Infrastructure/SOP/Raid Mismatch Count

What to do when a raid device has a mismatch count

Contact Information

Owner: Fedora Infrastructure Team

Contact: #fedora-admin, sysadmin-main

Location: All

Servers: Physical hosts

Purpose: Provides database connection to many of our apps.

Description

In some situations a raid device may indicate there is a count mismatch as listed in:

/sys/block/mdX/md/mismatch_cnt

Anything other than 0 is considered not good. Though if the number is low it's probably nothing to worry about. To correct this situation try the directions below

Correction

More than anything these steps are to A) Verify there is no problem and B) make the error go away. If step 1 and step 2 don't correct the problems, PROCEED WITH CAUTION. The steps below, however, should be relatively safe.

Step 1

Issue a repair (replace mdX with the questionable raid device):

echo repair > /sys/block/mdX/md/sync_action

Depending on the size of the array and disk speed this can take a while. Watch the progress with:

cat /proc/mdstat

Once that's done proceed to Step 2.

Step 2

Issue a check. It's this check that will reset the mismatch count if there are no problems. Again replace mdX with your actual raid device.

echo check > /sys/block/mdX/md/sync_action

Just as before, you can watch the progress with:

cat /proc/mdstat