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

[Jun Li]

On Fri, 12/05 18:01, Max Reitz wrote:
> 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.

Although "-o preallocation=falloc or -o preallocation=full" can create a qcow2
format image successfully, but when perform "qemu-img resize file.qcow2
+500M", then use the extend 500M disk image still hit the same issue(as rbd
block driver does not support growable file).

So use "-o preallocation=falloc or -o preallocation=full", it not a good
solution.

Regards,
Jun Li

> 
> 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
>