If you have enabled space reservation, you can use the vol options command to set fractional reserve, which reduces the size of the reserved space in the volume.
Fractional reserve can be set to any percentage from zero to 100. However, you can only set fractional reserve to a non-zero value when you are not using the autodelete function.
It is generally recommended to use the autodelete function, but there may occasionally be circumstances under which fractional reserve can be used, including:
- When Snapshot copies cannot be deleted
- When preserving existing Snapshot copies is more important than creating new ones
Fractional reserve can be used on the following types of volumes:
- Traditional volumes
- FlexVol volumes with a space guarantee of volume
Note: 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.
The default setting for fractional reserve is 100 percent. This means that when you create space-reserved LUNs, you can be sure that writes to those LUNs will always succeed without deleting Snapshot copies, even if all of the space-reserved LUNs are completely overwritten.
Setting fractional reserve to less than 100 percent causes the space reservation held for all space-reserved LUNs in that volume to be reduced to that percentage. Writes to the space-reserved LUNs in that volume are no longer unequivocally guaranteed.
Fractional reserve is generally used for volumes that hold LUNs with a small percentage of data overwrite.
Note: If you are using fractional reserve in environments in which write errors due to lack of available space are unexpected, you must monitor your free space and take corrective action to avoid write errors. Data ONTAP provides tools for monitoring available space in your volumes.
Note: Reducing the space reserved for overwrites (by using fractional reserve) does not affect the size of the space-reserved LUN. You can write data to the entire size of the LUN. The space reserved for overwrites is used only when the original data is overwritten.
Example
If you create a 500-GB space-reserved LUN, then Data ONTAP ensures that 500 GB of free space always remains available for that LUN to handle writes to the LUN.
If you then set fractional reserve to 50 for the LUN's containing volume, then Data ONTAP reserves 250 GB, or half of the space it was previously reserving for overwrites with fractional reserve set to 100. If more than half of the LUN is overwritten, then subsequent writes to the LUN could fail due to insufficient free space in the volume.
Note: When more than one LUN in the same volume has space reservations enabled, and fractional reserve for that volume is set to less than 100 percent, Data ONTAP does not limit any space-reserved LUN to its percentage of the reserved space. In other words, if you have two 100-GB LUNs in the same volume with fractional reserve set to 30, one of the LUNs could use up the entire 60 GB of reserved space for that volume.
Refer to the Technical Report on thin provisioning below for more detailed information on using fractional reserve .
