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QEMU NBD protocol support
=========================
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 two available metadata context
types. The first is related to exposing the contents of a dirty
bitmap alongside the associated disk contents. That metadata context
is named with 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``, set when the extent is "dirty"
The second is related to exposing the source of various extents within
the image, with a single metadata context named::
qemu:allocation-depth
In the allocation depth context, the entire 32-bit value represents a
depth of which layer in a thin-provisioned backing chain provided the
data (0 for unallocated, 1 for the active layer, 2 for the first
backing layer, and so forth).
For ``NBD_OPT_LIST_META_CONTEXT`` the following queries are supported
in addition to the specific ``qemu:allocation-depth`` and
``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
Features by version
-------------------
The following list documents which qemu version first implemented
various features (both as a server exposing the feature, and as a
client taking advantage of the feature when present), to make it
easier to plan for cross-version interoperability. Note that in
several cases, the initial release containing a feature may require
additional patches from the corresponding stable branch to fix bugs in
the operation of that feature.
2.6
``NBD_OPT_STARTTLS`` with TLS X.509 Certificates
2.8
``NBD_CMD_WRITE_ZEROES``
2.10
``NBD_OPT_GO``, ``NBD_INFO_BLOCK``
2.11
``NBD_OPT_STRUCTURED_REPLY``
2.12
``NBD_CMD_BLOCK_STATUS`` for ``base:allocation``
3.0
``NBD_OPT_STARTTLS`` with TLS Pre-Shared Keys (PSK),
``NBD_CMD_BLOCK_STATUS`` for ``qemu:dirty-bitmap:``, ``NBD_CMD_CACHE``
4.2
``NBD_FLAG_CAN_MULTI_CONN`` for shareable read-only exports,
``NBD_CMD_FLAG_FAST_ZERO``
5.2
``NBD_CMD_BLOCK_STATUS`` for ``qemu:allocation-depth``
7.1
``NBD_FLAG_CAN_MULTI_CONN`` for shareable writable exports
8.2
``NBD_OPT_EXTENDED_HEADERS``, ``NBD_FLAG_BLOCK_STATUS_PAYLOAD``
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