True NAS Scale Raid Z2 improper expansion

[ruchirys]

True NAS Scale 25.04.2.4

Initially we started the system with 4 Nos 20TB disks in raidz2 configuration.

Only 1 single pool is created

As the pool was about to get full, to increase the pool size we added additional 5 disks to the pool

They were added using the extend vdev method

Once 1 disk was added and volume expansion was completed, next disk was added

We are not able to get the full usable space after expansion.

Instead of expected 120TiB the GUI shows only 79.42TiB.

When we try to copy the data, it does not allow to copy beyond that.

All the 9 disks are online.

zpool list gives following

NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
Data 164T 128T 35.4T - - 7% 78% 1.00x ONLINE /mnt
boot-pool 232G 3.80G 228G - - 9% 1% 1.00x ONLINE -

zpool status given following output

pool: Data
state: ONLINE
scan: scrub canceled on Thu Oct 23 10:35:34 2025
expand: expanded raidz2-0 copied 55.3T in 23:40:16, on Wed Oct 8 21:31:21 2025
config:

NAME                                      STATE     READ WRITE CKSUM
Data                                      ONLINE       0     0     0
  raidz2-0                                ONLINE       0     0     0
    c4d7e7d6-8c89-4031-82d3-d334bf0ef9b1  ONLINE       0     0     0
    71e3fdbf-9af8-42f5-9e37-6d38a72d3ff6  ONLINE       0     0     0
    55fe3a3e-77f3-444f-b001-988ced60efe6  ONLINE       0     0     0
    a0f1cab2-1048-4acc-bc2e-b7c48ef3db45  ONLINE       0     0     0
    e2107252-6f8f-4dbd-8658-13145ee06228  ONLINE       0     0     0
    f3844f44-8592-4258-b841-2b3b1b653a3a  ONLINE       0     0     0
    ea868532-805b-4f4f-86bc-5ce6441af5cf  ONLINE       0     0     0
    fe5b5998-5a81-4fea-aaec-2b555e1e1765  ONLINE       0     0     0
    c649c21a-4e2d-4e91-ab0a-e106c116852b  ONLINE       0     0     0

errors: No known data errors

pool: boot-pool
state: ONLINE
scan: scrub repaired 0B in 00:00:10 with 0 errors on Wed Oct 29 03:45:12 2025
config:

NAME        STATE     READ WRITE CKSUM
boot-pool   ONLINE       0     0     0
  mirror-0  ONLINE       0     0     0
    sdk3    ONLINE       0     0     0
    sdi3    ONLINE       0     0     0

errors: No known data errors

Storage dashboard gives 79.42 TiB as usable space

Requesting some one to provide a resolution for the same

[IvanVolosyuk]

Seem like an issue with assumed parity as discussed here: #15232 (comment)

Assuming you had 100% utilization of 4x20 raidz2 pool (probably unrealistic), with new parity you will be able to write:
(7./9.) * (9 * 20 - 40) = 108T (maximum free space estimate when storing large files)
while reporting will use old parity:
(2./4.) * (9 * 20 - 40) = 70T (reported free space)
As you had a bit of free space before expansion the numbers you see a bit higher.

If you will do a send/recv your for your old data - you can improve space efficiency, but not free space reporting.
You can get up-to:
40 * (7/9 - 2/4) = 11T (extract space)