qemu-e2k/docs/tools
Eric Blake 58a6fdcc9e nbd/server: Allow MULTI_CONN for shared writable exports
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>
2022-05-12 13:10:52 +02:00
..
index.rst docs: standardize directory index to --- with overline 2021-09-13 13:56:26 +02:00
qemu-img.rst qemu-img: properly list formats which have consistency check implemented 2022-05-04 15:55:23 +02:00
qemu-nbd.rst nbd/server: Allow MULTI_CONN for shared writable exports 2022-05-12 13:10:52 +02:00
qemu-pr-helper.rst docs: standardize book titles to === with overline 2021-09-13 13:56:26 +02:00
qemu-storage-daemon.rst qsd: Add --daemonize 2022-03-04 18:14:40 +01:00
qemu-trace-stap.rst docs/tools/qemu-trace-stap.rst: Do not hard-code the QEMU binary name 2022-01-05 11:10:13 +01:00
virtfs-proxy-helper.rst
virtiofsd.rst virtiofsd: Add docs/helper for killpriv_v2/no_killpriv_v2 option 2022-04-21 12:05:15 +02:00