Fractional reserve is an attribute of a volume that enables you to determine how much space Data ONTAP reserves for overwrites for LUNs and space-reserved files.
The fractional reserve setting is expressed as a percentage.
Data ONTAP provides several features that use block sharing, including Snapshot copies, deduplication, compression, and FlexClone operations. When one or more of these features is in use, overwrites to that volume consume more space because the original block might be in use.
When fractional reserve is set to 100 percent (for a volume with a guarantee of volume or file), Data ONTAP ensures that there is always enough space in the volume for all of the data in all space-reserved LUNs or files in that volume to be completely overwritten. If there is insufficient space to support the use of a block-sharing feature, you are prevented from doing so--but you are always able to overwrite all space-reserved files and LUNs.
Setting fractional reserve to less than 100 percent causes the space held for overwrites to all space-reserved files and LUNs in that volume to be reduced to that percentage. Writes to the space-reserved files and LUNs in that volume are no longer unequivocally guaranteed.
Reducing the fractional reserve percentage does not affect the size of a space-reserved LUN. You can write data to the entire size of the LUN.
When more than one LUN in the same volume has 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.
Fractional reserve is generally used for volumes that hold files or LUNs with a small percentage of data overwrite.
If you create a 500-GB reserved LUN in a volume with a guarantee type of volume, 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 0 for the LUN's containing volume and then take a Snapshot copy, which locks the used blocks in the LUN, then Data ONTAP does not reserve any space in the volume for future overwrites to the LUN. Any subsequent overwrites to the LUN could fail due to insufficient free space in the volume, unless you configure the volume to automatically provide more free space, and provide sufficient free space in the aggregate.