How 100 percent fractional reserve affects available space

What happens when the fractional overwrite option is set to 100 percent

The following example shows how the default fractional reserve setting of 100 affects available space in a 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 200 GB of space intended for overwrite reserve. This space is actually reserved only when you take a Snapshot copy by using either the `snap` command or snapshot-methods, such as SnapMirror. For example, if you take a Snapshot copy in the volume shown in the illustration, the original 200 GB of data in the LUN are locked in the Snapshot copy. The reserve space guarantees that you can write over the original 200 GB of data inside the LUN even after you take the Snapshot copy. It guarantees that an application storing data in the LUN always has 500 GB of space available for writes.
2	The following illustration shows that the volume still has enough space for the following: 500-GB LUN (containing 200 GB of data) 200 GB intended reserve space for overwrites An additional 200 GB of other data At this point, there is enough space for one Snapshot copy.

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 200 GB of space intended for overwrite reserve. This space is actually reserved only when you take a Snapshot copy by using either the snap command or snapshot-methods, such as SnapMirror.

For example, if you take a Snapshot copy in the volume shown in the illustration, the original 200 GB of data in the LUN are locked in the Snapshot copy. The reserve space guarantees that you can write over the original 200 GB of data inside the LUN even after you take the Snapshot copy. It guarantees that an application storing data in the LUN always has 500 GB of space available for writes.

The following illustration shows that the volume still has enough space for the following:

500-GB LUN (containing 200 GB of data)
200 GB intended reserve space for overwrites
An additional 200 GB of other data

At this point, there is enough space for one Snapshot copy.

How the volume runs out of free space

The following two examples show how the volume might run out of free space when the fractional overwrite option is set to 100 percent.

Example 1

Stage	Status
1	The following illustration shows the 1-TB volume with a 500-GB LUN that contains 200 GB of data. There are 200 GB intended for overwrite reserve. At this point, you have not taken a Snapshot copy, and the volume has 500 GB of available space.
2	The following illustration shows the volume after you write 400 GB of other data. Data ONTAP reports that the volume is full when you try to take a Snapshot copy. This is because the 400 GB of other data does not leave enough space for the intended overwrite reserve. The Snapshot copy requires Data ONTAP to reserve 200 GB of space, but you have only 100 GB of available space.

Example 2

Stage	Status
1	A 1-TB volume has a 500-GB LUN that contains 200 GB of data. There are 200 GB of intended reserve space in the free area of the volume.
2	The following illustration shows the volume with a Snapshot copy. The volume has 200 GB reserved for overwrites to the original data and 300 GB of free space remaining for other data.
3	The following illustration shows the volume after you write 300 GB of other data (not in the LUN) to the volume. The volume reports that it is full because you have used all free space, but you can write data to the LUN indefinitely.
4	The following illustration shows the volume after you write another 100 GB of data to the LUN. At this point, the volume does not have enough space for another Snapshot copy. The second Snapshot copy requires 300 GB of reserve space because the total size of the data in the LUN is 300 GB.