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

To: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Subject: Re: RAID6 r-m-w, op-journaled fs, SSDs
From: Emmanuel Florac <eflorac@xxxxxxxxxxxxxx>
Date: Sun, 1 May 2011 11:11:52 +0200
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4DBC68DA.1090708@xxxxxxxxxxxxxxxxx>
Organization: Intellique
References: <19900.10868.583555.849181@xxxxxxxxxxxxxxxxxx> <20110430180213.6dcfc41c@xxxxxxxxxxxxxxx> <4DBC68DA.1090708@xxxxxxxxxxxxxxxxx>
Le Sat, 30 Apr 2011 14:54:02 -0500 vous écriviez:

> 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.

Yes, probably. To give some meat to the argument :

Using XFS mounted nobarrier on an 8 drives RAID-6 array with WB cache :
30000 journal (file creation/deletion) operations/s

Using XFS with barriers on the same RAID : 7000 journal operations/s

Using XFS nobarrier with WT cache : 700 journal op/s.

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