[Qemu-devel] qcow2: Can create qcow2 image format on rbd server

[Jun Li <junmuzi@gmail.com>]

Currently, qemu-img can not create qcow2 image format on rbd server. Analysis
the code as followings:
when create qcow2 format image:
qcow2_create2
  bdrv_create_file(filename, opts, &local_err);  --> Here will create a 0 size
  file(e.g: file1) on rbd server.
  ...
  ret = bdrv_pwrite(bs, 0, header, cluster_size); --> So here can not write
  qcow2 header into file1 due to the file1's length is 0. Seems
  qemu_rbd_aio_writev can not write beyond EOF.
  ...

As above analysis, there are two methods to solve the above bz as followings:
1, When create file1, just create a fixed-size file1 on rbd server(not 0 size).
2, When write the qcow2 header into file1, just let qemu_rbd_aio_writev can
enlarge the file1. So should add qemu_rbd_truncate inside qemu_rbd_aio_writev.
qemu_rbd_truncate will call rbd_resize, but seems rbd_resize is
synchronous function. If so, when do bdrv_pwrite, guest will hang. This is not
our expected.

For method 1, maybe it's not corresponding to the original principle of qcow2.
Yes, it's very easy to solve the above bz. Nevertheless, I just want to use
method 2 to solve above issue.

For method 2, could anyone give some suggestions on howto realize a
asynchronous rbd_resize. Thanks.

Above analysis also based on stefan's hints. Thanks.

Best Regards,
Jun Li