[Beowulf] hardware RAID versus mdadm versus LVM-striping

[Jon Forrest]

On 1/19/2010 6:21 AM, Joe Landman wrote:

> Rarely. Fake raid will generally not have any RAM cache or battery
> backup capability.

Not only that, but it won't have any hardware to do
parity calculations. (It might be hard to recognize
such hardware).

> In some instances, fake raid is *ok* for OS drives (RAID1 only), if the
> bios is smart enough to use it correctly, the underlying fake raid
> driver is relatively stable, and you have reasonable disks.

It doesn't take much extra work to do RAID0 or RAID1
so whether this is done by a fake raid driver
or the md raid driver probably isn't significant
from the resource usage point of view.

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.

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.

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.

Cordially,


-- 
Jon Forrest
Research Computing Support
College of Chemistry
173 Tan Hall
University of California Berkeley
Berkeley, CA
94720-1460
510-643-1032
jlforrest at berkeley.edu