Btrfs RAID1: How to replace a disk drive that is physically no more there?

Turns out that this is a limitation of btrfs as of beginning of 2017. To get the filesystem mounted rw again, one needs to patch the kernel. I have not tried it though. I am planing to move away from btrfs because of this; one should not have to patch a kernel to be able to replace a faulty disk.

Click on the following links for details:

• Kernel patch here
• Full email thread

Please leave a comment if you still suffer from this problem as of 2020. I believe that people would like to know if this has been fixed or not.

Update: I moved to good old mdadm and lvm and am very happy with my RAID10 4x4 Tb (8 Tb total space), as of 2020-10-20. It is proven, works well, not resource intensive and I have full trust in it.

replace needs the filesystem to be mounted rw to operate.

In a degraded BTRFS RAID1 filesytem, you have one and only one chance to mount the filesystem rw using -o degraded:

   degraded
       (default: off)

       Allow mounts with less devices than the RAID profile constraints
       require. A read-write mount (or remount) may fail when there are
       too many devices missing, for example if a stripe member is
       completely missing from RAID0.

After rw mount, find the devid of the missing device:

btrfs filesystem show /mountpoint

Replace the missing with the new device:

btrfs replace start -B <devid> /dev/new-disk /mountpoint

Check the status:

btrfs replace status /mountpoint

replace will resume on reboot.

Add the new drive to the filesystem with btrfs device add /dev/sdd /mountpoint then remove the missing drive with btrfs dev del missing /mountpoint remounting the filesystem may be required before btrfs dev del missing will work.