Re: wishful thinking about atomic, multi-sector or full MD stripewidth, writes in storage

From: Ric Wheeler
Date: Thu Sep 03 2009 - 20:40:11 EST

On 09/03/2009 07:50 PM, Krzysztof Halasa wrote:
Ric Wheeler<rwheeler@xxxxxxxxxx> writes:

The whole thread above is about software MD using commodity drives
(S-ATA or SAS) without battery backed write cache.
Yes. However, you mentioned external RAID arrays disable disk caches.
That's why I asked if they are using SATA or SCSI/etc. disks, and if
they have battery-backed cache.

Sorry for the confusion - they disable the write caches on the component drives normally, but have their own write cache which is not disabled in most cases.

Also, when you enable the write cache (MD or not) you are buffering
multiple MB's of data that can go away on power loss. Far greater
(10x) the exposure that the partial RAID rewrite case worries about.
The cache is flushed with working barriers. I guess it should be
superior to disabled WB cache, in both performance and expected disk

True - barriers (especially on big, slow s-ata drives) usually give you an overall win. SAS drives it seems to make less of an impact, but then you always need to benchmark your workload on anything to get the only numbers that really matter :-)


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