Re: [Qemu-devel] your mail

[Jun Li]

Please ignore it. Sorry...

On Sat, 12/06 18:54, Jun Li wrote:
> address@hidden
> Bcc: 
> Subject: Re: [Qemu-devel] qcow2: Can create qcow2 image format on rbd server
> Reply-To: 
> In-Reply-To: <address@hidden>
> 
> 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.
> 
> Sure. If bdrv_create_file(filename, opts, &local_err) create a fixed-size(not
> 0 size) just as using "preallocation=falloc or preallocation=full", it will
> create a fixed-size file on rbd server. So it won't exist above issue.
> 
> > 
> > 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).
> > 
> 
> Here just want to use rbd_resize to realize rbd growable.
> 
> > 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).
> > 
> 
> Yes, you are right. Also talked with stefan. Here just want to ask Josh Durgin
> whether it has other solutions or rbd can support asynchronous rbd_resize.
> 
> Regards,
> Jun Li
> 
> > 
> > >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
> >