When space reservation is enabled for one or more LUNs, Data ONTAP reserves enough space in the volume (traditional or FlexVol) so that writes to those LUNs do not fail because of a lack of disk space.
For example, if you create a 100-GB space reserved LUN in a 500-GB volume, that 100 GB of space is immediately allocated, leaving 400 GB remaining in the volume. In contrast, if space reservation is disabled on the LUN, all 500 GB in the volume remain available until writes are made to the LUN.
Space reservation is an attribute of the LUN; it is persistent across storage system reboots, takeovers, and givebacks. Space reservation is enabled for new LUNs by default, but you can create a LUN with space reservations disabled or enabled. After you create the LUN, you can change the space reservation attribute by using the lun set reservation command.
When a volume contains one or more LUNs with space reservation enabled, operations that require free space, such as the creation of Snapshot copies, are prevented from using the reserved space. If these operations do not have sufficient unreserved free space, they fail. However, writes to the LUNs with space reservation enabled will continue to succeed.