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

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

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

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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