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Re: [Xen-users] SW RAID powered Xen node - knowledgeable feedback much n

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Subject: Re: [Xen-users] SW RAID powered Xen node - knowledgeable feedback much needed
From: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
Date: Tue, 26 Oct 2010 21:17:43 +0100
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John Madden wrote:
Adding HW raid would mean an 60% price increase which would lead to 60%
more VEs on the node. Would the 60% increase in VEs count but with HW RAID
be preferable over the SW RAID with less VEs?

Why would adding hardware raid result in a 60% increase in guests?

I suspect he meant that if the price goes up 60% to pay for hardware raid, then they need to put 60% more guests on it to pay for it.

So I suppose the question would be, if you were having to do that, would the extra performance of a hardware controller support that - and I think the answer is no.

As you say, a hardware raid controller isn't necessarily faster than software raid. They "tend" to be simply because a good one will have a good processor and hardware to accelerate certain operations (such as parity computation for RAID 5). The processor may not be better than or even as good as, the one in the system - but they tend to win out by being dedicated and so available for full performance even when the system processor(s) is heavily loaded.

The other way that hardware raid can gain is by having battery backed cache and using write-back to cache (and delay) disk writes. That can have a significant effect on systems with a heavy disk I/O with significant writes. Such a cache in software raid wouldn't be safe as I'm not aware of any battery backed memory being available and supported for such use - and losing the wrote cache would almost certainly cause corruption in the event of a power failure or unclean shutdown.

Simon Hobson

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