Nathan M ,
Unallocated space is space that is designated for a pool but has not yet been allocated by a volume within that pool.
For Example :
You have total 5TB of physical space available.
Let us assume , you configure a DiskGroup in PoolA with 4TB capacity.
You have created 1 volumes in the PoolA with 3TB
4TB-3TB = 1TB is the unallocated space.
Please refer to SMU user guide for complete details :
https://support.hpe.com/hpsc/doc/public/display?docId=a00017707en_us
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