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>
master
Eric Blake 9 months ago committed by Kevin Wolf
parent a5fced4021
commit 58a6fdcc9e
  1. 1
      MAINTAINERS
  2. 5
      blockdev-nbd.c
  3. 1
      docs/interop/nbd.txt
  4. 3
      docs/tools/qemu-nbd.rst
  5. 3
      include/block/nbd.h
  6. 10
      nbd/server.c
  7. 8
      qapi/block-export.json
  8. 145
      tests/qemu-iotests/tests/nbd-multiconn
  9. 5
      tests/qemu-iotests/tests/nbd-multiconn.out
  10. 2
      tests/qemu-iotests/tests/nbd-qemu-allocation.out

@ -3367,6 +3367,7 @@ F: qemu-nbd.*
F: blockdev-nbd.c
F: docs/interop/nbd.txt
F: docs/tools/qemu-nbd.rst
F: tests/qemu-iotests/tests/*nbd*
T: git https://repo.or.cz/qemu/ericb.git nbd
T: git https://src.openvz.org/scm/~vsementsov/qemu.git nbd

@ -44,6 +44,11 @@ bool nbd_server_is_running(void)
return nbd_server || qemu_nbd_connections >= 0;
}
int nbd_server_max_connections(void)
{
return nbd_server ? nbd_server->max_connections : qemu_nbd_connections;
}
static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
{
nbd_client_put(client);

@ -68,3 +68,4 @@ NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
* 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
NBD_CMD_FLAG_FAST_ZERO
* 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
* 7.1: NBD_FLAG_CAN_MULTI_CONN for shareable writable exports

@ -139,8 +139,7 @@ driver options if :option:`--image-opts` is specified.
.. option:: -e, --shared=NUM
Allow up to *NUM* clients to share the device (default
``1``), 0 for unlimited. Safe for readers, but for now,
consistency is not guaranteed between multiple writers.
``1``), 0 for unlimited.
.. option:: -t, --persistent

@ -1,5 +1,5 @@
/*
* Copyright (C) 2016-2020 Red Hat, Inc.
* Copyright (C) 2016-2022 Red Hat, Inc.
* Copyright (C) 2005 Anthony Liguori <anthony@codemonkey.ws>
*
* Network Block Device
@ -346,6 +346,7 @@ void nbd_client_put(NBDClient *client);
void nbd_server_is_qemu_nbd(int max_connections);
bool nbd_server_is_running(void);
int nbd_server_max_connections(void);
void nbd_server_start(SocketAddress *addr, const char *tls_creds,
const char *tls_authz, uint32_t max_connections,
Error **errp);

@ -1,5 +1,5 @@
/*
* Copyright (C) 2016-2021 Red Hat, Inc.
* Copyright (C) 2016-2022 Red Hat, Inc.
* Copyright (C) 2005 Anthony Liguori <anthony@codemonkey.ws>
*
* Network Block Device Server Side
@ -1642,7 +1642,6 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
int64_t size;
uint64_t perm, shared_perm;
bool readonly = !exp_args->writable;
bool shared = !exp_args->writable;
BlockDirtyBitmapOrStrList *bitmaps;
size_t i;
int ret;
@ -1693,11 +1692,12 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
exp->description = g_strdup(arg->description);
exp->nbdflags = (NBD_FLAG_HAS_FLAGS | NBD_FLAG_SEND_FLUSH |
NBD_FLAG_SEND_FUA | NBD_FLAG_SEND_CACHE);
if (nbd_server_max_connections() != 1) {
exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
}
if (readonly) {
exp->nbdflags |= NBD_FLAG_READ_ONLY;
if (shared) {
exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
}
} else {
exp->nbdflags |= (NBD_FLAG_SEND_TRIM | NBD_FLAG_SEND_WRITE_ZEROES |
NBD_FLAG_SEND_FAST_ZERO);

@ -22,7 +22,9 @@
# recreated on the fly while the NBD server is active.
# If missing, it will default to denying access (since 4.0).
# @max-connections: The maximum number of connections to allow at the same
# time, 0 for unlimited. (since 5.2; default: 0)
# time, 0 for unlimited. Setting this to 1 also stops
# the server from advertising multiple client support
# (since 5.2; default: 0)
#
# Since: 4.2
##
@ -51,7 +53,9 @@
# recreated on the fly while the NBD server is active.
# If missing, it will default to denying access (since 4.0).
# @max-connections: The maximum number of connections to allow at the same
# time, 0 for unlimited. (since 5.2; default: 0)
# time, 0 for unlimited. Setting this to 1 also stops
# the server from advertising multiple client support
# (since 5.2; default: 0).
#
# Returns: error if the server is already running.
#

