question about MD raid rebuild performance degradation even with speed_limit_min/speed_limit_max set.

[Jason Keltz <jas@cse.yorku.ca>]

Hi.

I'm creating a 22 x 2 TB SATA disk MD RAID10 on a new RHEL6 system. I've 
experimented with setting "speed_limit_min" and "speed_limit_max" kernel 
variables so that I get the best balance of performance during a RAID 
rebuild of one of the RAID1 pairs. If, for example, I set 
speed_limit_min AND speed_limit_max to 80000 then fail a disk when there 
is no other disk activity, then I do get a rebuild rate of around 80 
MB/s. However, if I then start up a write intensive operation on the MD 
array (eg. a dd, or a mkfs on an LVM logical volume that is created on 
that MD), then, my write operation seems to get "full power", and my 
rebuild drops to around 25 MB/s. This means that the rebuild of my 
RAID10 disk is going to take a huge amount of time (>12 hours!!!). When 
I set speed_limit_min and speed_limit_max to the same value, am I not 
guaranteeing the rebuild speed? Is this a bug that I should be reporting 
to Red Hat, or a "feature"?

Thanks in advance for any help that you can provide...

Jason.