Understanding space reservation for volumes and LUNs

What space reservation is

Data ONTAP uses space reservation to guarantee that space is available for completing writes to a LUN or for overwriting data in a LUN. When you create a LUN, Data ONTAP reserves enough space in the traditional or FlexVol volume so that write operations to those LUNs do not fail because of a lack of disk space on the storage system. Other operations, such as taking a Snapshot copy or the creation of new LUNs, can occur only if there is enough available unreserved space; these operations are restricted from using reserved space.

What happens when space reservation is disabled

You can create LUNs with space reservation enabled or disabled. If you disable space reservation, write operations to a LUN might fail due to insufficient disk space and the host application or operating system might crash. When write operations fail, Data ONTAP displays system messages (one message per file) on the console, or sends these messages to log files and other remote systems, as specified by its /etc/syslog.conf configuration file.

See How to use individual commands for information about creating LUNs with space reservation enabled or disabled.

What fractional reserve is

Fractional reserve controls the amount of space Data ONTAP reserves in a traditional or FlexVol volume to enable overwrites to space-reserved LUNs. When you create a space-reserved LUN, fractional reserve is by default set to 100 percent. This means that Data ONTAP automatically reserves 100 percent of the total LUN size for overwrites. For example, if you create a 500-GB space-reserved LUN, Data ONTAP by default ensures that the host-side application storing data in the LUN always has access to 500 GB of space.

You can reduce the amount of space reserved for overwrites to less than 100 percent when you create LUNs in the following types of volumes:

Traditional volumes
FlexVol volumes that have the guarantee option set to volume

If the guarantee option for a FlexVol volume is set to file, then fractional reserve for that volume is set to 100 percent and is not adjustable.

For detailed information about how guarantees affect fractional reserve, see How guarantees on FlexVol volumes affect fractional reserve.

How the total LUN size affects reserved space

The amount of space reserved for overwrites is based on the total size of all space-reserved LUNs in a volume. LUNs that do not have space reservation enabled are not included in the total LUN size. For example, if there are two 200-GB LUNs in a volume (400 GB total), and the fractional_reserve option is set to 50 percent, then Data ONTAP guarantees that the volume has 200 GB available for overwrites to those LUNs (400 GB total * 50% = 200 GB).

Note
Fractional overwrite is set at the volume level. It does not control how the total amount of space reserved for overwrites in a volume is applied to individual LUNs in that volume.

Command for setting fractional reserve

You use the following command to set fractional reserve:

vol options vol-name fractional_reserve pct

pct is the percentage of the LUN you want to reserve for overwrites. The default setting is 100. For traditional volumes and FlexVol volumes with the volume guarantee, you can set pct to any value from 0 to 100. For FlexVol volumes with the file guarantee, pct is set to 100 by default and is not adjustable.

Example

The following command sets the fractional reserve space on a volume named testvol to 50 percent:

vol options testvol fractional_reserve 50

How space reservation settings persist

Space reservation settings persist across reboots, takeovers, givebacks, and snap restores. A single file SnapRestore® action on a volume maintains the fractional reserve setting of the volume and the space reservation settings of the LUNs in that volume. A single file SnapRestore of a LUN restores the space reservation setting of that LUN, provided there is enough space in the volume.

If you revert from Data ONTAP 7.0 to Data ONTAP 6.5, or from Data ONTAP 6.5 to 6.4, the space reservation option remains on. If you revert from Data ONTAP 6.4 to 6.3, the space reservation option is set to Off.

How revert operations affect fractional reserve

Fractional reserve is available in Data ONTAP 6.5.1 or later. Data ONTAP 6.4.x versions do not support setting the amount of reserve space to less than 100 percent of the total LUN size. If you want to revert from Data ONTAP 6.5.1 to Data ONTAP 6.4.x, and are using fractional reserve, make sure you have enough available space for 100 percent overwrite reserve. If you do not have enough space when you revert, Data ONTAP displays the following prompt:

You have an over committed volume. You are required to set the 
fractional_reserve to 100. This can be done by either disabling 
space reservations on all objects in the volume or making more 
space available for full reservations or deleting all the Snapshot 
copies in the volume.