From: | Rob Wultsch <wultsch(at)gmail(dot)com> |
---|---|
To: | Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov> |
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 20:06:11 |
Message-ID: | AANLkTikh+5JUhWZvCg8=X-zNQDd=mh4xCBm=cAca5Wc3@mail.gmail.com |
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On Fri, Oct 22, 2010 at 12:05 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> 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
>
Ignoring (briefly) the cost in terms of performance of the different
system, not needing full_page_writes would make geographically
dispersed replication possible for certain cases where it is not
currently (or at least rather painful).
--
Rob Wultsch
wultsch(at)gmail(dot)com
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