From: Greg Smith <greg@2ndquadrant.com>
To: Craig James <craig_james@emolecules.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: limiting performance impact of wal archiving.
Date: Tue, 10 Nov 2009 12:26:03 -0500
Message-ID: <4AF9A22B.3090506@2ndquadrant.com> (raw)
In-Reply-To: <4AF99DC2.9040607@emolecules.com>
References: <8a1bfe660911100355l34a557ffj8244079c1fb689b5@mail.gmail.com>
<hdbrv8$p35$1@ger.gmane.org>
<8a1bfe660911100700l2a44f638m97ea6136b65deb21@mail.gmail.com>
<hdbvqt$7v8$1@ger.gmane.org>
<8a1bfe660911100729q42ac732cga9bbadd52e2533dd@mail.gmail.com>
<4AF99666.5090705@2ndquadrant.com>
<8a1bfe660911100852l5573ab30ne91c6d8937dc836b@mail.gmail.com>
<4AF99DC2.9040607@emolecules.com>
Craig James wrote:
> On Tue, Nov 10, 2009 at 5:35 PM, Greg Smith <greg@2ndquadrant.com> wrote:
>> Given the current quality of Linux code, I hesitate to use anything
>> but ext3
>> because I consider that just barely reliable enough even as the most
>> popular
>> filesystem by far. JFS and XFS have some benefits to them, but none so
>> compelling to make up for how much less testing they get. That said,
>> there
>> seem to be a fair number of people happily running high-performance
>> PostgreSQL instances on XFS.
>
> I thought the common wisdom was to use ext2 for the WAL, since the WAL
> is a journal system, and ext3 would essentially be journaling the
> journal. Is that not true?
Using ext2 means that you're still exposed to fsck errors on boot after
a crash, which doesn't lose anything but you have to go out of your way
to verify you're not going to get stuck with your server down in that
case. The state of things on the performance side is nicely benchmarked
at
http://www.commandprompt.com/blogs/joshua_drake/2008/04/is_that_performance_i_smell_ext2_vs_ext3_on_...
Sure, it jumps from 85MB/s to 115MB/s if you use ext2, but if noatime
had been used I think even some of that fairly small gap would have
closed. My experience is that it's really hard to saturate even a
single disk worth of bandwidth with WAL writes if there's a dedicated
WAL volume. As such, I'll use ext3 until it's very clear that's the
actual bottleneck, and only then step back and ask if converting to ext2
is worth the performance boost and potential crash recovery mess. I've
never actually reached that point in a real-world situation, only in
simulated burst write tests.
--
Greg Smith greg@2ndQuadrant.com Baltimore, MD
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To: pgsql-performance@postgresql.org
Cc: greg@2ndquadrant.com, craig_james@emolecules.com
Subject: Re: limiting performance impact of wal archiving.
In-Reply-To: <4AF9A22B.3090506@2ndquadrant.com>
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