[kwlug-disc] Should drives in a raid perform alike?

Richard Weait richard at weait.com
Fri Apr 30 11:48:04 EDT 2010

Given two disks in a RAID0:
- They are nominally identical disks, same manufacturer, same
capacity, same part number
- each has only a single, full disk partition (sdb1, sdc1)
- those partitions are RAID0'd together (md0)
- both drives connect to the same drive controller
- the operating system is held on a different drive.  (sda)

Would you expect drive performance to be noticeably different between
the two?  I didn't expect that.  Munin (stats graphing tool) reports

drive | iowait(ms)| util
sdb  |  253.66   | 43.19%
sdc  | 151.20    | 38.40%

Which suggests (?) that sdc is done faster than sdb.  Shouldn't they
be doing the same thing in the same time?  The graph makes this look
worse during the load on the left side of the graph.

Visual aids - munin graphs of disk performance during heavy load.

Thoughts?  Am I expecting too much similarity between drives?

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