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Re: [Libguestfs] [PATCH v2 4/4] v2v: reuse main guestfs for virtio win drivers iso



On Tue, Oct 13, 2015 at 02:36:10PM +0100, Richard W.M. Jones wrote:
> On Tue, Oct 13, 2015 at 03:50:54PM +0300, Roman Kagan wrote:
> > If we're given an ISO image as the source of virtio windows drivers, and
> > the backend supports hot-adding drives, reuse the same guestfs handle
> > that is open for accessing the guest being inspected, rather than
> > creating a new one (which would run another virtual machine instance and
> > is fairly resource-intensive).
> 
> I really don't like this specific patch.  Hotplugging is not very
> reliable in my experience.  The patch tries to work out if hotplugging
> is going to work by calling guestfs_add_drive and taking an alternate
> path, but I'm not so convinced that would work on other architectures
> (eg. hotplugging on aarch64 is in various states between not working
> at all and flaky).
> 
> Are you observing any benefit with this patch?  v2v conversion takes a
> long time - I can't believe the extra overhead of creating a handle
> could make much difference.

Frankly, I don't have convincing numbers to justify this patch.  In my
measurements with test conversions on an overwise idle workstation gives
on the order of 5% gain where statistical variance is just a little
less; OTOH LIBGUESTFS_BACKEND=direct gives statistically significant
30-40% improvement :)

We've been thinking of doing mass VM conversion where an extra qemu
instance for every v2v operation would create significant memory and io
overhead but I don't have a proof of that.

In view of hot-add being unstable as you say, I'm fine with dropping
this patch altogether.

Thanks,
Roman.


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