Re: qemu-nbd CMD_BLOCK_STATUS returns one extent in CentOS 8.0, all extents in 4.1.0 - is this a bug?

[Nir Soffer]

On Wed, Nov 27, 2019 at 8:06 PM Nir Soffer <address@hidden> wrote:
>
> I have a failing test[1] of ovirt-imageio nbd client[2], getting
> base:allocation from qemu-nbd.
>
> The failing test is:
>
> def test_base_allocation_some_data(nbd_server, user_file):
>     nbd_server.image = user_file.path
>     size = 1024**3
>     data_length = 4096
>     zero_length = size // 2 - data_length
>
>     with io.open(nbd_server.image, "wb") as f:
>         f.truncate(size)
>         f.write(b"x" * data_length)
>         f.seek(zero_length, os.SEEK_CUR)
>         f.write(b"x" * data_length)
>
>     nbd_server.start()
>
>     with nbd.open(nbd_server.url) as c:
>         extents = c.extents(0, size)

Changing this to:

        extents = []
        pos = 0

        while pos < size:
            res = c.extents(pos, size - pos)["base:allocation"]
            for ext in res:
                pos += ext.length
                extents.append(ext)

Solves the issue:
https://travis-ci.org/nirs/ovirt-imageio/jobs/617846324

So I guess I answered my question but I would like to get
your opinion anyway.

>
>         assert extents == {
>             "base:allocation": [
>                 nbd.Extent(data_length, False),
>                 nbd.Extent(zero_length, True),
>                 nbd.Extent(data_length, False),
>                 nbd.Extent(zero_length, True),
>             ]
>         }
>
> The test fails because on CentOS 8, qemu-nbd returns:
>
>     {'base:allocation': [Extent(length=4096, zero=False)]}
>
> We send CMD_BLOCK_STATUS with flags=0 (NBD_CMD_FLAG_REQ_ONE not set).
>
> The tests fail with:
> qemu-img-2.12.0-65.module_el8.0.0+189+f9babebb.5.x86_64
>
> And work with:
> qemu-img-4.1.0-5.fc30.x86_64
>
> It looks like qemu on the failing version return single extent instead
> of all extents
> in the requested range (offset=0, length=1g). Reading the NBD spec it seems 
> like
> the server is allowed to that, but since the behaviour is different in
> 4.1.0, maybe
> we are missing some patch in CentOS 8.0?
>
> Looks like I need to change the test to do multiple calls until the
> entire we have
> reponse for the entire range.
>
> [1] https://travis-ci.org/nirs/ovirt-imageio/jobs/617732045
> [2] https://gerrit.ovirt.org/c/105126
>
> Nir