Best choice for copy/clone/snapshot

From: Ross Boylan <ross@biostat.ucsf.edu>
To: kvm@vger.kernel.org
Cc: ross@biostat.ucsf.edu
Subject: Best choice for copy/clone/snapshot
Date: Tue, 12 May 2009 18:08:25 -0700	[thread overview]
Message-ID: <1242176905.15140.38.camel@iron.psg.net> (raw)

First, I have a feeling this might be a question I could ask on a qemu
list.  Is there a way for me to tell which questions should go where?
Is it OK to ask here?

As I install software onto a system I want to preserve its state--just
the disk state---at various points so I can go back.  What is the best
way to do this?

First, I think I could just make a copy of the virtual disk, although I
haven't seen this suggested anywhere.  I assume this will work if the VM
is off; are there other circumstances in which it is safe?  Since my
original virtual disk file isn't really occupying its nominal space, I
assume this will be true of the copy too.

Second, kvm-img could create a copy on write image.  There are several
things I don't understand about this.  Suppose I go
kvm-img -b A.img  B.img

If I then go on and use A.img as I did before, changing what is on disk,
have I screwed up B.img?

Do A.img or B.img have to be qcow2 format?  I created a raw image for
portability.

Suppose I work for awhile installing new stuff on B.img, and then want
to preserve the state.  Is
kvm-img -b B.img C.img
sensible, or is this kind of recursive operation (B.img is already the
copy on write version of A.img) not OK?

Does ‘commit [-f fmt] filename’, documented as
        Commit the changes recorded in filename in its base image.
mean commit the recorded changes TO its base image?

Here are some other things I think I don't want to do.  Please let me
know if I'm mistaken.

-snapshot on the kvm command line: nothing persistent comes of this
(maybe if you commit you update the original image, but you don't get
2).

snapshot in the monitor: this snapshots the non-disk state of the VM;
further, that state is not guaranteed to work if you later change what
is on the disk.  I think kvm-img snapshot also accesses these
facilities.

Yours in confusion :)
Ross

P.S. Please cc me.
-- 
Ross Boylan                                      wk:  (415) 514-8146
185 Berry St #5700                               ross@biostat.ucsf.edu
Dept of Epidemiology and Biostatistics           fax: (415) 514-8150
University of California, San Francisco
San Francisco, CA 94107-1739                     hm:  (415) 550-1062