qemu-e2k/include/block
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
..
accounting.h block: add empty account cookie type 2019-10-10 10:56:18 +02:00
aio-wait.h qemu/atomic.h: rename atomic_ to qatomic_ 2020-09-23 16:07:44 +01:00
aio.h util/event-loop-base: Introduce options to set the thread pool size 2022-05-09 10:43:23 +01:00
aio_task.h block: introduce aio task pool 2019-10-10 10:56:17 +02:00
block-common.h block: fix preallocate filter: don't do unaligned preallocate requests 2022-03-07 09:19:20 +01:00
block-copy.h block/block-copy: add block_copy_reset() 2022-03-07 09:33:30 +01:00
block-global-state.h block: Classify bdrv_get_flags() as I/O function 2022-05-04 15:55:23 +02:00
block-hmp-cmds.h Clean up header guards that don't match their file name 2022-05-11 16:49:06 +02:00
block-io.h block: Classify bdrv_get_flags() as I/O function 2022-05-04 15:55:23 +02:00
block.h include/block/block: split header into I/O and global state API 2022-03-04 18:18:25 +01:00
block_backup.h block/backup: drop unused synchronization interface 2018-12-14 11:52:40 +01:00
block_int-common.h block: introduce snapshot-access block driver 2022-03-07 09:33:31 +01:00
block_int-global-state.h Clean up decorations and whitespace around header guards 2022-05-11 16:50:32 +02:00
block_int-io.h block/io: introduce block driver snapshot-access API 2022-03-07 09:33:31 +01:00
block_int.h include/block/block_int: split header into I/O and global state API 2022-03-04 18:18:25 +01:00
blockjob.h include/block/blockjob.h: global state API 2022-03-04 18:18:25 +01:00
blockjob_int.h include/block/blockjob_int.h: split header into I/O and GS API 2022-03-04 18:18:25 +01:00
dirty-bitmap.h block/dirty-bitmap: introduce bdrv_dirty_bitmap_status() 2022-03-07 09:33:30 +01:00
export.h block/export: Move blk to BlockExport 2020-10-02 15:46:40 +02:00
fuse.h fuse: Allow exporting BDSs via FUSE 2020-12-11 17:52:39 +01:00
nbd.h nbd/server: Allow MULTI_CONN for shared writable exports 2022-05-12 13:10:52 +02:00
nvme.h hw/nvme: 64-bit pi support 2022-03-03 09:30:21 +01:00
qapi.h qapi: Allow getting flat output from 'query-named-block-nodes' 2020-02-20 16:43:42 +01:00
qdict.h include: move qdict_{crumple,flatten} declarations 2022-04-21 17:03:51 +04:00
raw-aio.h linux-aio: add `dev_max_batch` parameter to laio_io_unplug() 2021-11-02 13:03:35 +01:00
replication.h replication: move include out of root directory 2021-05-26 14:49:46 +02:00
reqlist.h block/reqlist: add reqlist_wait_all() 2022-03-07 09:33:30 +01:00
snapshot.h include/block/snapshot: global state API + assertions 2022-03-04 18:18:25 +01:00
thread-pool.h util/event-loop-base: Introduce options to set the thread pool size 2022-05-09 10:43:23 +01:00
throttle-groups.h block/throttle-groups: throttle_group_co_io_limits_intercept(): 64bit bytes 2021-02-03 08:14:00 -06:00
write-threshold.h write-threshold: deal with includes 2021-05-14 16:14:10 +02:00