[nSLUG] raid performance
nslug at fop.ns.ca
Fri Jun 23 08:17:26 ADT 2006
On Thu, 22 Jun 2006, Rory wrote:
> Having recently switched to 2.6.15 kernel we are finding that the RAID
> performance completely sucks. Specifically RAID 10, on Dell 2850 with a
> Perc4 controller. This is an LSI based controller and therefore uses
> the megaraid driver.
I've only used the Compaq SmartArray hardware RAID controller under 2.6,
and that was fine - software raid gives reasonable performance, too. That
said, I've only used it with RAID 0, 1 and 5, though I don't see a
hardware device doing RAID 10 being anything too different. One thing to
check is whether your kernel is compiled for desktop style performance -
pre-emptive low latency kernel and a 1000Hz timer tick could cause
problems. If so, disabling pre-emption and setting the timer tick to 100Hz
may boost the performance to levels you had before.
Something else that may be worth playing with (if available) are the block
device settings in /sys/block/sd?/queue; see the kernel documentation in
linux/Documentation/block/biodoc.txt for details (in fact, I think all the
files in that directory would be worth a read for clues to increase
performance). You may need to twiddle with schedulers again; I don't know
your workload, but I would suspect sticking with the deadline scheduler
would be the best place to start testing.
Do you have any actual figures showing the performance difference between
the kernel versions, and if there's anything else going on to interfere?
vmstat, iostat and bonnie results might be worth a look. The fact you're
seeing the same problem with software raid that disappears when you go
back to 2.6 suggests to me there's a config setting somewhere that needs
tweaking. Finding it would be, of course, the proverbial needle in the
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