RCS and SCCS (and CMS)

[g-rh at cca.UUCP]

	... Re the usual argument for RCS and what SCCS really does.

>There are several barely relevant performance implications of the
>differences between this scheme and what RCS does:
>
>    + for RCS:  For retrieving the latest version (RCS is betting that
>	this is the most common case), RCS is likely to be faster;

	Actually, if you are using the RCS type scheme, you can gain
a real factor of speed by storing the raw latest file by itself since
file extraction is a straight copy -- this can be a big win if the
hardware supports DMA.  However RCS doesn't do this.  There are arguments
for not doing this -- if anybody mucks with the raw file you have just
bitten the big wienie.

>    + for SCCS:  Doing RCS check-ins (storing new versions) should be
>	distinctly slower than doing SCCS deltas;

	I see no reason for this statement -- in either case you need
to (a) extract the comparison version, and (b) do the difference.  This
part is the same with RCS having an edge on extraction.  You then have to
rewrite the history controlled file.  In the SCCS scheme you rewrite the
entire interleaved file.  In the RCS scheme you write the new version and
the deltas; these occupy the same space (more or less).  There is a potential
gain in the RCS scheme in that the rewrite can be done as concatenations.

>This is one of the few areas where VMS (gasp!) really does do better
>than Unix (IMHO) -- DEC's CMS (though it has brain damage in some ways)
>really does have some features that would make it, on the whole, better
>than either SCCS or RCS, if you could use it on Unix instead of VMS.

	CMS is intermediate between simple version control systems (which
operate on a single file, e.g. RCS and SCCS) and real configuration manage-
ment systems which have means for dealing with collections of files.  It
has a modicum of CM features.  It also has a simple merge facility.  The
original CMS implementation used a base plus all successive deltas and had
horrid problems with speed; the current implementation is decent.  
-- 

In the fields of Hell where the grass grows high
Are the graves of dreams allowed to die.
	Richard Harter, SMDS  Inc.