Hi Johnnie.black - thanks for your response. This is the output I get. :
Linux 4.19.88-Unraid.
Last login: Sat Jan 25 10:22:33 +0100 2020 on /dev/pts/0.
root@unRaid:~# btrfs fi usage -T /mnt/cache
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
Overall:
Device size: 1.31TiB
Device allocated: 0.00B
Device unallocated: 1.31TiB
Device missing: 0.00B
Used: 0.00B
Free (estimated): 0.00B (min: 8.00EiB)
Data ratio: 0.00
Metadata ratio: 0.00
Global reserve: 16.00MiB (used: 0.00B)
Data Metadata System
Id Path RAID5 RAID5 RAID5 Unallocated
-- --------- --------- --------- -------- -----------
1 /dev/sdc1 1.00GiB 512.00MiB 32.00MiB 445.60GiB
2 /dev/sde1 1.00GiB 512.00MiB 32.00MiB 445.60GiB
3 /dev/sdb1 1.00GiB 512.00MiB 32.00MiB 445.60GiB
-- --------- --------- --------- -------- -----------
Total 2.00GiB 1.00GiB 64.00MiB 1.30TiB
Used 768.00KiB 112.00KiB 16.00KiB
I wiped the 3 cache disks so yes, used space is almost nil. But not getting a correct free space indication is a bit of a bummer as one might run out of free space without prior warning I suppose.