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 two available metadata context 21types. The first is related to exposing the contents of a dirty 22bitmap alongside the associated disk contents. That metadata context 23is named with the following form: 24 25 qemu:dirty-bitmap:<dirty-bitmap-export-name> 26 27Each dirty-bitmap metadata context defines only one flag for extents 28in reply for NBD_CMD_BLOCK_STATUS: 29 30 bit 0: NBD_STATE_DIRTY, set when the extent is "dirty" 31 32The second is related to exposing the source of various extents within 33the image, with a single metadata context named: 34 35 qemu:allocation-depth 36 37In the allocation depth context, the entire 32-bit value represents a 38depth of which layer in a thin-provisioned backing chain provided the 39data (0 for unallocated, 1 for the active layer, 2 for the first 40backing layer, and so forth). 41 42For NBD_OPT_LIST_META_CONTEXT the following queries are supported 43in addition to the specific "qemu:allocation-depth" and 44"qemu:dirty-bitmap:<dirty-bitmap-export-name>": 45 46* "qemu:" - returns list of all available metadata contexts in the 47 namespace. 48* "qemu:dirty-bitmap:" - returns list of all available dirty-bitmap 49 metadata contexts. 50 51= Features by version = 52 53The following list documents which qemu version first implemented 54various features (both as a server exposing the feature, and as a 55client taking advantage of the feature when present), to make it 56easier to plan for cross-version interoperability. Note that in 57several cases, the initial release containing a feature may require 58additional patches from the corresponding stable branch to fix bugs in 59the operation of that feature. 60 61* 2.6: NBD_OPT_STARTTLS with TLS X.509 Certificates 62* 2.8: NBD_CMD_WRITE_ZEROES 63* 2.10: NBD_OPT_GO, NBD_INFO_BLOCK 64* 2.11: NBD_OPT_STRUCTURED_REPLY 65* 2.12: NBD_CMD_BLOCK_STATUS for "base:allocation" 66* 3.0: NBD_OPT_STARTTLS with TLS Pre-Shared Keys (PSK), 67NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE 68* 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports, 69NBD_CMD_FLAG_FAST_ZERO 70* 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth" 71* 7.1: NBD_FLAG_CAN_MULTI_CONN for shareable writable exports 72