File system snapshots are inexpensive and easy to create in ZFS. Most likely, snapshots will be common in most ZFS environments. For information about ZFS snapshots, see Chapter 7, Working With ZFS Snapshots and Clones.
The presence of snapshots can cause some unexpected behavior when you attempt to free space. Typically, given appropriate permissions, you can remove a file from a full file system, and this action results in more space becoming available in the file system. However, if the file to be removed exists in a snapshot of the file system, then no space is gained from the file deletion. The blocks used by the file continue to be referenced from the snapshot.
As a result, the file deletion can consume more disk space, because a new version of the directory needs to be created to reflect the new state of the namespace. This behavior means that you can get an unexpected ENOSPC or EDQUOT when attempting to remove a file.