[Qemu-devel] qcow2 and rbd driver question

[Stefano Garzarella]

Hi Kevin,
I'm working on an issue where I'm not able to create a qcow2 image with
the rbd driver (https://bugzilla.redhat.com/show_bug.cgi?id=1171007).

What I found is the following:
- when qcow2 uses the rbd driver, the new file is created (rbd_create)
  with the size equals to 0. (qemu_opt_get_size_del(opts,
  BLOCK_OPT_SIZE, 0) returns 0 in qemu_rbd_co_create_opts())
- the file is truncated (implemented with rbd_resize) to 0 before to
  write the qcow2 header.
- the "size" parameter passed to rbd_create() or rbd_resize() is
  interpreted as the maximum size of the file, this means that all
  writes that exceed that size, fails and returns -22.

As a workaround, I'm checking if the write operation exceeds the maximum
size and then I'll resize the file. It works, but I'm not sure it is the
right way.
I also checked the RBD API, but I didn't find any useful function or
flags that allow writes to increase file size.

Any suggestions?

Thank you very much,
Stefano