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

[Kevin Wolf <kwolf@redhat.com>]

Am 08.12.2014 um 14:50 hat Jun Li geschrieben:
> 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?

Are you sure that the rbd libraries are thread-safe and can be used in
this way?

Kevin