Re: [libvirt] libguestfs integration: rich disk access for libvirt applications
On Tue, Sep 27, 2011 at 12:20:31PM +0100, Daniel P. Berrange wrote:
One other point worth mentioning is that libguestfs.so does not want
to directly link to libvirt.so, and vica-verca, to ensure we both
avoid pulling major new dependancy chains for all users.
Actually libguestfs.so in Fedora links to libvirt.so today. What we
don't want is libvirt to be required *for libguestfs to compile*.
At the moment if libvirt is not available, we disable one API at
compile time (using #ifdef HAVE_LIBVIRT etc). I don't this discussion
is affected by this.
If I were ignoring the requirement that libguestfs does not link to
libvirt, then you could quite likely make all this happen with only
a simple additional API in libvirt. We need an API to let a client
open a connection to a <channel> device, using the virStreamPtr
API.
If the guests were not running, libguestfs would use virDomainCreate
to spawn a transient, auto-detroy guest, with a custom kernel/initrd
that runs the appliance, and an additional <channel> device, but with
all other parts of the guest XML unchanged. This would ensure all the
lock manager, sVirt and secret stuff 'just works'. If the guest is
already running, libguestfs would just query the XML to find the
<channel> device configuration. Then it could just use a new API
like virDomainOpenChannel(virStreamPtr, const char *channelid) to
get a stream to talk to the guestfs daemon with.
I'm with you up to here, but there's a practical problem: How do we
create the appliance kernel/initrd/root disk on the server side? (I'm
assuming that libvirt doesn't forward these large objects from the
client to the server.) Normally these objects are created by running
febootstrap-supermin-helper.
To do this I would create what I call a bridging library, to be
named 'libvirt-guestfs.so'.
See above, although we have converged on similar designs, but for
different reasons.
FWIW as outlined in the other email, I think we can do this without a
bridging library, and just making changes behind the scenes in
guestfs_add_domain[1], which would be transparent to callers.
Rich.
[1] http://git.annexia.org/?p=libguestfs.git;a=blob;f=src/virt.c;hb=HEAD
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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