[Libguestfs] virt-sysprep future

[Richard W.M. Jones]

On Mon, Mar 26, 2012 at 03:59:04PM +0800, Wanlong Gao wrote:
> I'd like to know when will you work out the virt-sysprep in Ocaml ? :-D

I meant to write something about our investigations last week.
Here we go ...

Requirements:

* Compatible with the current virt-sysprep, but easier to maintain and
  faster.

* It should work in the "local" and over-libguestfs cases, which means
  using FUSE.

* The current version is a shell script, so it's customizable by
  simple editing.  I'd like to keep some customizability in the new
  version.

These requirements could all be satisfied if we had:

* Some kinds of "mount-local" ability, ie. integrating FUSE with the
  API in some way.  [i]

* Custom scripts:
    virt-sysprep --script foo.sh  # runs foo.sh on the guest

The problem is how to do the "mount-local".  We analyzed this last
week on IRC and came up with two possible methods:

(1) We add FUSE support directly to the API, linking with libfuse.
This is essentially the same as what was outlined in link [i] back in
December 2011.

(2) We add an attach-method "fd:<N>" which allows the file descriptor
of the appliance to be passed over to another process.  In this case
we would allow guestmount to be passed a file descriptor (eg.
"guestmount --attach-fd=<NN>") which it would then use exclusively.

The problem with (1) is that we are not confident that it's a good
idea to use libfuse as a true library.  It doesn't appear that anyone
else is using it this way.

The problem with (2) is that it's complex and isn't future compatible
with remote libvirt support [ii].

I suspect that we'll need to implement both, and be prepared to throw
away a few implementations, before we know really which is the best
way to go about this.

Rich.

[i] https://www.redhat.com/archives/libguestfs/2011-December/thread.html#00136

[ii] https://www.redhat.com/archives/libvir-list/2011-September/msg01049.html

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org