[Libguestfs] [PATCH v4 0/3] v2v: Add -o rhv-upload output mode.

[Richard W.M. Jones]

On Thu, Mar 08, 2018 at 12:13:01PM +0000, Nir Soffer wrote:
> On Wed, Mar 7, 2018 at 12:18 AM Richard W.M. Jones <rjones at redhat.com>
> wrote:
> 
> > Previous versions:
> >   v3: https://www.redhat.com/archives/libguestfs/2018-March/msg00000.html
> >   v2:
> > https://www.redhat.com/archives/libguestfs/2018-February/msg00177.html
> >   v1:
> > https://www.redhat.com/archives/libguestfs/2018-February/msg00139.html
> >
> > This completely rethinks the approach taken by the previous patches.
> >
> > Instead of trying to involve qemu's curl driver, this uses a small
> > Python 3 nbdkit plugin to interface between qemu and the oVirt server.
> >
> > The data path is:
> >
> >   qemu-img convert  ------->   nbdkit   ------->   oVirt imageio
> >                       nbd                 https
> >
> 
> What is the advantage of this for raw files? Why not:
> 
>     v2v -> ovirt imageio?

Not sure I understand what you mean?  virt-v2v always runs ‘qemu-img
convert’ to do the copy (not conversion), so the question is how do we
connect qemu-img to the oVirt server.  One way would be to extend qemu
so it knows how to write on an https connection (which it does not do
now) but that has a number of disadvantages, as well as being hard to
implement.

> And how qcow2 files will be handled?

We'll add ‘-O qcow2’ to the qemu-img convert command line, and qemu
will then write out a qcow2 file.  However it's not quite so
straightforward (and in fact I didn't get it to work yet).  qemu will
try to first read from the target (invoking pread calls in the nbdkit
Python plugin which will try to read from oVirt over https).
Unfortunately it fails here for a couple of reasons:

(1) My pread method is broken.  I saw your suggested fixes to it (and
pwrite) and will try those later.

(2) In any case it won't work because the disk at this point is empty
and full of zeroes, and it's looking for a qcow2 header.  To fix this
we'll have to write a qcow2 header to the disk first (TBD).

> when I tried nbdkit few month ago I could not make it handle qcow2
> files. Maybe I had to write a plugin?

NBD (the protocol) doesn't "know" about qcow2 files.  You can serve
any file you want as a range of bytes, including qcow2, but that
requires whatever is consuming those bytes to then do the qcow2
en-/decoding.  (Which means effectively the client has to be qemu
because nothing else can parse qcow2 reliably).  In the qemu-img
convert case above this all works because qemu-img (ie. qemu) is the
client, and it does the encoding of qcow2, and we're just shuffling a
byte stream to oVirt imageio.

> We considered using this flow when we download/upload images,
> to support on-the-fly image conversion:
> 
>     raw file -> qemu-img convert -> nbdkit -> qcow2 stream -> imageio ->
> http client
> 
> And same for uploading, e.g. uploading qcow2 and writing raw image.
>
> If this is possible using nbdkit plugin, can we ruse the same plugin in
> different applications, or we must implement the plugin in each application?

This should be possible (modulo fixing issues (1) & (2) above).  The
plugin I have written is very specific to the virt-v2v task, but it
could be evolved into something which would handle this case.

nbdkit is designed around the idea that you can make small plugins in
familiar scripting languages for specific tasks.

I did think about having a generic "oVirt plugin" which we'd ship with
upstream nbdkit, but making it generic enough to handle a useful range
of cases seemed difficult.

> > There are two Python scripts included.  One is the nbdkit plugin.  The
> > other creates the VM.  As with the previous patches, these scripts get
> > embeded in virt-v2v at compile time, so effectively there is no API
> > contract between virt-v2v & the Python code.
> >
> > With this patch series I am able to (mostly) successfully convert VMs
> > from local disk to oVirt 4.2, with full end-to-end streaming.  There
> > is some room for optimization -- in particular uploads are currently
> > rather slow because we rely on qemu-img batching small requests into
> > large ones which it doesn't do well, and instead the nbdkit plugin
> > could batch small writes into larger ones.  Also I noticed (but only
> > one time) that very long transfers would cause the oVirt ticket to
> > expire, even though we were writing the whole time.
> >
> 
> On the host, the ticket is extended regularly, based on the activity.
> 
> On the proxy we currently have 3600 seconds timeout, and the ticket
> is never extended. I think we should have the same mechanism as
> we do on the host.

I have only seen this once and never again, so hopefully it was just a
network blip causing a > 3600 second timeout.

If it happens again I'll take a closer look.

Thanks for the review, it does seem like pwrite is rather broken.
Unfortunately my ovirt node hangs hard when I try to boot any guest
(seems like a kernel or even hardware bug) so I have never been able
to test that the transferred guest works :-(

Rich.

> Nir
> 
> There are still a few unresolved issues (see patch 3/3) so this is not
> > quite ready to go upstream yet, but can still be reviewed.  Patches 1
> > & 2 are the same as posted before.
> >
> > I did not yet test qcow2 uploads.  Those are "interestingly" different
> > because qcow2 will require us to read from the remote oVirt server as
> > well as just stream/write to it.  The pread method for that is written
> > but has not been tested.
> >
> > Rich.
> >
> > _______________________________________________
> > Libguestfs mailing list
> > Libguestfs at redhat.com
> > https://www.redhat.com/mailman/listinfo/libguestfs
> >

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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