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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:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/md/raid1.c?id=9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc

fixed by upstream commit:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/md?id=d1901ef099c38afd11add4cfb3312c02ef21ec4a

"
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

commit 9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc
    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):


How reproducible:
100%

Steps to Reproduce:
1. create mirror
2. set writeonly on one leg
3. perform reads.

Actual results:
reads from both legs of mirror

Expected results:
reads avoided on leg of mirror marked writeonly

Additional info:

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.

https://rhn.redhat.com/errata/RHSA-2016-0855.html