When space reservation is enabled for one or more files, Data ONTAP reserves enough space in the volume (traditional or FlexVol) so that writes to those files do not fail because of a lack of disk space.
File space reservation is an attribute of the file or LUN; it is persistent across storage system reboots, takeovers, and givebacks. To guarantee that writes to a specific file or LUN will always succeed, you enable space reservations for that file or LUN by using the file reservation command. (LUNs have space reservations enabled by default.)
When a volume contains one or more files with space reservation enabled, operations that require free space, such as the creation of Snapshot copies or new files, are prevented from using the reserved space. If these operations do not have sufficient unreserved free space, they fail. However, writes to the files or LUNs with space reservation enabled will continue to succeed.