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.
read-only has POSIX file open-like semantics: if the device is itself
read only but the read-only property is false, blkio_start() fails.
With qemu-storage-daemon, you must set writable=on on the
vhost-user-blk export for the device to be writable.
Alberto