file or directory is removed, or when an existing file is opened with truncation. Non-current versions remain in the filesystem namespace, under the name of the original file but with a suffix attached consisting of a semicolon and version sequence number. All but the current version are hidden from directory reads (unless the SHOWVERSIONS environment variable is set), but versions are otherwise accessible for all normal operations. The environment variable and general accessibility allow versions to be managed with the usual filesystem utilities, though there is also an "undelete" command that can be used to purge and restore files, enable and disable versioning on directories, etc. Others Subversion has a feature called "autoversioning" where a WebDAV source with a subversion backend can be mounted as a file system on systems that support this kind of mount (Linux, Windows and others do) and saves to that file system generate new revisions on the revision control system. The commercial Clearcase configuration management and revision control software has also supported "MVFS" (multi version file system) on HP-UX, AIX and Windows since the early 1990s. Related software The following are not versioning filesystems, but allow similar functionality. APFS and ZFS support instantaneous snapshots and clones. Btrfs supports snapshots. HAMMER in DragonFlyBSD has the ability to store revisions in the filesystem. NILFS, which supports snapshotting. Plan 9's Fossil file system can provide a similar feature, taking periodic snapshots (often hourly) and making them available in . Fossil can forever archive a snapshot into Venti (usually one snapshot each day) and make them available in . If multiple changes are made to a file during the interval between snapshots, only the most recent will be recorded in the next snapshot. Write Anywhere File Layout - NetApp's storage solutions implement a file system called WAFL, which uses snapshot technology to keep different versions of all files in a volume around. pdumpfs, authored by Satoru Takabayashi, is a simple daily backup system similar to Plan 9's /n/dump, implemented in Ruby. It functions as a snapshotting tool, which makes it possible to copy a whole directory to another location by using hardlinks. Used regularly, this can produce an effect similar to versioning. Microsoft Windows Shadow Copy - is a feature introduced by Microsoft with Windows Server 2003. Shadow Copy allows for taking manual or automatic backup copies or snapshots of a file or folder on a specific volume at