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Re: moving pg_xlog -- yeah, it's worth it!

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, "Aidan Van Dyk" <aidan(at)highrise(dot)ca>
Cc: "Jesper Krogh" <jesper(at)krogh(dot)cc>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: moving pg_xlog -- yeah, it's worth it!
Date: 2010-02-11 18:19:07
Message-ID: 4B73F5BB020000250002F1D5@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-performance
Aidan Van Dyk <aidan(at)highrise(dot)ca> wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:
>> Hmm, so maybe the performance benefit is not from it being on a
>> separate array, but from it being RAID1 instead of RAID5?
> 
> Or the cumulative effects of:
> 1) Dedicated spindles/Raid1
> 2) More BBU cache available (I can't imagine the OS pair writing
>    much)
> 3) not being queued behind data writes before getting to
>    controller
> 3) Not waiting for BBU cache to be available (which is shared with
>    all data writes) which requires RAID5 writes to complete...
> 
> Really, there's *lots* of variables here.  The basics being that
> WAL on the same FS as data, on a RAID5, even with BBU is worse
> than WAL on a dedicated set of RAID1 spindles with it's own BBU.
> 
> Wow!
 
Sure, OK, but what surprised me was that a set of 15 read-only
queries (with pretty random reads) took almost twice as long when
the WAL files were on the same file system.  That's with OS writes
being only about 10% of reads, and *that's* with 128 GB of RAM which
keeps a lot of the reads from having to go to the disk.  I would not
have expected that a read-mostly environment like this would be that
sensitive to the WAL file placement.  (OK, I *did* request the
separate file system for them anyway, but I thought it was going to
be a marginal benefit, not something this big.)
 
-Kevin

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