Date: Wed, 01 Apr 2009 20:57:57 +1300
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: Raid 10 chunksize
In-reply-to: <C5F14970.3CA1%scott@richrelevance.com>
To: Scott Carey <scott@richrelevance.com>
Cc: Stef Telford <stef@ummon.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Message-id: <49D31E85.8050405@paradise.net.nz>
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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!

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

Cheers

Mark