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.
Rich.
--
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