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Re: 16-bit page checksums for 9.2

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: (view raw or whole thread)
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 

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:

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

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