RAID 1 Mismatches

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I have three disks in my system, divided into a mix of RAID-1 and RAID-5 
partitions.  I have /boot as a RAID-1 on MD0 (SDA1, SDB1), / as RAID-1 on MD1 
(SDA2, SDB2), and the remainder as RAID-5 with LVM (SDA3, SDB3, SDC3).  The 
intent is that for boot and root, if SDA fails, I can boot off SDB.

Every week I get an "Anacron job 'cron.weekly'" e-mail telling of: "WARNING: 
mismatch_cnt is not 0 on /dev/md1".  If I look at 
/sys/block/md1/md/mismatch_cnt, I will see a number, usually 128, sometimes 
64.  If I manually invoke a scan, I will see the same number.  Note that 
neither MD0 or MD3 report any errors.

If I set SDB1 bad, then remove then re-add it, it will rebuild fine, and a scan 
shows no errors.  Scanning the messages (current and historical), I see no 
reports of medium errors reported for any of SD[ABC].  

I have several questions:

1) Where can the errors be coming from?  I would understand if a drive were 
reporting errors.  Could it be during boot, one of the R-1 members is being 
written too before MD is started?

2) Sans drive errors messages, how to determine which drive is out of sync.  I 
have resynced SDA2 to SDB2, but that is basically a coin flip; if SDB2 were 
correct, then I may have damaged files on SDA2.  How can I determine where the 
mismatches occur, and then determine the file(s) potentially affected?



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