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According to the NBD spec, a server that advertises NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will not see any cache inconsistencies: when properly separated by a single flush, actions performed by one client will be visible to another client, regardless of which client did the flush. We always satisfy these conditions in qemu - even when we support multiple clients, ALL clients go through a single point of reference into the block layer, with no local caching. The effect of one client is instantly visible to the next client. Even if our backend were a network device, we argue that any multi-path caching effects that would cause inconsistencies in back-to-back actions not seeing the effect of previous actions would be a bug in that backend, and not the fault of caching in qemu. As such, it is safe to unconditionally advertise CAN_MULTI_CONN for any qemu NBD server situation that supports parallel clients. Note, however, that we don't want to advertise CAN_MULTI_CONN when we know that a second client cannot connect (for historical reasons, qemu-nbd defaults to a single connection while nbd-server-add and QMP commands default to unlimited connections; but we already have existing means to let either style of NBD server creation alter those defaults). This is visible by no longer advertising MULTI_CONN for 'qemu-nbd -r' without -e, as in the iotest nbd-qemu-allocation. The harder part of this patch is setting up an iotest to demonstrate behavior of multiple NBD clients to a single server. It might be possible with parallel qemu-io processes, but I found it easier to do in python with the help of libnbd, and help from Nir and Vladimir in writing the test. Signed-off-by: Eric Blake <eblake@redhat.com> Suggested-by: Nir Soffer <nsoffer@redhat.com> Suggested-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru> Message-Id: <20220512004924.417153-3-eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
72 lines
2.7 KiB
Plaintext
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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