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Regarding how deltas are stored in CVS

From: Doug Lee
Subject: Regarding how deltas are stored in CVS
Date: Fri, 23 Apr 2004 17:20:59 -0400
User-agent: Mutt/1.5.4i

It was mentioned on another thread that CVS stores the HEAD revision
in its entirety, older trunk revisions as backward deltas from the
HEAD, and branches as forward deltas from the branch point.

If I read this right, a common case will take a lot more space than I
thought.  My current project is an example:  I was just asked to go
back and apply some current features to an old revision of a set of
files.  When I did that, I put in as much of the current code as I
could without allowing any code to go in that could have side effects
outside those intended.  Some files I could literally copy from the
HEAD revision (though I did it via the appropriate merge commands);
others came close but not quite there, and of course still others
remain very different from the head of the trunk.

But I think what people are saying is that a copy of the head of
the trunk, when placed on a branch, will basically create a
reverse-duplicate set of deltas (perhaps condensed into a smaller
number of them of course) to the set between the head of the trunk
and the branch point.  Somehow I'd been assuming that CVS would
maybe notice that the head of my branch and the head of the trunk
produced identical contents for some files and might only store one

I don't yet know enough about the RCS file format, but I wonder if the
format itself would support deltas forming a different graph than is
currently formed, to optimize file size and/or number of deltas from
point to point.

Doug Lee           address@hidden
Bartimaeus Group   address@hidden
"I before E, except after C, or when sounded like A, as in neighbor
and weigh, except for when weird foreign concierges seize neither
leisure nor science from the height of society."

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