nbd.txt (2815B)
1 QEMU supports the NBD protocol, and has an internal NBD client (see 2 block/nbd.c), an internal NBD server (see blockdev-nbd.c), and an 3 external NBD server tool (see qemu-nbd.c). The common code is placed 4 in nbd/*. 5 6 The NBD protocol is specified here: 7 https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md 8 9 The following paragraphs describe some specific properties of NBD 10 protocol realization in QEMU. 11 12 = Metadata namespaces = 13 14 QEMU supports the "base:allocation" metadata context as defined in the 15 NBD protocol specification, and also defines an additional metadata 16 namespace "qemu". 17 18 == "qemu" namespace == 19 20 The "qemu" namespace currently contains two available metadata context 21 types. The first is related to exposing the contents of a dirty 22 bitmap alongside the associated disk contents. That metadata context 23 is named with the following form: 24 25 qemu:dirty-bitmap:<dirty-bitmap-export-name> 26 27 Each dirty-bitmap metadata context defines only one flag for extents 28 in reply for NBD_CMD_BLOCK_STATUS: 29 30 bit 0: NBD_STATE_DIRTY, set when the extent is "dirty" 31 32 The second is related to exposing the source of various extents within 33 the image, with a single metadata context named: 34 35 qemu:allocation-depth 36 37 In the allocation depth context, the entire 32-bit value represents a 38 depth of which layer in a thin-provisioned backing chain provided the 39 data (0 for unallocated, 1 for the active layer, 2 for the first 40 backing layer, and so forth). 41 42 For NBD_OPT_LIST_META_CONTEXT the following queries are supported 43 in 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 53 The following list documents which qemu version first implemented 54 various features (both as a server exposing the feature, and as a 55 client taking advantage of the feature when present), to make it 56 easier to plan for cross-version interoperability. Note that in 57 several cases, the initial release containing a feature may require 58 additional patches from the corresponding stable branch to fix bugs in 59 the 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), 67 NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE 68 * 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports, 69 NBD_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