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Re: RAID6 r-m-w, op-journaled fs, SSDs

To: xfs@xxxxxxxxxxx
Subject: Re: RAID6 r-m-w, op-journaled fs, SSDs
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Sat, 30 Apr 2011 14:54:02 -0500
In-reply-to: <20110430180213.6dcfc41c@xxxxxxxxxxxxxxx>
References: <19900.10868.583555.849181@xxxxxxxxxxxxxxxxxx> <20110430180213.6dcfc41c@xxxxxxxxxxxxxxx>
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.17) Gecko/20110414 Thunderbird/3.1.10
On 4/30/2011 11:02 AM, Emmanuel Florac wrote:
Le Sat, 30 Apr 2011 16:27:48 +0100 vous écriviez:

How bad can this be when the journal is say internal for a
filesystem that is held on wide-stride RAID6 set?

Not that bad because typically the journal is small enough to fit
entirely in the controller cache.

I suspect very
very bad, with apocalyptic read-modify-write storms, eating IOPS.

Not if you're using write-back cache.

Just having write back cache isn't magic by itself. The cache management algorithm and configuration thereof are often as important, if not more, than the total cache size on the RAID HBA or SAN controller.

Poor cache management, I'd guess, is one reason why you see Areca RAID cards with 1-4GB cache DRAM whereas competing cards w/ similar price/performance/features from LSI, Adaptec, and others sport 512MB.

--
Stan

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