3.12: raid-1 mismatch_cnt question

[Justin Piszcz]

Hi,

I run two SSDs in a RAID-1 configuration and I have a swap partition on a
third SSD.  Over time, the mismatch_cnt between the two devices grows higher
and higher.

Once a week, I run a check and repair against the md devices to help bring
the mismatch_cnt down.  When I run the check and repair, the system is live
so there are various logs/processes writing to disk.  The system also has
ECC memory and there are no errors reported.

The following graph is the mismatch_cnt from June 2013 to current; each drop
represents a check+repair.  In September, I dropped the kernel/vm caches
before running check/repair and that seemed to help a bit.
http://home.comcast.net/~jpiszcz/20131104/md_raid_mismatch_cnt.png

My question is: is this normal or should the mismatch_cnt always be 0 unless
there is a HW or md/driver issue?

Justin.

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