Ok, we're making balance progress now... thank you for the help. The
COW aspect makes sense regarding why the rebalance was stuck.
$ sudo btrfs fi usage /.BACKUPS/
Overall:
Device size: 10.92TiB
Device allocated: 7.19TiB
Device unallocated: 3.72TiB
Device missing: 0.00B
Used: 7.18TiB
Free (estimated): 1.87TiB (min: 1.87TiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 512.00MiB (used: 0.00B)
Data,RAID1: Size:3.59TiB, Used:3.58TiB
/dev/sda1 1.78TiB
/dev/sdb1 3.59TiB
/dev/sdc1 1.79TiB
/dev/sdj1 17.00GiB
Metadata,RAID1: Size:4.00GiB, Used:3.83GiB
/dev/sda1 1.00GiB
/dev/sdb1 4.00GiB
/dev/sdc1 2.00GiB
/dev/sdj1 1.00GiB
System,RAID1: Size:32.00MiB, Used:720.00KiB
/dev/sdb1 32.00MiB
/dev/sdj1 32.00MiB
Unallocated:
/dev/sda1 38.07GiB
/dev/sdb1 44.03GiB
/dev/sdc1 24.00GiB
/dev/sdj1 3.62TiB
