Re: [PATCH 2/2] nbd: Add new qemu:joint-allocation metadata context

[Eric Blake <eblake@redhat.com>]

On Thu, Jun 10, 2021 at 04:16:27PM +0300, Nir Soffer wrote:
> On Thu, Jun 10, 2021 at 2:52 AM Nir Soffer <nsoffer@redhat.com> wrote:
> >
> > On Wed, Jun 9, 2021 at 9:01 PM Eric Blake <eblake@redhat.com> wrote:
> 
> I posted a work in progress patch implementing support for
> qemu:joint-allocaition
> in oVirt:
> https://gerrit.ovirt.org/c/ovirt-imageio/+/115197
> 
> The most important part is the nbd client:
> https://gerrit.ovirt.org/c/ovirt-imageio/+/115197/1/daemon/ovirt_imageio/_internal/nbd.py
> 
> With this our tests pass with qemu-nbd build with Eric patch:
> https://gerrit.ovirt.org/c/ovirt-imageio/+/115197/1/daemon/test/client_test.py

Note that you _don't_ have to use qemu:joint-allocation; you could get
the same information by using the already-existing
qemu:allocation-depth.  But your patch DOES make it obvious that it
was a lot simpler to use a single context (the bulk of your tweak is
trying an additional NBD_OPT_SET_META_CONTEXT), than it would be to
handle parallel contexts (where you would have to tweak your
NBD_CMD_BLOCK_STATUS handler to cross-correlate information).

> 
> We may need to use qemu:joint-allocation only for qcow2 images, and
> base:allocation
> for raw images, because allocation depth reporting is not correct for
> raw images. Since

Allocation depth (aka how deep in the backing chain is data allocated)
IS correct for raw images (the raw image IS the source of all data);
it is just not useful.

> we control the qemu-nbd in both cases this should not be an issue. But
> it would be
> better if allocation depth would work for any kind of image, and we always use
> qemu:joint-allocation.

Maybe the thing to do is improve the documentation and try to avoid
ambiguous terminalogy; in qemu:allocation-depth, a return of depth 0
should be called "absent", not "unallocated".  And in libnbd, a
base:allocation of 0 should be "data" or "normal", not "allocated".
But I don't see how qemu will ever advertise an allocation depth of 0
for a raw image, because raw images have no backing chain to defer to.
If you are trying to recreate a sparse raw image, then you really do
want to pay attention to NBD_STATE_HOLE and NBD_STATE_ZERO, as those
are more accurate pictures of the properties of extents within the raw
file.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org