after reading the (old) chapter on shared filesystems, some
admittedly basic questions on COW files. (i'm fairly sure i know the
answers here but i just want to play it safe.)
1) each filesystem i use as part of my UML session can have its own
COW file, right? as in:
... ubda=root.cow,root.img ubdc=usr.cow,usr.img ... etc etc ...
2) once i establish a COW file for a given backing file, i can use
"chmod" to protect that backing file from inadvertant changes, yes?
that seems to be the case based on my testing.
3) is there a way to "examine" the contents of a COW file? i'm
reminded of the "minifo" (file overlay) filesystem -- is there a way
to peek inside a COW file to see some sort of file-based "diff"
against the backing file?
4) is there a way to re-synchronize a COW file against a changed
backing file?
regarding 4), here's my thinking. let's say i had what i thought
was a good root filesystem and i set up a number of guest UML sessions
against it using COW files for each guest. later, i realize there's a
critical file missing from that root FS.
i *could* add the missing file individually to each guest, of
course, but it would be nicer to be able to add it to the single root
FS and simply reconcile all those existing COW files to a slightly new
root FS (under the assumption that none of those COW files would ever
have made changes in the area that the new file is going into). can
that be done? (i'm guessing no.)
and if not, that's why i asked about multiple filesystems. if i
wanted a number of guests to share, say, /usr, i'd introduce that as a
separate filesystem, protect it with read-only, then just let each
guest mount it read-only. that way, if i realized i'd forgotten an
important executable or shared lib, i'd just stop the guests, mount
and modify the usr.img file, then start the guests again, right?
rday
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry:
Have classroom, will lecture.
http://crashcourse.ca Waterloo, Ontario, CANADA
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