When space reservation is enabled, the amount of space available on a volume containing LUNs determines whether Snapshot copy creation can take place.
When you first create a LUN with space reservation enabled, it is granted a space reservation equal to its size. This reserved space is subtracted from the total available disk space on the storage system volume on which the LUN resides.
When you create a Snapshot copy of the storage system volume holding the LUN, that Snapshot copy locks down all the disk blocks occupied by live data.
By monitoring the remaining available space in the storage system volume, the available space in the volume determines whether Snapshot copy creation is allowed. When the amount of available space on the storage system volume falls to below the threshold you set to prevent overwriting space reserved LUNs, Snapshot creation is blocked.