BTRFS added a new device in RAID1 mode, how to remove it?

Question

I was out of disk space so I cleaned up my old snapshots like I usually do. This time, df -h / insisted on showing "no space available" while btrfs fi df / shows a couple of GB's of space, so I decided to run a btrfs balance.

btrfs balance failed due to "not enough space" errors, I searched the net and directed to this blog post which advises to workaround this problem by temporarily adding a small device (even a USB pendrive) to be able to run btrfs balance, then remove it when done.

I added my pendrive to my rootfs:

btrfs dev add /dev/sdc1 /

No errors shown and now I had some free space. I run btrfs balance and when it's finished, I tried to remove the pendrive:

sudo btrfs dev del /dev/sdc1 /
ERROR: error removing device '/dev/sdc1': unable to go below two devices on raid1

BTRFS somehow decided to use the drive in RAID1 profile for Metadata and System data:

$ sudo btrfs fi usage / 
Overall:
    Device size:         864.53GiB
    Device allocated:        857.05GiB
    Device unallocated:        7.48GiB
    Device missing:          0.00B
    Used:            830.28GiB
    Free (estimated):         24.93GiB  (min: 21.23GiB)
    Data ratio:               1.00
    Metadata ratio:           2.00
    Global reserve:      512.00MiB  (used: 0.00B)

Data,single: Size:828.01GiB, Used:810.56GiB
   /dev/mapper/masa-root     828.01GiB

Data,RAID1: Size:2.02GiB, Used:2.01GiB
   /dev/mapper/masa-root       2.02GiB
   /dev/sdc1       2.02GiB

Data,DUP: Size:2.00GiB, Used:1.94GiB
   /dev/sdc1       4.00GiB

Metadata,RAID1: Size:2.97GiB, Used:131.61MiB
   /dev/mapper/masa-root       2.97GiB
   /dev/sdc1       2.97GiB

Metadata,DUP: Size:7.50GiB, Used:5.78GiB
   /dev/mapper/masa-root      15.01GiB

System,RAID1: Size:32.00MiB, Used:128.00KiB
   /dev/mapper/masa-root      32.00MiB
   /dev/sdc1      32.00MiB

Unallocated:
   /dev/mapper/masa-root       1.97GiB
   /dev/sdc1       5.51GiB

How can I properly remove this temporary device?

Answer 1

From man btrfs device:

REMOVE DEVICE

Device removal must satisfy the profile constraints, otherwise the command fails. For example:

$ btrfs device remove /dev/sda /mnt
  ERROR: error removing device '/dev/sda': unable to go below two devices on raid1

In order to remove a device, you need to convert the profile in this case:

$ btrfs balance start -mconvert=dup -dconvert=single /mnt
$ btrfs device remove /dev/sda /mnt