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.

Example

Consider what would happen in the example if all files in the active file system were deleted. Before the deletion, the df output was as follows:

Filesystem 	        kbytes  used    avail  capacity
/vol/vol0/          3000000 3000000 0       100%
/vol/vol0/.snapshot 1000000 500000  500000   50%

After the deletion, the df command generates the following output:

Filesystem          kbytes  used    avail  capacity
/vol/vol0/          3000000 2500000 500000   83%
/vol/vol0/.snapshot 1000000 3500000 0       350%

The entire 3,000,000 KB (3 GB) in the active file system is still being used by Snapshot copies, along with the 500,000 KB (0.5 GB) that were being used by Snapshot copies before, making a total of 3,500,000 KB (3.5 GB) of Snapshot data. This is 2,500,000 KB (2.5 GB) more than the space reserved for Snapshot copies; therefore, 2.5 GB of space that would be available to the active file system is now unavailable to it. The post-deletion output of the df command lists this unavailable space as “used” even though no files are stored in the active file system.

Parent topic: What the Snapshot reserve is