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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> |
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| .. | ||
| acpi.json | ||
| audio.json | ||
| authz.json | ||
| block-core.json | ||
| block-export.json | ||
| block.json | ||
| char.json | ||
| common.json | ||
| compat.json | ||
| control.json | ||
| crypto.json | ||
| dump.json | ||
| error.json | ||
| introspect.json | ||
| job.json | ||
| machine-target.json | ||
| machine.json | ||
| meson.build | ||
| migration.json | ||
| misc-target.json | ||
| misc.json | ||
| net.json | ||
| opts-visitor.c | ||
| pci.json | ||
| pragma.json | ||
| qapi-clone-visitor.c | ||
| qapi-dealloc-visitor.c | ||
| qapi-forward-visitor.c | ||
| qapi-schema.json | ||
| qapi-type-helpers.c | ||
| qapi-util.c | ||
| qapi-visit-core.c | ||
| qdev.json | ||
| qmp-dispatch.c | ||
| qmp-event.c | ||
| qmp-registry.c | ||
| qobject-input-visitor.c | ||
| qobject-output-visitor.c | ||
| qom.json | ||
| rdma.json | ||
| replay.json | ||
| rocker.json | ||
| run-state.json | ||
| sockets.json | ||
| string-input-visitor.c | ||
| string-output-visitor.c | ||
| tpm.json | ||
| trace-events | ||
| trace.h | ||
| trace.json | ||
| transaction.json | ||
| ui.json | ||
| yank.json | ||