@ -0,0 +1,145 @@
#!/usr/bin/env python3
# group: rw auto quick
#
# Test cases for NBD multi-conn advertisement
#
# Copyright (C) 2022 Red Hat, Inc.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
import os
from contextlib import contextmanager
import iotests
from iotests import qemu_img_create, qemu_io
disk = os.path.join(iotests.test_dir, 'disk')
size = '4M'
nbd_sock = os.path.join(iotests.sock_dir, 'nbd_sock')
nbd_uri = 'nbd+unix:///{}?socket=' + nbd_sock
@contextmanager
def open_nbd(export_name):
h = nbd.NBD()
try:
h.connect_uri(nbd_uri.format(export_name))
yield h
finally:
h.shutdown()
class TestNbdMulticonn(iotests.QMPTestCase):
def setUp(self):
qemu_img_create('-f', iotests.imgfmt, disk, size)
qemu_io('-c', 'w -P 1 0 2M', '-c', 'w -P 2 2M 2M', disk)
self.vm = iotests.VM()
self.vm.launch()
result = self.vm.qmp('blockdev-add', {
'driver': 'qcow2',
'node-name': 'n',
'file': {'driver': 'file', 'filename': disk}
})
self.assert_qmp(result, 'return', {})
def tearDown(self):
self.vm.shutdown()
os.remove(disk)
try:
os.remove(nbd_sock)
except OSError:
pass
@contextmanager
def run_server(self, max_connections=None):
args = {
'addr': {
'type': 'unix',
'data': {'path': nbd_sock}
}
}
if max_connections is not None:
args['max-connections'] = max_connections
result = self.vm.qmp('nbd-server-start', args)
self.assert_qmp(result, 'return', {})
yield
result = self.vm.qmp('nbd-server-stop')
self.assert_qmp(result, 'return', {})
def add_export(self, name, writable=None):
args = {
'type': 'nbd',
'id': name,
'node-name': 'n',
'name': name,
}
if writable is not None:
args['writable'] = writable
result = self.vm.qmp('block-export-add', args)
self.assert_qmp(result, 'return', {})
def test_default_settings(self):
with self.run_server():
self.add_export('r')
self.add_export('w', writable=True)
with open_nbd('r') as h:
self.assertTrue(h.can_multi_conn())
with open_nbd('w') as h:
self.assertTrue(h.can_multi_conn())
def test_limited_connections(self):
with self.run_server(max_connections=1):
self.add_export('r')
self.add_export('w', writable=True)
with open_nbd('r') as h:
self.assertFalse(h.can_multi_conn())
with open_nbd('w') as h:
self.assertFalse(h.can_multi_conn())
def test_parallel_writes(self):
with self.run_server():
self.add_export('w', writable=True)
clients = [nbd.NBD() for _ in range(3)]
for c in clients:
c.connect_uri(nbd_uri.format('w'))
self.assertTrue(c.can_multi_conn())
initial_data = clients[0].pread(1024 * 1024, 0)
self.assertEqual(initial_data, b'\x01' * 1024 * 1024)
updated_data = b'\x03' * 1024 * 1024
clients[1].pwrite(updated_data, 0)
clients[2].flush()
current_data = clients[0].pread(1024 * 1024, 0)
self.assertEqual(updated_data, current_data)
for i in range(3):
clients[i].shutdown()
if __name__ == '__main__':
try:
# Easier to use libnbd than to try and set up parallel
# 'qemu-nbd --list' or 'qemu-io' processes, but not all systems
# have libnbd installed.
import nbd # type: ignore
iotests.main(supported_fmts=['qcow2'])
except ImportError:
iotests.notrun('libnbd not installed')

@ -0,0 +1,5 @@
...
----------------------------------------------------------------------
Ran 3 tests
OK

@ -17,7 +17,7 @@ wrote 2097152/2097152 bytes at offset 1048576
exports available: 1
export: ''
size: 4194304
flags: 0x58f ( readonly flush fua df multi cache )
flags: 0x48f ( readonly flush fua df cache )
min block: 1
opt block: 4096
max block: 33554432

Loading…
Cancel
Save