Volume size rules

Storage system volumes that will hold LUNs must be large enough to hold all the LUNs in the volume, as well any Snapshot copies if Snapshot copies are created.

The following factors govern the appropriate minimum size for a volume that holds a LUN:

If the LUNs are space reserved, then the volume must be more than twice the combined size of all the LUNs on the volume if a Snapshot copy of the volume is created. This enables the volume to hold the LUNs in a fractional reservation area.
No matter how much the contents of the LUNs change between Snapshot copies, the entire contents of the disks are written to the volume.
The volume must also provide enough additional space to hold the number of Snapshot copies you intend to keep online.
The amount of space consumed by a Snapshot copy depends on the amount of data that changes after the Snapshot copy is taken. The maximum number of Snapshot copies is 255 per storage system volume.