Message-ID: <4C08030A.20801@2ndquadrant.com>
Date: Thu, 03 Jun 2010 15:31:22 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Scott Marlowe <scott.marlowe@gmail.com>
CC: Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	Craig James <craig_james@emolecules.com>,
	Matthew Wakeling <matthew@flymine.org>, pgsql-performance@postgresql.org
Subject: Re: Weird XFS WAL problem
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Scott Marlowe wrote:
> I think it's a case of the quickest, simplest answer to semi-new tech.
>  Not sure what to do with barriers?  Just flush the whole cache.
>   

Well, that really is the only useful thing you can do with regular SATA 
drives; the ATA command set isn't any finer grained than that in a way 
that's useful for this context.  And it's also quite reasonable for a 
RAID controller to respond to that "flush the whole cache" call by 
flushing its cache.  So it's not just the simplest first answer, I 
believe it's the only answer until a better ATA command set becomes 
available.

I think this can only be resolved usefully for all of us at the RAID 
firmware level.  If the controller had some logic that said "it's OK to 
not flush the cache when that call comes in if my battery is working 
fine", that would make this whole problem go away.  I don't expect it's 
possible to work around the exact set of concerns Kevin listed any other 
way, because as he pointed out the right thing to do is very dependent 
on the battery health, which the OS also doesn't know (again, would 
require some new command set verbage).

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us