Fractional reserve is a volume option that enables you to determine how much space Data ONTAP reserves for Snapshot copy overwrites for LUNs, as well as for space-reserved files when all other space in the volume is used.
The fractional reserve setting defaults to 100%, but you can use the vol options command to set fractional reserve to any percentage from zero to 100.
It is best 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 or
- FlexVol volumes with a space guarantee of or none. You can only set fractional reserve for a volume with a space guarantee of none with Data ONTAP version 7.3.3 and later and version 8.0.1 and later.
Note: If the
guarantee option for a FlexVol volume is set to
none or
volume, then fractional reserve for that volume can be set to the desired value. For the vast majority of configurations, you should set fractional reserve to zero when the
guarantee option is set to
none because it greatly simplifies space management. 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.
If fractional reserve is set to 100%, 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, which is why you should use snap autodelete or vol autogrow for these volumes.
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.
See the Technical Report on thin provisioning for detailed information on using fractional reserve .
