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Re: ext4 finally doing the right thing

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Aidan Van Dyk <aidan(at)highrise(dot)ca>
Cc: Greg Stark <stark(at)mit(dot)edu>, pgsql-performance(at)postgresql(dot)org, Jeff Davis <pgsql(at)j-davis(dot)com>
Subject: Re: ext4 finally doing the right thing
Date: 2010-01-21 14:49:05
Message-ID: 4B586961.20401@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-performance
Aidan Van Dyk wrote:
> Sure, if your WAL is on the same FS as your data, you're going to get
> hit, and *especially* on ext3...
>
> But, I think that's one of the reasons people usually recommend putting
> WAL separate.

Separate disks can actually concentrate the problem.  The writes to the 
data disk by checkpoints will also have fsync behind them eventually, so 
splitting out the WAL means you just push the big write backlog to a 
later point.  So less frequently performance dives, but sometimes 
bigger.  All of the systems I was mentioning seeing >10 second pauses on 
had a RAID-1 pair of WAL disks split from the main array.

-- 
Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com  www.2ndQuadrant.com


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