What is a good choice of sata-ii raid controller for freebsd?
wb at freebie.xs4all.nl
Sun Feb 11 14:29:47 UTC 2007
On Sun, Feb 11, 2007 at 03:04:44PM +0100, Patrick M. Hausen wrote..
> 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.
For that purpose a sensibly designed battery-backup write cache works
wonders. We have tons of customers running RAID5 for DBMS use.
It all really depends on what your needs are as far as I/O goes
whether RAID5 will do it for you or not. Do not automatically dismiss
it. RAID0+1 might be faster, but comes at a substantially higher price
Wilko Bulte wilko at FreeBSD.org
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