I had 2x 2TB BTRFS RAID 1 cache pool. I added a 4TB drive, after looking at a BTRFS calculator it said I would have around 4TB usable.

But Unraid is showing me much less usable space.

/dev/nvme0n1p1 3.7T 423G 1.7T 21% /mnt/cache

1.7TB + 423G does not equal 3.7TB.

Scrub and balance are all showing as fine and without errors.

Overall:
    Device size:                   7.28TiB
    Device allocated:            890.06GiB
    Device unallocated:            6.41TiB
    Device missing:                  0.00B
    Device slack:                  3.50KiB
    Used:                        847.11GiB
    Free (estimated):              3.22TiB      (min: 3.22TiB)
    Free (statfs, df):             1.62TiB
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)
    Multiple profiles:                  no

Data,RAID1: Size:442.00GiB, Used:422.42GiB (95.57%)
   /dev/nvme0n1p1        220.00GiB
   /dev/nvme2n1p1        222.00GiB
   /dev/nvme1n1p1        442.00GiB

Metadata,RAID1: Size:3.00GiB, Used:1.14GiB (37.88%)
   /dev/nvme0n1p1          2.00GiB
   /dev/nvme2n1p1          1.00GiB
   /dev/nvme1n1p1          3.00GiB

System,RAID1: Size:32.00MiB, Used:112.00KiB (0.34%)
   /dev/nvme0n1p1         32.00MiB
   /dev/nvme1n1p1         32.00MiB

Unallocated:
   /dev/nvme0n1p1          1.60TiB
   /dev/nvme2n1p1          1.60TiB
   /dev/nvme1n1p1          3.20TiB

Any ideas please?

Edited December 9, 2025Dec 9 by nebb00

Using an odd number of different capacity devices with Btrfs will cause stats issues, but the capacity can still be used; stats will be more correct as the used space increases.

20 minutes ago, JorgeB said:

Using an odd number of different capacity devices with Btrfs will cause stats issues, but the capacity can still be used; stats will be more correct as the used space increases.

ah, this might be an issue as I use mover based on % used... balls