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 these steps do not make the mismatch go away then there is likely a problem with one of the disks. At that point PROCEED WITH CAUTION. The steps below though should be fairly 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