Mdraid Strange Surprises…

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I installed 2 new data servers with a big (12TB) RAID6 mdraid.

I formated the whole arrays with bad blocks checks. One server is moderately used (nfs on one md), while the other not.

One week later, after the raid-check from cron, I get on both servers

a few block_mismatch… 1976162368 on the used one and a tiny bit less

on the other…

2 thoughts on - Mdraid Strange Surprises…

  • I think there should not be any count mismatches with raid 6, but… md raid 1 is another beast. Such count mismatches can happen fairly easily.

    a page in the virtual memory is modified, eventually it sends it to both disks. One disk is a little bit slower, and you have your potential mismatch. As I understand, the raid check does not care about virtual memory but acts on physical disk sectors. If the raid check checks a block in the very moment where one disk has written it but the other disk *not yet* then…, well, you get it?

    I do not know exactly about md’s raid 6, but I always thougt that this false positive mismatch count thing was raid 1 specific. Because of this all I also would tend to turn off the weekly raid check cronjob for md raid 1

    $ chkconfig –list mdmonitor mdmonitor 0:off 1:off 2:on 3:on 4:on 5:on 6:off

    configure it with a working email address. And there *are* entries in /var/log/messages. Could it be that this happened a long time ago, you did not notice and the log files rotated out?

  • From: Markus Falb

    Ok, so basicaly we should just ignore these mismatches…

    Already running…

    There are entries… but nothing alarming… By example: