23.8.8.2 Snapshot Caveats

VC's snapshot facilities are modeled on RCS's named-configuration support. They use RCS's native facilities for this, so snapshots made using RCS through VC are visible even when you bypass VC.

With CVS, Meta-CVS, and Subversion, VC also uses the native mechanism provided by that back end to make snapshots and retrieve them (tags for CVS and Meta-CVS, copies for Subversion).

For SCCS, VC implements snapshots itself. The files it uses contain name/file/version-number triples. These snapshots are visible only through VC.

There is no support for VC snapshots using GNU Arch yet.

A snapshot is a set of checked-in versions. So make sure that all the files are checked in and not locked when you make a snapshot.

File renaming and deletion can create some difficulties with snapshots. This is not a VC-specific problem, but a general design issue in version control systems that no one has solved very well yet.

If you rename a registered file, you need to rename its master along with it (the command vc-rename-file does this automatically). If you are using SCCS, you must also update the records of the snapshot, to mention the file by its new name (vc-rename-file does this, too). An old snapshot that refers to a master file that no longer exists under the recorded name is invalid; VC can no longer retrieve it. It would be beyond the scope of this manual to explain enough about RCS and SCCS to explain how to update the snapshots by hand.

Using vc-rename-file makes the snapshot remain valid for retrieval, but it does not solve all problems. For example, some of the files in your program probably refer to others by name. At the very least, the makefile probably mentions the file that you renamed. If you retrieve an old snapshot, the renamed file is retrieved under its new name, which is not the name that the makefile expects. So the program won't really work as retrieved.