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da...@lang.hm  
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 More options Apr 1 2009, 4:38 pm
Newsgroups: pgsql.performance
From: da...@lang.hm
Date: Wed, 1 Apr 2009 13:38:34 -0700 (PDT)
Local: Wed, Apr 1 2009 4:38 pm
Subject: Re: [PERFORM] Raid 10 chunksize
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On Wed, 1 Apr 2009, Mark Kirkwood wrote:
> Scott Carey wrote:

>> A little extra info here >>  md, LVM, and some other tools do not allow the
>> file system to use write barriers properly.... So those are on the bad list
>> for data integrity with SAS or SATA write caches without battery back-up.
>> However, this is NOT an issue on the postgres data partition.  Data fsync
>> still works fine, its the file system journal that might have out-of-order
>> writes.  For xlogs, write barriers are not important, only fsync() not
>> lying.

>> As an additional note, ext4 uses checksums per block in the journal, so it
>> is resistant to out of order writes causing trouble.  The test compared to
>> here was on ext4, and most likely the speed increase is partly due to that.

> [Looks at  Stef's  config - 2x 7200 rpm SATA RAID 0]  I'm still highly
> suspicious of such a system being capable of outperforming one with the same
> number of (effective) - much faster - disks *plus* a dedicated WAL disk
> pair... unless it is being a little loose about fsync! I'm happy to believe
> ext4 is better than ext3 - but not that much!

given how _horrible_ ext3 is with fsync, I can belive it more easily with
fsync turned on than with it off.

David Lang

> However, its great to have so many different results to compare against!

> Cheers

> Mark

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