Description of problem:
Customer encountered mdraid broken writemostly feature - both disks in mirror are read from fairly equally even with writemostly set on just one leg.
writemostly feature broken by upsteam commit:
fixed by upstream commit:
md/raid1: fix read balance when a drive is write-mostly.
When a drive is marked write-mostly it should only be the
target of reads if there is no other option.
This behaviour was broken by
md/raid1: read balance chooses idlest disk for SSD
which causes a write-mostly device to be *preferred* is some cases.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. create mirror
2. set writeonly on one leg
3. perform reads.
reads from both legs of mirror
reads avoided on leg of mirror marked writeonly
Patch(es) available on kernel-2.6.32-584.el6
Verified on kernel-2.6.32-584.el6:
I used two disks: sdb/sdc for test, after set one disk: sdc with writemostly, the MB_read of iostat for sdc becomes 0.
The patch from commet 6 also exist on 2.6.32-584.el6.
Change to verified.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.