ndquadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
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