software raid and multiple cores

[Peter M. Petrakis]

On 11/26/2011 04:35 AM, Mark van Harmelen wrote:
> Hi
> 
> Thanks to those who contributed to a discussion on software raid
> recently, it changed my mind about the universal desirability of
> hardware raid cards.
> 
> So now I'm intent on building a sw raid based machine, but mostly am
> interested in performance under disc load.
> 
> Basic question: given a number of spare cores (be they hyperthreaded
> or not) is ubuntu's sofware raid clever enough to be able to deal
> with multiple read and write requests simultaneously, one request per
> spare core? Or would I expect only to see one of the spare core
> utilised?

Request level might be the the wrong level to look at, Linux already has
lots of queue level optimizations. What's really going to matter here is
efficiently handling parity calculations and the effective RAID
real estate (stripes). There does appear to be a  multi-core optimization
for MD RAID5/6 and it's disabled by default on Ubuntu kernels.

http://lxr.linux.no/linux+v3.1.3/drivers/md/Kconfig#L157

config MULTICORE_RAID456
	bool "RAID-4/RAID-5/RAID-6 Multicore processing (EXPERIMENTAL)"
	depends on MD_RAID456
	depends on SMP
	depends on EXPERIMENTAL
	---help---
	  Enable the raid456 module to dispatch per-stripe raid operations to a
	  thread pool.

	  If unsure, say N.

$ grep MULTICORE_RAID456 /boot/config-3.0.0-1*
/boot/config-3.0.0-12-generic:# CONFIG_MULTICORE_RAID456 is not set
/boot/config-3.0.0-13-generic:# CONFIG_MULTICORE_RAID456 is not set

It looks like a work in progress, I hope you have nothing to lose on those disks.
See the linux-raid list for more information: http://marc.info/?l=linux-raid .

Peter

> 
> thanks mark
> 
>