[nSLUG] raid performance

Dop Ganger 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 
haystack... :-)

Cheers... Dop.


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