On Wed, 22 Dec 2010, Stephan wrote:
Does anybody have an idea what causes the poor perfomance on hardware RAID-sets?
Write performance can be significantly reduced due to lack of write caching in some raid adapters. Typically, raid adapters will require some kind of battery backup in order to enable write caching. The only adapters I'm very familiar with in this regards are the Compaq/HP Smart Array adapters (ciss(4)). The Smart Array adapters will not allow you to enable write cache if there is no battery, but I have been able to enable write cache on a Dell M600 Blade with mpt(4) by using a linux lsiutil program. The adapter on the M600 does not have battery backup, so there's more potential for data loss on loss of power.
I saw the same with the ciss driver which is as slow on 5.1 but very fast on 5.99.39 (up to 180MB/s write speed). The changelog tells about "tagged queueing"...
I found out through experimentation that the Smart Array adapters will perform much better with no write cache if multiple commands are queued simultaneously to the adapter. Due to the way NetBSD queues command via the scsipi mid-layer, and the ciss(4) driver not notifying the mid-layer that it can accept multiple command, ciss(4) performed very poorly until I was able to fix it. The mpt(4) driver has code that attempts to enable tagged queueing, but as best I can tell, there isn't any way it could ever be enabled. I'm also not at all certain that the mpt adapter performance would improve by queueing multiple commands, and I don't have a system at this time to do any testing on. I was working with someone with a Dell system , but she has been rather involved in other things recently. If I can ever get access to the M600 blade again, I can try to figure out how to make the driver enable tagged queueing and see if that makes any difference, but that blade is currently being used for other testing with a much highor priority than my playing around. -- Michael L. Hitch mhi...@montana.edu Computer Consultant Information Technology Center Montana State University Bozeman, MT USA
