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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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| _templates | ||
| about | ||
| config | ||
| devel | ||
| interop | ||
| specs | ||
| sphinx | ||
| sphinx-static | ||
| spin | ||
| system | ||
| tools | ||
| user | ||
| block-replication.txt | ||
| bypass-iommu.txt | ||
| COLO-FT.txt | ||
| colo-proxy.txt | ||
| conf.py | ||
| defs.rst.inc | ||
| hyperv.txt | ||
| igd-assign.txt | ||
| image-fuzzer.txt | ||
| index.rst | ||
| memory-hotplug.txt | ||
| meson.build | ||
| multi-thread-compression.txt | ||
| multiseat.txt | ||
| nvdimm.txt | ||
| pci_expander_bridge.txt | ||
| pcie_pci_bridge.txt | ||
| pcie_sriov.txt | ||
| pcie.txt | ||
| pvrdma.txt | ||
| qcow2-cache.txt | ||
| qdev-device-use.txt | ||
| qemu-option-trace.rst.inc | ||
| qemupciserial.inf | ||
| rdma.txt | ||
| replay.txt | ||
| spice-port-fqdn.txt | ||
| throttle.txt | ||
| u2f.txt | ||
| xbzrle.txt | ||
| xen-save-devices-state.txt | ||