FS union

Lou Grinzo (lgrinzo@stny.lrun.com)
Wed, 16 Jun 1999 08:32:57 -0400


One possibility that I don't think anyone has mentioned is
giving the user or programmer finer-grained control over the
name space for the unioned FS's. In other words, there could
easily be times when you would want to make just certain files
in the underlying FS take precedence in the name space.

The basic model talked about so far, of "last mounted, first
found" is good. But adding a way (from the CL as well as
a sys call) to explicitly say "if fred.c is in the underlying FS,
make it take precedence over the fred.c, if any, in the top
FS" would be very handy. Imagine being able to union the
source trees for two versions of a project, and selectively
use some files from the underlying project's FS in the top
project, without having to cp source files or change make
files, for example.

You'd want a corresponding way to undo this, and "sink"
fred.c again, as well as some other tools or options on
existing tools, to help the disambiguate the name space.

One other thing - has a decision been made about the
number of FS's that can be unioned (unified?) at once?
Everyone keeps talking about two FS's in examples (as
I did above), but is there a technical reason to limit it to
two, or will it be more?

Lou

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