What is a good choice of sata-ii raid controller for freebsd?

Patrick M. Hausen hausen at punkt.de
Sun Feb 11 14:04:47 UTC 2007


On Fri, Feb 09, 2007 at 06:15:53PM +0300, Artem Kuchin wrote:

> Under gmirror OS must issue two commands to write to disks and some
> commands to check/set mark that mirrored data is intact.
> Under hardware RAID OS issue sonly one command to write and no
> checking command, since raid controller handles this async.
> So, software OS raid must be slower than controller based raid anyway.

Yes. The OS has got to do a bit more work that is otherwise done
by the CPU on the RAID controller.

For modern CPUs this extra work is measurably neglegible.

One guy that I happen to know, who was responsible for the database
backend servers of Germany's biggest web mail provider at the time,
ran extensive benchmarks. Result: for RAID 1, RAID 0 and RAID 1+0
there is no difference in "hardware RAID" vs. OS mirroring and
striping. He used Linux, but I'd bet a huge amount that his
findings can be transferred to arbitrary current operating systems.

RAID 5 and RAID 6 are different beasts alltogether, but you do
not want RAID 5 for transaction heavy systems, anyway. When you
are running a huge DB that is not "read mostly", you want to have
your working set in memory. If the database needs to write to disk,
eventually, it's all about latency. And latency on RAID 5 is
horrendous, regardless if implemented in "hardware RAID" or not.

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