When you create a LUN, Data ONTAP reserves space in the volume containing that LUN so that write operations to that LUN do not fail due to lack of disk space. With fractional reserve, this space is set to less than 100 percent of the total size of the LUNs.
SnapDrive creates and manages LUNs with space reservation enabled. Operations such as creating a Snapshot copy or creating new LUNs can occur only if there is enough available unreserved space. These operations are restricted from using reserved space.
While space reservation is enabled at the LUN level, fractional overwrite reserve amounts are configured at the volume level; that is, fractional space reservation does not control how the total amount of space reserved for overwrites in a volume is applied to individual LUNs in that volume.
The volume has the guarantee option set to volume rather than file. Fractional reserve is supported by Data ONTAP 7.1 or later. For more detailed information, see the SAN Administration Guide for your version of Data ONTAP.
Additional space that is not space-reservation-enabled on the volume is automatically reserved for overwriting blocks that belong to a LUN. By default this additional space is equal to 100 percent of the total size of all space-reserved LUNs in the volume. If space reservation is disabled, write operations to a LUN might fail due to insufficient disk space in the volume and the host application might terminate, report I/O errors, or experience unexpected behavior.
With fractional reserve, the space reserved for overwrites is set to less than 100 percent and the space that is preallocated for space reservation is reduced to that percentage. Fractional reserve is generally used for volumes with LUNs that store data with a low rate of change.