[Beowulf] hardware RAID versus mdadm versus LVM-striping

Tony Travis a.travis at abdn.ac.uk
Tue Jan 19 13:22:45 EST 2010


Jon Forrest wrote:
> [...]
> The only advantage I can think of for fake raid is
> that there's usually a BIOS of sorts in the fake
> raid card that lets you manipulate the raid units.
> This might be more convenient than having to boot
> Linux and mess with mdadm commands.

Hello, Jon and Joe.

I use Adaptec 'host' RAID controllers as a way of adding SATA ports to 
old motherboards that don't have any on-board, configure them in SATA 
(i.e. non-RAID) mode and build "md" software RAID's using the ports.

> For RAID levels that require parity calculations, then
> having a hardware RAID card is a win because the card
> does a lot of work and hides both the parity calculations
> and required IOs from the host system. On the third hand,
> if you have a system with lots of CPU and I/O capacity
> that wouldn't otherwise get used, then it could be argued
> that a hardware RAID card is an unnecessary expense.

It has been argued before that, these days, "md" software RAID often 
performs better because the 'host' CPU is considerably more powerful 
than the embedded processor on a 'hardware' RAID controller. However, 
one point that is often overlooked, and the reason I chose a hybrid 
approach is that AFAIK "md" RAID's do not support hot-swap. I would be 
very interested to know if anyone is using hot-swap "md" RAID's in 
production servers: I do realise that development work is going on.

> In the old days it was easier to decide to go with
> hardware RAID. These days it's best to do test with
> both hardware and software RAID, and then see if
> the measured improvements of hardware RAID (if any)
> justify its expense. Of course, in any production system
> you'll want a few extra RAID cards lying around just
> in case.

Yes, I agree with that!

A great virtue of "md" RAID's is that they are independant of the 
underlying disk controller, and you can easily replace broken 
controllers or motherboards. If you don't have a spare RAID controller 
supporting the proprietary format your shiny 'hardware' RAID is using 
then you can't access your data :-(

Bye,

   Tony.
-- 
Dr. A.J.Travis, University of Aberdeen, Rowett Institute of Nutrition
and Health, Greenburn Road, Bucksburn, Aberdeen AB21 9SB, Scotland, UK
tel +44(0)1224 712751, fax +44(0)1224 716687, http://www.rowett.ac.uk
mailto:a.travis at abdn.ac.uk, http://bioinformatics.rri.sari.ac.uk/~ajt
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