Re: How to salvage a degraded mdadm RAID1 with as little data loss as possible?

On 2026-06-21, Paul Leiber wrote:

> I managed to rebuild the md RAID1 using the data on the seemingly failed
> device (partition_1). First, I did a dd dump of partition_2 (currently in
> usage) in order not to lose data. Then, I recreated the RAID1 using the data
> from partition_1:
>
> mdadm --stop /dev/md0 # This stops the degraded RAID1
> mdadm --assemble --update=uuid /dev/md0 /dev/mapper/partition_1 # This creates a new RAID1 using the partition_1, a new array UUID is required in order for --assembly to work
> mdadm --manage --add /dev/md0 /dev/mapper/partition_2 # This adds partition_2 to the RAID1, contents of partition_1 are replicated to partition_2 automatically

Your commands are strange : the partitions should be the disk partitions
from /dev and not mapped ones. Or you have another layer ? From where
come /dev/mapper/partition_1 and /dev/mapper/partition_2 ?

Beside this it is much quicker and safer to go this way :
- do not stop the md array (and thus the assemble is not needed)
- remove the failed partition
  mdadm --manage /dev/md0 --remove "failed partition"
- add the new clean partition
  mdadm --manage /dev/md0 --add "good partition"
- and let mdadm sync the array