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Re: BBU Cache vs. spindles

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Rob Wultsch" <wultsch(at)gmail(dot)com>
Cc: "Greg Smith" <greg(at)2ndquadrant(dot)com>,<jd(at)commandprompt(dot)com>, "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com>, "Bruce Momjian" <bruce(at)momjian(dot)us>, "Steve Crawford" <scrawford(at)pinpointresearch(dot)com>, <pgsql-performance(at)postgresql(dot)org>, "Ben Chobot" <bench(at)silentmedia(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: BBU Cache vs. spindles
Date: 2010-10-22 19:05:39
Message-ID: 4CC19A330200002500036CF8@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-performancepgsql-www
Rob Wultsch <wultsch(at)gmail(dot)com> wrote:
 
> I would think full_page_writes=off + double write buffer should be
> far superior, particularly given that the WAL is shipped over the
> network to slaves.
 
For a reasonably brief description of InnoDB double write buffers, I
found this:
 
http://www.mysqlperformanceblog.com/2006/08/04/innodb-double-write/
 
One big question before even considering this would by how to
determine whether a potentially torn page "is inconsistent". 
Without a page CRC or some such mechanism, I don't see how this
technique is possible.
 
Even if it's possible, it's far from clear to me that it would be an
improvement.  The author estimates (apparently somewhat loosely)
that it's a 5% to 10% performance hit in InnoDB; I'm far from
certain that full_page_writes cost us that much.  Does anyone have
benchmark numbers handy?
 
-Kevin

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