[Discuss] RAID question

[Drew]

> That was
> my other main concern about software RAID; it seemed to dump a
> hardware job on the software, with all the usual performance
> penalties.  When software tries to emulate hardware, it always
> slows things down by some fraction.

I'm going to call this statement out as erroneous. Modern CPU's are
overpowered for a lot of work loads and the spare CPU horsepower gives
hardware RAID a run for it's money. Utilizing the CPU also permits
larger cache sizes (most HW RAID has 1-2GB cache max, mdadm (linux
software raid) can access a good portion of main RAM) which improve
performance by an extra margin.

mdadm does require a bit more setup for the end user but the final
benefit is portability. Your RAID is no longer tied to vendor specific
hardware, meaning if a HBA packs it in, whatever you can find to move
the affected drives to will work fine. Whereas with hardware RAID, you
have to get either the same card or a newer card and hope the new card
can see the configuration on the drives.

As an aside, vendors like QNAP rely on mdadm quite heavily. Their EC
series NAS/SAN chassis (which we use at work in production) use mdadm
exclusively, no hardware RAID in those chassis.

I will also note there is a distinction between software RAID, and
fake-RAID (software assisted RAID). Software RAID, like mdadm, is pure
software. As long as the linux OS can see the drives, mdadm can RAID
them. Fake-RAID on the other hand hides the individual drives behind a
raid controller but offloads the actual work to the CPU. Fake-RAID is
what you usually see on consumer PC's with 'RAID' supplied by a bios
switchable drive controller. And Fake-RAID is in many ways the worst
of each. :)


-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie

"This started out as a hobby and spun horribly out of control."
-Unknown