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Re: Write barriers and hardware RAID

To: J Pälve <xtc@xxxxxxx>, xfs@xxxxxxxxxxx
Subject: Re: Write barriers and hardware RAID
From: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Date: Tue, 21 Jul 2009 10:12:26 +0200
In-reply-to: <alpine.LRH.1.10.0907210918150.14559@xxxxxxx>
Organization: it-management http://it-management.at
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On Dienstag 21 Juli 2009 J Pälve wrote:
> Thank you for your clarification. I'm still wondering about this
> mention in the FAQ:
> "With a RAID controller with battery backed controller cache and
> cache in write back mode, you should turn off barriers - they are
> unnecessary in this case, and if the controller honors the cache
> flushes, it will be harmful to performance."
> If you issue a cache flush request to one of these controllers that
> honor them, will it only flush its own cache or also issue a cache
> flush request to the disks? In the latter case, wouldn't write
> barriers work correctly even with both controller and disk cache
> enabled?

If you use a RAID controller but then it allows cache flushes from the 
host to really flush it's cache, it's basically the same as setting the 
whole controller to "write through" mode (where he doesn't buffer 
anything). Your expensive RAID controller then acts like a dump switch 
and your only advantage is that you can connect several disks with 
parity. But your performance will be a mess. I can't believe anybody 
wanting performance will do that.
( We have some "up to 4 people" companies with HP servers where the 3-
disk RAID-5 is setup this way. As long as they only store word documents 
and such it's enough. We setup the whole controller write through then, 
battery backup is of course not needed. But disk write cache is always 
set to off, to prevent data loss on power fail.)

Back to your question: If there exists a hardware RAID controller 
honoring flushes, it should also flush the disks (I'd expect that). But 
I don't know any controller to really do that. If you do, please tell 

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