Since one of the SSDs in my cache pool started reporting SMART errors I thought it was time to replace it. Since there' a bug in the current Unraid version I thought I could just remove the drive so it wouldn't cause issues before adding a new one later. After stopping the array, removing the failing drive, and starting it again I was greeted by an "Filesystem Unmountable Error". Stopping the array and starting in maintenance mode and hitting the check button I got:
bad tree block 1083446640640, bytenr mismatch, want=1083446640640, have=0
Couldn't read tree root
ERROR: cannot open file system
Opening filesystem to check...
The troubleshooting steps from the FAQ all output some variation of the root being unable to be opened.
root@WaterVapor:/# mount -o degraded,usebackuproot,ro /dev/mapper/sdm1 /x
mount: /x: wrong fs type, bad option, bad superblock on /dev/mapper/sdm1, missing codepage or helper program, or other error.
root@WaterVapor:/# btrfs restore -D -v /dev/mapper/sdm1 /mnt/btrfs_save/
bad tree block 1083446640640, bytenr mismatch, want=1083446640640, have=0
Couldn't read tree root
Could not open root, trying backup super
warning, device 4 is missing
bad tree block 1021977804800, bytenr mismatch, want=1021977804800, have=0
ERROR: cannot read chunk root
Could not open root, trying backup super
warning, device 4 is missing
bad tree block 1021977804800, bytenr mismatch, want=1021977804800, have=0
ERROR: cannot read chunk root
Could not open root, trying backup super
root@WaterVapor:/# btrfs check --repair /dev/mapper/sdm1
enabling repair mode
WARNING:
Do not use --repair unless you are advised to do so by a developer
or an experienced user, and then only after having accepted that no
fsck can successfully repair all types of filesystem corruption. Eg.
some software or hardware bugs can fatally damage a volume.
The operation will start in 10 seconds.
Use Ctrl-C to stop it.
10 9 8 7 6 5 4 3 2 1
Starting repair.
Opening filesystem to check...
bad tree block 1083446640640, bytenr mismatch, want=1083446640640, have=0
Couldn't read tree root
ERROR: cannot open file system
Does this mean the cache is FUBAR or is there still something that can be tried? Seems that even the dangerous repair option just gives up.