When space reservation is enabled, the amount of space available on a LUN volume 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. As data is written to the LUN, the space occupied by that data is subtracted from the remaining available volume space and added to the used volume space.
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, space reservations determine 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.