Re: [Libguestfs] [PATCH nbdkit] New plugin: blkio [incomplete]

On Wed, Jan 11, 2023 at 11:34:50AM -0500, Stefan Hajnoczi wrote: > On Sat, Jan 07, 2023 at 07:44:37PM +0000, Richard W.M. Jones wrote: > > > > This is upstream in nbdkit now: > > https://gitlab.com/nbdkit/nbdkit/-/tree/master/plugins/blkio > > > > Another question: > > > > (6) vhost-user + "read-only" property acts a bit strangely. After > > opening virtio-blk-vhost-user it throws an EROFS error if you try to > > "blkio_start" it. However if you set the "read-only" property to true > > then it's OK to start it, and subsequent read operations work too. > > > > I would expect that any device can be started without needing to set > > this property, and in general I don't understand why vhost-user is r/o > > since everything I read about it seems to indicate it's a general > > purpose writable protocol. > > I guess the vhost-user-blk server is read-only and that's why it's > refusing to blkio_start() unless readonly was set to true. > > If you're using qemu-storage-daemon, set the writable=on parameter on > the export. So it turned out to be the case. Alberto pointed out that unless you set writable=on explicitly, q-s-d makes the export read-only. This was the reverse of what I expected. But it is covered in the q-s-d man page. But about libblkio: The problem is basically you must know if the export is read-only or not before you connect. For a readonly vhost-user export this is the observed behaviour: blkio_create ==> OK blkio_get_bool ("read-only") ==> returns false blkio_connect ==> OK blkio_get_bool ("read-only") ==> returns false blkio_start ==> fails "Device is read-only" At least after connecting it seems we should be able to find out that the export is read-only.