Exporting qcow2 images as raw data from ova file with qemu-nbd

[Nir Soffer]

I'm trying to export qcow2 images from ova format using qemu-nbd.

I create 2 compressed qcow2 images, with different data:

$ qemu-img info disk1.qcow2
image: disk1.qcow2
file format: qcow2
virtual size: 200 MiB (209715200 bytes)
disk size: 384 KiB
...

$ qemu-img info disk2.qcow2
image: disk2.qcow2
file format: qcow2
virtual size: 200 MiB (209715200 bytes)
disk size: 384 KiB
...

And packed them in a tar file. This is not a valid ova but good enough
for this test:

$ tar tvf vm.ova
-rw-r--r-- nsoffer/nsoffer 454144 2020-06-22 21:34 disk1.qcow2
-rw-r--r-- nsoffer/nsoffer 454144 2020-06-22 21:34 disk2.qcow2

To get info about the disks in ova file, we can use:

$ python -c 'import tarfile; print(list({"name": m.name, "offset":
m.offset_data, "size": m.size} for m in tarfile.open("vm.ova")))'
[{'name': 'disk1.qcow2', 'offset': 512, 'size': 454144}, {'name':
'disk2.qcow2', 'offset': 455168, 'size': 454144}]

First I tried the obvious:

$ qemu-nbd --persistent --socket=/tmp/nbd.sock --read-only --offset=512 vm.ova

And it works, but it exposes the qcow2 data. I want to raw data so I
can upload the guest
data to ovirt, where is may be converted to qcow2 format.

$ qemu-img info --output json "nbd+unix://?socket=/tmp/nbd.sock"
{
    "virtual-size": 209715200,
    "filename": "nbd+unix://?socket=/tmp/nbd.sock",
    "format": "qcow2",
 ...
}

Looking in qemu manual and qapi/block-core.json, I could construct this command:

$ qemu-nbd --persistent --socket=/tmp/nbd.sock --read-only
'json:{"driver": "qcow2", "file": {"driver": "raw", "offset": 512,
"size": 454144, "file": {"driver": "file", "filename": "vm.ova"}}}'

And it works:

$ qemu-img info --output json "nbd+unix://?socket=/tmp/nbd.sock"
{
    "virtual-size": 209715200,
    "filename": "nbd+unix://?socket=/tmp/nbd.sock",
    "format": "raw"
}

$ qemu-img map --output json "nbd+unix://?socket=/tmp/nbd.sock"
[{ "start": 0, "length": 104857600, "depth": 0, "zero": false, "data":
true, "offset": 0},
{ "start": 104857600, "length": 104857600, "depth": 0, "zero": true,
"data": false, "offset": 104857600}]

$ qemu-img map --output json disk1.qcow2
[{ "start": 0, "length": 104857600, "depth": 0, "zero": false, "data": true},
{ "start": 104857600, "length": 104857600, "depth": 0, "zero": true,
"data": false}]

$ qemu-img convert -f raw -O raw nbd+unix://?socket=/tmp/nbd.sock disk1.raw

$ qemu-img info disk1.raw
image: disk1.raw
file format: raw
virtual size: 200 MiB (209715200 bytes)
disk size: 100 MiB

$ qemu-img compare disk1.raw disk1.qcow2
Images are identical.

I wonder if this is the best way to stack a qcow2 driver on top of a
raw driver exposing
a range from a tar file.

I found similar example for gluster in:
docs/system/device-url-syntax.rst.inc

Richard suggested to try nbdkit tar plugin, but the plugin is not
available on RHEL,
and this adds additional dependency, when we already use qemu-nbd.

Nir