Qemu supports the NBD protocol, and has an internal NBD client (see
block/nbd.c), an internal NBD server (see blockdev-nbd.c), and an
external NBD server tool (see qemu-nbd.c). The common code is placed
in nbd/*.

The NBD protocol is specified here:
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md

The following paragraphs describe some specific properties of NBD
protocol realization in Qemu.

= Metadata namespaces =

Qemu supports the "base:allocation" metadata context as defined in the
NBD protocol specification, and also defines an additional metadata
namespace "qemu".


== "qemu" namespace ==

The "qemu" namespace currently contains only one type of context,
related to exposing the contents of a dirty bitmap alongside the
associated disk contents.  That context has the following form:

    qemu:dirty-bitmap:<dirty-bitmap-export-name>

Each dirty-bitmap metadata context defines only one flag for extents
in reply for NBD_CMD_BLOCK_STATUS:

    bit 0: NBD_STATE_DIRTY, means that the extent is "dirty"

For NBD_OPT_LIST_META_CONTEXT the following queries are supported
in addition to "qemu:dirty-bitmap:<dirty-bitmap-export-name>":

* "qemu:" - returns list of all available metadata contexts in the
            namespace.
* "qemu:dirty-bitmap:" - returns list of all available dirty-bitmap
                         metadata contexts.
