Libguestfs - Jan 2023 - [PATCH nbdkit] New plugin: blkio [incomplete]

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.

Stefan
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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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