Remove drive from soft RAID

Question

I have a dedicated server with 3 SSD drives in RAID 1. Output of cat /proc/mdstat:

    Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md4 : active raid1 sdc4[2] sdb4[1] sda4[0]
      106738624 blocks [3/3] [UUU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

md2 : active raid1 sdc2[2] sda2[0] sdb2[1]
      5497792 blocks [3/3] [UUU]
      
md1 : active raid1 sda1[0] sdc1[2] sdb1[1]
      259008 blocks [3/3] [UUU]
      
unused devices: <none>

¿How can a drive be safely removed from the soft raid without loosing any data? I would like to remove a drive from the array in order to reformat it and use it independently, while keeping the most important data mirrored.

Solution

You've got a three-way mirror there: each drive has a complete copy of all data. Assuming the drive you want to remove is /dev/sdc, and you want to remove it from all three arrays, you'd perform the following steps for /dev/sdc1, /dev/sdc2, and /dev/sdc4.

Step 1: Remove the drive from the array. You can't remove an active device from an array, so you need to mark it as failed first.

mdadm /dev/md1 --fail /dev/sdc1
mdadm /dev/md1 --remove /dev/sdc1

Step 2: Erase the RAID metadata so the kernel won't try to re-add it:

wipefs -a /dev/sdc1

Step 3: Shrink the array so it's only a two-way mirror, not a three-way mirror with a missing drive:

mdadm --grow /dev/md1 --raid-devices=2

You may need to remove the write-intent bitmap from /dev/md4 before shrinking it (the manual isn't clear on this), in which case you'd do so just before step 3 with mdadm --grow /dev/md4 --bitmap=none, then put it back afterwards with mdadm --grow /dev/md4 --bitmap=internal.

Comments

[Nobody]

Does this preserve the data on the disk which you remove? I.e. can you afterwards access the data which was on the RAID from the separate disk too?

[Mark]

@Nobody, if you want to read the data off the removed drive, you should skip step 2. Virtually all the data is still there after running wipefs (it only erases a few key bytes required for filesystem recognition), but reading it becomes an exercise in data recovery rather than just a matter of plugging the drive in.