Re: BBU Cache vs. spindles

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, jd(at)commandprompt(dot)com, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>, Steve Crawford <scrawford(at)pinpointresearch(dot)com>, pgsql-performance(at)postgresql(dot)org, Ben Chobot <bench(at)silentmedia(dot)com>
Subject: Re: BBU Cache vs. spindles
Date: 2010-12-01 03:54:59
Message-ID: 201012010354.oB13sxQ08756@momjian.us
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Greg Smith wrote:
> Tom Lane wrote:
> > You've got entirely too simplistic a view of what the "delta" might be,
> > I fear. In particular there are various sorts of changes that involve
> > inserting the data carried in the WAL record and shifting pre-existing
> > data around to make room, or removing an item and moving remaining data
> > around. If you try to replay that type of action against a torn page,
> > you'll get corrupted results.
> >
>
> I wasn't sure exactly how those were encoded, thanks for the
> clarification. Given that, it seems to me there are only two situations
> where full_page_writes is safe to turn off:
>
> 1) The operating system block size is exactly the same database block
> size, and all writes are guaranteed to be atomic to that block size.

Is that true? I have no idea. I thought everything was done at the
512-byte block level.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +

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