[Beowulf] hardware RAID versus mdadm versus LVM-striping

Joe Landman landman at scalableinformatics.com
Tue Jan 19 10:43:30 PST 2010

Tony Travis wrote:

> 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.

Not entirely correct.  SATA where the hot swap (bring device in/out) 
logic is.  And it does (at least in modern kernels) support physical 
removal/addition of devices.  The MD system itself is event driven.  You 
can "automate" device removal/insertion into a unit, and rebuild the 
RAID as needed ... to a degree.  The issue we run into is that 
occasionally, we have to force a bus scan on the scsi buses to see new 
SATA drives.  Once that is done, some of our other tools automate the 
incorporation of the new disk within the RAID.

>> 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 :-(

In the many RAID cases we have dealt with over the years, we haven't run 
into this as an issue.  That is, while touted as a real tangible benefit 
of MD RAID, it is of dubious real value in most of the cases we have 

Really the benefit is that of being against the change of business 
conditions for your RAID vendor.  If you plan on keeping the same array 
active until it dies (4-10 years), this could be a consideration. 
However, you also have to worry about disk availability/compatibility, 
etc.  That is, its not *just* a RAID card issue, its a full stack issue.

MD allows you to reduce the risk in various portions of this stack.

> Bye,
>   Tony.

Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics Inc.
email: landman at scalableinformatics.com
web  : http://scalableinformatics.com
phone: +1 734 786 8423 x121
fax  : +1 866 888 3112
cell : +1 734 612 4615

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