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
@ -0,0 +1,145 @@ |
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#!/usr/bin/env python3 |
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# group: rw auto quick |
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# |
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# Test cases for NBD multi-conn advertisement |
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# |
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# Copyright (C) 2022 Red Hat, Inc. |
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# |
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# This program is free software; you can redistribute it and/or modify |
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# it under the terms of the GNU General Public License as published by |
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# the Free Software Foundation; either version 2 of the License, or |
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# (at your option) any later version. |
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# |
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# This program is distributed in the hope that it will be useful, |
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# but WITHOUT ANY WARRANTY; without even the implied warranty of |
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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# GNU General Public License for more details. |
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# |
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# You should have received a copy of the GNU General Public License |
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# along with this program. If not, see <http://www.gnu.org/licenses/>. |
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|
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import os |
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from contextlib import contextmanager |
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import iotests |
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from iotests import qemu_img_create, qemu_io |
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|
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|
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disk = os.path.join(iotests.test_dir, 'disk') |
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size = '4M' |
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nbd_sock = os.path.join(iotests.sock_dir, 'nbd_sock') |
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nbd_uri = 'nbd+unix:///{}?socket=' + nbd_sock |
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|
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|
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@contextmanager |
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def open_nbd(export_name): |
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h = nbd.NBD() |
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try: |
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h.connect_uri(nbd_uri.format(export_name)) |
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yield h |
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finally: |
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h.shutdown() |
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|
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class TestNbdMulticonn(iotests.QMPTestCase): |
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def setUp(self): |
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qemu_img_create('-f', iotests.imgfmt, disk, size) |
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qemu_io('-c', 'w -P 1 0 2M', '-c', 'w -P 2 2M 2M', disk) |
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|
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self.vm = iotests.VM() |
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self.vm.launch() |
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result = self.vm.qmp('blockdev-add', { |
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'driver': 'qcow2', |
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'node-name': 'n', |
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'file': {'driver': 'file', 'filename': disk} |
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}) |
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self.assert_qmp(result, 'return', {}) |
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|
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def tearDown(self): |
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self.vm.shutdown() |
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os.remove(disk) |
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try: |
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os.remove(nbd_sock) |
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except OSError: |
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pass |
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|
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@contextmanager |
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def run_server(self, max_connections=None): |
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args = { |
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'addr': { |
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'type': 'unix', |
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'data': {'path': nbd_sock} |
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} |
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} |
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if max_connections is not None: |
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args['max-connections'] = max_connections |
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|
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result = self.vm.qmp('nbd-server-start', args) |
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self.assert_qmp(result, 'return', {}) |
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yield |
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result = self.vm.qmp('nbd-server-stop') |
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self.assert_qmp(result, 'return', {}) |
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def add_export(self, name, writable=None): |
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args = { |
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'type': 'nbd', |
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'id': name, |
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'node-name': 'n', |
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'name': name, |
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} |
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if writable is not None: |
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args['writable'] = writable |
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|
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result = self.vm.qmp('block-export-add', args) |
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self.assert_qmp(result, 'return', {}) |
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def test_default_settings(self): |
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with self.run_server(): |
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self.add_export('r') |
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self.add_export('w', writable=True) |
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with open_nbd('r') as h: |
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self.assertTrue(h.can_multi_conn()) |
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with open_nbd('w') as h: |
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self.assertTrue(h.can_multi_conn()) |
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|
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def test_limited_connections(self): |
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with self.run_server(max_connections=1): |
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self.add_export('r') |
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self.add_export('w', writable=True) |
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with open_nbd('r') as h: |
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self.assertFalse(h.can_multi_conn()) |
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with open_nbd('w') as h: |
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self.assertFalse(h.can_multi_conn()) |
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def test_parallel_writes(self): |
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with self.run_server(): |
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self.add_export('w', writable=True) |
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clients = [nbd.NBD() for _ in range(3)] |
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for c in clients: |
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c.connect_uri(nbd_uri.format('w')) |
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self.assertTrue(c.can_multi_conn()) |
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initial_data = clients[0].pread(1024 * 1024, 0) |
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self.assertEqual(initial_data, b'\x01' * 1024 * 1024) |
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updated_data = b'\x03' * 1024 * 1024 |
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clients[1].pwrite(updated_data, 0) |
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clients[2].flush() |
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current_data = clients[0].pread(1024 * 1024, 0) |
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self.assertEqual(updated_data, current_data) |
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for i in range(3): |
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clients[i].shutdown() |
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if __name__ == '__main__': |
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try: |
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# Easier to use libnbd than to try and set up parallel |
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# 'qemu-nbd --list' or 'qemu-io' processes, but not all systems |
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# have libnbd installed. |
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import nbd # type: ignore |
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iotests.main(supported_fmts=['qcow2']) |
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except ImportError: |
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iotests.notrun('libnbd not installed') |
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@ -0,0 +1,5 @@ |
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... |
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---------------------------------------------------------------------- |
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Ran 3 tests |
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|
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OK |
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