When reservations are enabled for one or more LUNs, Data ONTAP reserves enough space in the volume so that writes to those LUNs do not fail because of a lack of disk space.
Reservations are an attribute of the LUN; they are persistent across storage system reboots, takeovers, and givebacks. Reservations are enabled for new LUNs by default, but you can create a LUN with reservations disabled or enabled. After you create a LUN, you can change the reservation attribute by using the command.
When a volume contains one or more LUNs with reservations 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 reservations enabled will continue to succeed.
You can enable file and LUN reservations for files and LUNs contained by volumes with volume guarantees of any value. However, if the containing volume has a volume guarantee of none, file and LUN reservations do not provide protection against out-of-space errors.