On Tue, Jun 20 2017, Peter Sangas wrote:

quoted

From: NeilBrown [mailto:neilb@suse.com]
Sent: Monday, June 19, 2017 9:15 PM
Subject: RE: mismatch_cnt > 0 during initial sync?

This is a recovery, not a resync.  They are different.

 
Thank you for pointing this out and the nice explanation.    

From reading this list and other sources issuing a repair command as
follows:

"echo repair > /sys/block/md2/md/sync_action"

might be ill-advised since it's luck of the draw whether or not the
operation gets the right data instead of the bad data.

Is this correct? 

How do you define "right data" and "bad data"?
What is your threat-model which explains how the two blocks are
different?

In most likely cases, neither version if the data is more correct than
the other.  In some others, your hardware is broken and needs to be
replaced.

If you have no understanding of why there might be a difference, then
issuing a repair is not likely to be worse than not issuing a repair.
If you do have that understanding, then use that to make your decision.

NeilBrown

For reference I have a RAID1 with 3 disks.  mdadm - v3.3 - 3rd September
2013

Thank you,
Pete