If you have enabled space reservation for files or LUNs, you can reduce the size of the reserved space by using fractional reserve. You set fractional reserve using the vol options command.
Fractional reserve is an option on the volume, and it 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 files or LUNs, you can be sure that writes to those files or LUNs will always succeed, even if all of the space-reserved files or LUNs are completely overwritten.
Setting fractional reserve to less than 100 percent causes the space reservation held for all space-reserved files in that volume to be reduced to that percentage. Writes to the space-reserved files 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 or file itself. You can write data to the entire size of the LUN or file. 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 releases the reservation on 250 GB, or half of the space it was previously reserving for overwrites. 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.
Any files or LUNs in that volume that do not have space reservations enabled do not contribute to the amount of space that is reserved for the volume. For example, if you created a 10-GB file in the same volume but did not enable space reservations on that file, no further space would be reserved. If you later enabled space reservations on that file, an extra 5 GB would be reserved (because fractional reserve was already set to 50 percent).
Note: When more than one file or LUN in the same volume have space reservations enabled, and fractional reserve for that volume is set to less than 100 percent, Data ONTAP does not limit any space-reserved file or 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.
