Date: Mon, 06 Jun 2005 13:27:40 -0400
From: Michael Stone <mstone+postgres@mathom.us>
Subject: Re: Performance nightmare with dspam (urgent) (resolved)
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To: Postgresql Performance <pgsql-performance@postgresql.org>
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On Mon, Jun 06, 2005 at 10:52:09AM -0500, John A Meinel wrote:
>pg_xlog benefits from being super fast. Because it has to be fully
>synced before the rest of the data can be committed. Yes they are small,
>but if you can make it fast, you eliminate that overhead. It also
>benefits from having it's own spindle, because you eliminate the seek
>time. (Since it is always appending)

Eliminating the seeks is definately a win. 

>Anyway, my point is that pg_xlog isn't necessarily tiny. Many people
>seem to set it as high as 100-200, and each one is 16MB.

It's not the size of the xlog, it's the size of the write. Unless you're
writing out a stripe size of data at once you're only effectively
writing to one disk pair at a time anyway. (Things change if you have a
big NVRAM cache to aggregate the writes, but you'd need a *lot* of
transaction activity to exceed the 50MB/s or so you could get from the
single raid1 pair in that scenario.)

Mike Stone