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 >
