COW may also be used as the underlying mechanism for snapshots, such as those provided by logical volume management, file systems such as Btrfs and ZFS, and database servers such as Microsoft SQL Server. Typically, the snapshots store only the modified data, and are stored close to the main array, so they are only a weak form of incremental backup and cannot substitute for a full backup. Some systems also use a COW technique to avoid the fuzzy backups, otherwise incurred when any file in the set of files being backed up changes during that backup.