ClearCase was basically a spinoff of Apollo's DSEE (same engineers), which didn't need kernel patches, because Domain/OS actually had an object filesystem and s

ClearCase was basically a spinoff of Apollo's DSEE (same engineers), which didn't need kernel patches, because Domain/OS actually had an object filesystem and so the reference-to-repo was just a filesystem object. (As with ClearCase, it wasn't so much the software cost as the 1-3 FTEs you needed to be sacrificial experts to tune and manage it.) (insert "mimic a fraction of our power" meme here, though that applies to a lot of Apollo stuff :-) RCS had tags but they were single file tags; the two big things CVS gave you were "this collection of RCS files means something as a whole" (which made tags and branches meaningful) and "you can sort-of-efficiently update from a repo on the other side of the country". (The former was the entire point: CVS was written so Solbourne could maintain their multi-cpu-SunOS fork and still pull changes from upstream; the latter was from Cygnus having a similar upstream+local problem and developers distributed across the planet.)