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

[Jun Li]

On Mon, 12/08 11:07, Kevin Wolf wrote:
> Am 05.12.2014 um 16:32 hat Jun Li geschrieben:
> > 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).
> 
> This is not a solution. Even if you might be able to create an image
> successfully, using qcow2 without a backend that allows the image file
> to grow is bound to fail sooner or later.
> 
> So yes, you'll want to extend the rbd block driver to grow the file
> asynchronously when writing beyond EOF.
> 

Hi Kevin,

  Although rbd_resize is synchronous, I just want to create a new child-thread
to realize asynchronous(Maybe just like userspace aio: libaio) rbd_resize.
Just like following:

qemu_rbd_aio_writev()
{
    if (BlockDriverState->file->growable == 1) {
        pthread_create(&thread_id, NULL, child_thread, NULL);
    }

    rbd_start_aio();
}

child_thread()
{
    ...
    rbd_resize();
    rbd_start_aio();
    ...
}

Currently, seems do not have original asynchronous rbd_resize. Besides, rbd 
block
driver do not support growable file. So I want to use above method to realize
asynchronous rbd_resize() in our qemu level.

What's your opinion?

Regards,
Jun Li