-Qemu supports the NBD protocol, and has an internal NBD client (see
+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/*.
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md
The following paragraphs describe some specific properties of NBD
-protocol realization in Qemu.
+protocol realization in QEMU.
= Metadata namespaces =
-Qemu supports the "base:allocation" metadata context as defined in the
+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:
+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, means that the extent is "dirty"
+ 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 "qemu:dirty-bitmap:<dirty-bitmap-export-name>":
+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