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

[Max Reitz <mreitz@redhat.com>]

On 2014-12-05 at 16:32, Jun Li wrote:
> 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).

Should be possible by using -o preallocation=falloc or -o 
preallocation=full.

I can't say a lot about making rbd growable because I know near to 
nothing about rbd; but there are protocols which really simply don't 
support writes beyond the end of file, and where that's intended (for 
instance, while nbd somehow does support it when using the qemu nbd 
server, normally (strictly according to the protocol) it does not); so 
for these protocols, you have to use a preallocated image file or an 
image format which does not grow on writes (such as raw).

Of course, while that may be a solution for nbd, it doesn't sound like a 
good solution for rbd, so writes beyond the EOF should probably be 
supported there (although once again, I don't know rbd well enough to 
judge that).

Max

> 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