Date: Thu, 11 Feb 2010 13:04:21 -0500
From: Aidan Van Dyk <aidan@highrise.ca>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	Jesper Krogh <jesper@krogh.cc>, pgsql-performance@postgresql.org
Subject: Re: moving pg_xlog -- yeah, it's worth it!
Message-ID: <20100211180421.GE14128@oak.highrise.ca>
References: <4B71358E020000250002F0E4@gw.wicourts.gov>
	<4B718BDB.8080807@krogh.cc>
	<4B7139DD020000250002F0E9@gw.wicourts.gov>
	<4B73EE95020000250002F1CC@gw.wicourts.gov>
	<20100211175752.GB3145@alvh.no-ip.org>
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* Alvaro Herrera <alvherre@commandprompt.com> [100211 12:58]:
> 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!

;-)


-- 
Aidan Van Dyk                                             Create like a god,
aidan@highrise.ca                                       command like a king,
http://www.highrise.ca/                                   work like a slave.

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