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90 lines
3.0 KiB
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QEMU NBD protocol support
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=========================
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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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-------------------
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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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------------------
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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:
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``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:``
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returns list of all available metadata contexts in the namespace
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``qemu:dirty-bitmap:``
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returns list of all available dirty-bitmap metadata contexts
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Features by version
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-------------------
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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
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``NBD_OPT_STARTTLS`` with TLS X.509 Certificates
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2.8
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``NBD_CMD_WRITE_ZEROES``
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2.10
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``NBD_OPT_GO``, ``NBD_INFO_BLOCK``
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2.11
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``NBD_OPT_STRUCTURED_REPLY``
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2.12
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``NBD_CMD_BLOCK_STATUS`` for ``base:allocation``
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3.0
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``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
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``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
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``NBD_CMD_BLOCK_STATUS`` for ``qemu:allocation-depth``
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7.1
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``NBD_FLAG_CAN_MULTI_CONN`` for shareable writable exports
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8.2
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``NBD_OPT_EXTENDED_HEADERS``, ``NBD_FLAG_BLOCK_STATUS_PAYLOAD``
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