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

[Stan Hoeppner]

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