Littleboy,  This can happen when there isn't enough evenly-distributed free space in the pool. Being that you're (likely) at critical capacity usage, rebalance can't take place. You'll be unable to even them out unless you can get the used capacity below the critical threshold again, whether by reducing used capacity, increasing unused capacity (add disks), or raising the critical threshold beyond the used capacity.  As you're very very close to the limit, I don't know that you'll be able to raise the threshold any further. I'd add an SDS or disks if you can't remove/shrink a volume.
Here is the Storage Pool message: [root[NAME] ~]# scli --query_storage_pool --protection_domain_name Fab1 --storage_pool_name HDD Storage Pool HDD (Id: eed4df9d00000000) has 8 volumes and 0 Bytes available for volume allocation         The number of parallel rebuild/rebalance jobs: 2         Rebuild is enabled and using No-Limit policy         Rebalance is enabled and using No-Limit policy         Background device scanner: Mode: device_only, Bandwidth Limit 1024 KBps per device         Zero padding is disabled         Spare policy: 15% out of total         Uses RAM Read Cache         RAM Read Cache write handling mode is 'passthrough'         56.2 TB (57555 GB) total capacity         90.4 GB (92580 MB) unused capacity         0 Bytes snapshots capacity         47.7 TB (48832 GB) in-use capacity         0 Bytes thin capacity         47.7 TB (48832 GB) protected capacity         0 Bytes failed capacity         0 Bytes degraded-failed capacity         0 Bytes degraded-healthy capacity         0 Bytes unreachable-unused capacity         0 Bytes active rebalance capacity         0 Bytes pending rebalance capacity         0 Bytes active fwd-rebuild capacity         0 Bytes pending fwd-rebuild capacity         0 Bytes active bck-rebuild capacity         0 Bytes pending bck-rebuild capacity         0 Bytes rebalance capacity         0 Bytes fwd-rebuild capacity         0 Bytes bck-rebuild capacity         0 Bytes active moving capacity         0 Bytes pending moving capacity         0 Bytes total moving capacity         8.4 TB (8633 GB) spare capacity         47.7 TB (48832 GB) at-rest capacity         0 Bytes decreased capacity           Primary-reads                            0 IOPS 0 Bytes per-second         Primary-writes                           0 IOPS 0 Bytes per-second         Secondary-reads                          0 IOPS 0 Bytes per-second         Secondary-writes                         0 IOPS 0 Bytes per-second         Backward-rebuild-reads                   0 IOPS 0 Bytes per-second         Backward-rebuild-writes                  0 IOPS 0 Bytes per-second         Forward-rebuild-reads                    0 IOPS 0 Bytes per-second         Forward-rebuild-writes                   0 IOPS 0 Bytes per-second         Rebalance-reads                          0 IOPS 0 Bytes per-second         Rebalance-writes                         0 IOPS 0 Bytes per-second   Volumes summary:         8 thick-provisioned volumes. Total size: 23.8 TB (24416 GB)         7 volumes are unmapped But i can not create the volume : [root[NAME] ~]# scli --add_volume --protection_domain_name Fab1 --storage_pool_name HDD --size_gb 8 --volume_name HDD9 Error: MDM failed command.  Status: Insufficient resources Thanks a lot!
Can not create volume by use "Unused" capacity.