1Qemu supports the NBD protocol, and has an internal NBD client (see 2block/nbd.c), an internal NBD server (see blockdev-nbd.c), and an 3external NBD server tool (see qemu-nbd.c). The common code is placed 4in nbd/*. 5 6The NBD protocol is specified here: 7https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md 8 9The following paragraphs describe some specific properties of NBD 10protocol realization in Qemu. 11 12= Metadata namespaces = 13 14Qemu supports the "base:allocation" metadata context as defined in the 15NBD protocol specification, and also defines an additional metadata 16namespace "qemu". 17 18== "qemu" namespace == 19 20The "qemu" namespace currently contains only one type of context, 21related to exposing the contents of a dirty bitmap alongside the 22associated disk contents. That context has the following form: 23 24 qemu:dirty-bitmap:<dirty-bitmap-export-name> 25 26Each dirty-bitmap metadata context defines only one flag for extents 27in reply for NBD_CMD_BLOCK_STATUS: 28 29 bit 0: NBD_STATE_DIRTY, means that the extent is "dirty" 30 31For NBD_OPT_LIST_META_CONTEXT the following queries are supported 32in addition to "qemu:dirty-bitmap:<dirty-bitmap-export-name>": 33 34* "qemu:" - returns list of all available metadata contexts in the 35 namespace. 36* "qemu:dirty-bitmap:" - returns list of all available dirty-bitmap 37 metadata contexts. 38 39= Features by version = 40 41The following list documents which qemu version first implemented 42various features (both as a server exposing the feature, and as a 43client taking advantage of the feature when present), to make it 44easier to plan for cross-version interoperability. Note that in 45several cases, the initial release containing a feature may require 46additional patches from the corresponding stable branch to fix bugs in 47the operation of that feature. 48 49* 2.6: NBD_OPT_STARTTLS with TLS X.509 Certificates 50* 2.8: NBD_CMD_WRITE_ZEROES 51* 2.10: NBD_OPT_GO, NBD_INFO_BLOCK 52* 2.11: NBD_OPT_STRUCTURED_REPLY 53* 2.12: NBD_CMD_BLOCK_STATUS for "base:allocation" 54* 3.0: NBD_OPT_STARTTLS with TLS Pre-Shared Keys (PSK), 55NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE 56