What the Snapshot reserve is

The Snapshot reserve specifies a set percentage of disk space for Snapshot copies. By default, the Snapshot reserve is 20 percent of disk space.

The Snapshot reserve can be used only by Snapshot copies, not by the active file system. This means that if the active file system runs out of disk space, any disk space still remaining in the Snapshot reserve is not available for active file system use.

Note: Although the active file system cannot consume disk space reserved for Snapshot copies, Snapshot copies can exceed the Snapshot reserve and consume disk space normally available to the active file system.

Snapshot reserve management tasks
Snapshot reserve management involves the following tasks:
Using deleted active file disk space
When enough disk space is available for Snapshot copies in the Snapshot reserve, deleting files in the active file system frees disk space for new files, while the Snapshot copies that reference those files consume only the space in the Snapshot reserve.
Snapshot copies can exceed reserve
There is no way to prevent Snapshot copies from consuming disk space greater than the amount reserved for them; therefore, it is important to reserve enough disk space for Snapshot copies so that the active file system always has space available to create new files or modify existing ones.
Recovering disk space for file system use
Whenever Snapshot copies consume more than 100% of the Snapshot reserve, the system is in danger of becoming full. In this case, you can create files only after you delete enough Snapshot copies.

Parent topic: What Snapshot disk consumption is