Re: [9fans] caching/overlay/union filesystem

[Richard <ru@ohio.river.org>]

rob pike writes:
>	A user could have the distribution source on a CDROM and view it via a
>	cache file
>	server. This would store only the files that have changed - using copy
>	on write.
>	I suspose this could be extended to storing only the blocks that have
>	changed, or
>	even file diff(1) output, or wrap(8) file even...
>
>This is the essence of what BSD calls union directories.

and what Linux calls the Overlay File System, which hasnt been updated
for 2 years.
http://home.att.net/~artnaseef/ovlfs/ovlfs.html

Russ Cox writes:

>If you had such a thing, you could try out
>new wrap updates by doing
>
>	gunzip < /tmp/new.9gz >/tmp/new.9
>	archfs /tmp/new.9
>	stitch -b /tmp/new.9 /

another applications: many of us working at home get most of 
our software off of CDs.  it is wasteful to  back up this
stuff because one can install it again from CD very easily and
the CD's are even replaceable in case one loses one.

and yet most of us have made small changes and additions to this stuff.
most of those changes and additions can be restricted to one's home
directory or /etc and thus be kept separate from the stuff from CD, but
some changes have to go into eg /usr/lib/terminfo/ (excuse the Unixiness
of this example).

by keeping the "underlying" filesystem "pristine" (like it was right
after installing from the CD) and by keeping all local changes in an
overlay filesystem, one can back up just the local changes.  this makes
backup archives much much smaller, often letting you get by with backing
up onto zip drives or CDRWs or even to the Internet rather than to tape
(saving the cost of a tape drive).