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How file and LUN reservations work

Enabling reservations for a file or LUN tells Data ONTAP to reserve the space required for that file or LUN at creation time rather than as the space is required for writes. Disabling reservations enables you to overcommit the containing volume by creating LUNs that appear to provide more space than the volume can currently provide.

Reservations are an attribute of the file or LUN; they are persistent across storage system reboots, takeovers, and givebacks. Reservations are enabled for new LUNs by default, but you can create a file or LUN with reservations disabled or enabled. After you create a LUN, you change the reservation attribute by using the lun set reservation command. You change the reservation attribute for files by using the file reservation command.

When a volume contains one or more files or 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 files or LUNs with reservations enabled continue to succeed.

You can enable reservations for files and LUNs contained by volumes with volume guarantees of any value. However, if the volume has a guarantee of none, reservations do not have any effect.

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.