How reducing fractional reserve affects available space

When you can reduce fractional reserve

You can reduce fractional reserve to less than 100 percent for traditional volumes or for volumes that have the guarantee option set to volume.

What happens when the fractional reserve option is set to 50 percent

The following example shows how a fractional reserve setting of 50 percent affects available space in the same 1-TB volume with a 500-GB LUN.

Stage	Status
1	The following illustration shows a 1-TB volume with a 500-GB LUN after 200 GB of data are written to the LUN. The volume has 100 GB intended for overwrite reserve because the fractional reserve for this volume is set to 50 percent.
2	The following illustration shows the volume with an additional 300 GB of other data. The volume still has 100 GB of free space, which means there is space for one of the following: Writing up to 200 GB of new data to the LUN and maintaining the ability to take a Snapshot copy Writing up to 100 GB of other data and maintaining the ability to take a Snapshot copy Compare this example with the volume shown in Example 2, in which the same volume has an overwrite reserve of 100 percent, but the volume has run out of free space.

Stage

Status

The following illustration shows a 1-TB volume with a 500-GB LUN after 200 GB of data are written to the LUN. The volume has 100 GB intended for overwrite reserve because the fractional reserve for this volume is set to 50 percent.

The following illustration shows the volume with an additional 300 GB of other data. The volume still has 100 GB of free space, which means there is space for one of the following:

Writing up to 200 GB of new data to the LUN and maintaining the ability to take a Snapshot copy
Writing up to 100 GB of other data and maintaining the ability to take a Snapshot copy

Compare this example with the volume shown in Example 2, in which the same volume has an overwrite reserve of 100 percent, but the volume has run out of free space.

How a volume with fractional overwrite reserve runs out of free space

The following example shows how the volume might run out of space when the fractional reserve option is set to 50 percent.

Stage	Status
1	The following illustration shows a 1-TB volume with a 500-GB LUN after you write 500 GB to the LUN and then take a Snapshot copy. The volume has 250 GB reserved for overwrites to the LUN and 250 GB available for other data.
2	The following illustration shows that you have 50 GB of free space after you write 200 GB of other data (for example, files) to the volume. You try to write more than 300 GB of data to the LUN, and the write fails. The volume has 50 GB of free space plus 250 GB of space reserved for overwrites to the LUN. The volume has enough space for you to write no more than 300 GB of data to the LUN.