[CentOS] mismatch_cnt after 5.3 -> 5.4 upgrade

[Devin Reade]

S.Tindall <tindall.satwth at brandxmail.com> wrote:

> mismatch_cnt (/sys/block/md*/md/mismatch_cnt) is the number of
> unsynchronized blocks in the raid.

Understood.  

I did the repair/check on sync_action and it got rid of the problem. (Thanks)

What I _don't_ understand is why they were unsynchronized to begin with
(`cat /proc/mdstat` showed the array to be clean). Nor do I understand
the mechanism used by the 'repair' mechanism, and why I should believe
that it's using the correct data in its sync.  Although I've looked around,
I've not seen anything that describes how repair works and (specifically
for raid1) how it can tell which slice has the good data and which has the
bad data.

"Fixing" things without understanding what is going on under the covers
(at least conceptually) does not give me a warm fuzzy feeling :/

Devin
-- 
A zygote is a gamete's way of producing more gametes.  This may be the
purpose of the universe.				- Robert Heinlein