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Fixing a DegradedArray Event
Identify the failed device, remove it safely, replace it and monitor the rebuild until /proc/mdstat reports [UU].
What triggers a DegradedArray event?
The kernel notifies you as soon as mdadm detects a missing or failed disk. In the sample output below the array /dev/md1 went degraded because /dev/sdb3 dropped out and was marked with (F) inside /proc/mdstat.
A DegradedArray event had been detected on md device /dev/md/1.
md1 : active raid1 sdb3[2](F) sda3[0]
1948488512 blocks super 1.0 [2/1] [U_]
unused devices: <none>
root@server:~# mdadm --detail /dev/md1
Raid Level : raid1
Array Size : 1.8 TiB
Active Devices : 1
Failed Devices : 1
Number Major Minor RaidDevice State
2 8 19 - faulty spare /dev/sdb3
Diagnose
Use mdadm and /proc/mdstat to confirm the degraded state and the failed device.
- mdadm --detail /dev/md1
- Look for [U_] / (F) markers
- Identify the affected disk (e.g. /dev/sdb3)
Replace
Remove the failed device and reinsert a healthy disk before re-adding it to the array.
- mdadm --remove /dev/md1 /dev/sdb3
- Swap or re-enable the disk
- mdadm --add /dev/md1 /dev/sdb3
Monitor
Watch the rebuild until the array reports [UU] and the state goes from degraded to clean.
- watch -n 5 cat /proc/mdstat
- Wait for recovery percentages to finish
- Ensure no faulty devices remain
Typical command workflow
root@server:~# mdadm --remove /dev/md1 /dev/sdb3
root@server:~# mdadm --add /dev/md1 /dev/sdb3
root@server:~# cat /proc/mdstat
# recovery [>....................] 0.1% ...
Once synchronization finishes, validation ensures each RAID member reports healthy status.
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