question about MD raid rebuild performance degradation even with speed_limit_min/speed_limit

Jason Keltz

2014-10-20 21:07:38 UTC

Post by Jason Keltz
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.

I would like to add that I downloaded the latest version of Ubuntu, and
am running it on the same server with the same MD.
When I set speed_limit_min and speed_limit_max to 80000, I was able to
start two large dds on the md array, and the rebuild stuck at around 71
MB/s, which is close enough. This leads me to believe that the problem
above is probably a RHEL6 issue. However, after I stopped the two dd
operations, and raised both speed_limit_min and speed_limit_max to
120000, the rebuild stayed between 71-73 Mb/s for more than 10 minutes
.. now it seems to be at 100 MB/s... but doesn't seem to get any higher
(even though I had 120 MB/s and above on the RHEL system without any
load)... Hmm.

Jason.
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