mdadm --detail on degraded mirror increases Events number on RHEL6.4
Environment
- Red Hat Enterprise Linux (RHEL) 6.4
- mdadm-3.2.5-4.el6.x86_64
Issue
mdadm --detailon degraded mirror increases Events number on RHEL 6.4- Is the increase of the Events counter expected behavior?
Resolution
- This is expected behavior on
mdadm --detail(-D) on a degraded mirror. Usemdadm -Eto query the degraded mirror instead to not affect the Events counter.
Root Cause
- Is this expected behavior for
mdadm -D? - There is no special case code in the md kernel stack to handle
mdadm -Ddifferently than any other device access, so callingmdadm -Don an array will result in a transition from initial assembly to read/write mode, which triggers the event counter update. - Starting a device, running
mdadm -D, then stopping the device sounds like a probe of the device. If that's all the steps being done, then should usemdadm -Einstead. It doesn't require that the device be started, instead it queries the superblock on the constituent device directly without any array ever being started.
Diagnostic Steps
- Stop and start /dev/md0 just to show that it's in sync:
# mdadm -A -R /dev/md0 /dev/sdb /dev/sdc
mdadm: /dev/md0 has been started with 2 drives.
# mdadm -S /dev/md0
mdadm: stopped /dev/md0
- Start mirror with one out of two drives:
# mdadm -A -R /dev/md0 /dev/sdb
mdadm: /dev/md0 has been started with 1 drive (out of 2).
- Get the event id from the drive that's in the mirror:
# mdadm --examine /dev/sdb |grep Events
Events : 256
- Run
mdadm --detail /dev/md0lots of times and the event id increases (by two for every run of the command).
# mdadm --examine /dev/sdb |grep Events
Events : 1338
# mdadm --examine /dev/sdc |grep Events
Events : 256
-
The same issue doesn't happen when the mirror has both drives and is not degraded.
-
From this information: https://raid.wiki.kernel.org/index.php/RAID_superblock_formats
Incremented whenever the superblock is updated.
Used by mdadm in re-assembly to detect failed/out-of-sync component devices.
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