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72 lines
2.7 KiB
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72 lines
2.7 KiB
Plaintext
QEMU supports the NBD protocol, and has an internal NBD client (see
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block/nbd.c), an internal NBD server (see blockdev-nbd.c), and an
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external NBD server tool (see qemu-nbd.c). The common code is placed
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in nbd/*.
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The NBD protocol is specified here:
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https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md
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The following paragraphs describe some specific properties of NBD
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protocol realization in QEMU.
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= Metadata namespaces =
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QEMU supports the "base:allocation" metadata context as defined in the
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NBD protocol specification, and also defines an additional metadata
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namespace "qemu".
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== "qemu" namespace ==
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The "qemu" namespace currently contains two available metadata context
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types. The first is related to exposing the contents of a dirty
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bitmap alongside the associated disk contents. That metadata context
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is named with the following form:
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qemu:dirty-bitmap:<dirty-bitmap-export-name>
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Each dirty-bitmap metadata context defines only one flag for extents
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in reply for NBD_CMD_BLOCK_STATUS:
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bit 0: NBD_STATE_DIRTY, set when the extent is "dirty"
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The second is related to exposing the source of various extents within
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the image, with a single metadata context named:
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qemu:allocation-depth
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In the allocation depth context, the entire 32-bit value represents a
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depth of which layer in a thin-provisioned backing chain provided the
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data (0 for unallocated, 1 for the active layer, 2 for the first
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backing layer, and so forth).
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For NBD_OPT_LIST_META_CONTEXT the following queries are supported
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in addition to the specific "qemu:allocation-depth" and
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"qemu:dirty-bitmap:<dirty-bitmap-export-name>":
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* "qemu:" - returns list of all available metadata contexts in the
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namespace.
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* "qemu:dirty-bitmap:" - returns list of all available dirty-bitmap
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metadata contexts.
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= Features by version =
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The following list documents which qemu version first implemented
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various features (both as a server exposing the feature, and as a
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client taking advantage of the feature when present), to make it
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easier to plan for cross-version interoperability. Note that in
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several cases, the initial release containing a feature may require
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additional patches from the corresponding stable branch to fix bugs in
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the operation of that feature.
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* 2.6: NBD_OPT_STARTTLS with TLS X.509 Certificates
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* 2.8: NBD_CMD_WRITE_ZEROES
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* 2.10: NBD_OPT_GO, NBD_INFO_BLOCK
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* 2.11: NBD_OPT_STRUCTURED_REPLY
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* 2.12: NBD_CMD_BLOCK_STATUS for "base:allocation"
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* 3.0: NBD_OPT_STARTTLS with TLS Pre-Shared Keys (PSK),
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NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
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* 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
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NBD_CMD_FLAG_FAST_ZERO
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* 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
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* 7.1: NBD_FLAG_CAN_MULTI_CONN for shareable writable exports
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