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

Am 22.06.26 um 07:50 schrieb Michel Verdier:
> 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 ?
The partitions are LUKS encrpyted and hence decrypted before being assembled into the RAID array. Thus the mapped drives.
>
> 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
>

There was a partition which md claimed was failed (partition_1) which contained the newer database. There was a partition the degraded array was using (partition_2) which contained the older database. I wanted to keep the newer database on the seemingly failed device.

I was not sure if

1) it is possible to remove all devices from a md array (removing partition_2 would have resulted in an array without any partition)
2) adding a device with data on it I want to keep (partition_1) to an array is possible without losing the data (adding partition_2 to the freshly created md array resulted in the data on partition_2 being overwritten, as intended, but I wanted to avoid that partition_1 is overwritten with data from partition 2)

Hence I decided to reassemble the array starting with partition_1 and adding partition_2.

Are you positive that the procedure you recommend would have ended in the same result?