diff options
| -rw-r--r-- | nova/privsep/qemu.py | 35 | ||||
| -rw-r--r-- | nova/tests/unit/virt/libvirt/test_driver.py | 2 | ||||
| -rw-r--r-- | nova/tests/unit/virt/libvirt/test_utils.py | 2 | ||||
| -rw-r--r-- | nova/tests/unit/virt/test_images.py | 2 | ||||
| -rw-r--r-- | releasenotes/notes/make-disk-image-conversion-faster-c4abe83ae702888b.yaml | 8 |
5 files changed, 32 insertions, 17 deletions
diff --git a/nova/privsep/qemu.py b/nova/privsep/qemu.py index 30d0df39d3..8c22d689cb 100644 --- a/nova/privsep/qemu.py +++ b/nova/privsep/qemu.py @@ -35,23 +35,30 @@ def convert_image(source, dest, in_format, out_format, instances_path, # NOTE(mikal): this method is deliberately not wrapped in a privsep entrypoint def unprivileged_convert_image(source, dest, in_format, out_format, instances_path, compress): - # NOTE(mdbooth): qemu-img convert defaults to cache=unsafe, which means - # that data is not synced to disk at completion. We explicitly use - # cache=none here to (1) ensure that we don't interfere with other - # applications using the host's io cache, and (2) ensure that the data is - # on persistent storage when the command exits. Without (2), a host crash - # may leave a corrupt image in the image cache, which Nova cannot recover - # automatically. - # NOTE(zigo): we cannot use -t none if the instances dir is mounted on a - # filesystem that doesn't have support for O_DIRECT, which is the case - # for example with tmpfs. This simply crashes "openstack server create" - # in environments like live distributions. In such case, the best choice - # is writethrough, which is power-failure safe, but still faster than - # writeback. + # NOTE(mdbooth, kchamart): `qemu-img convert` defaults to + # 'cache=writeback' for the source image, and 'cache=unsafe' for the + # target, which means that data is not synced to disk at completion. + # We explicitly use 'cache=none' here, for the target image, to (1) + # ensure that we don't interfere with other applications using the + # host's I/O cache, and (2) ensure that the data is on persistent + # storage when the command exits. Without (2), a host crash may + # leave a corrupt image in the image cache, which Nova cannot + # recover automatically. + + # NOTE(zigo, kchamart): We cannot use `qemu-img convert -t none` if + # the 'instance_dir' is mounted on a filesystem that doesn't support + # O_DIRECT, which is the case, for example, with 'tmpfs'. This + # simply crashes `openstack server create` in environments like live + # distributions. In such cases, the best choice is 'writeback', + # which (a) makes the conversion multiple times faster; and (b) is + # as safe as it can be, because at the end of the conversion it, + # just like 'writethrough', calls fsync(2)|fdatasync(2), which + # ensures to safely write the data to the physical disk. + if nova.privsep.utils.supports_direct_io(instances_path): cache_mode = 'none' else: - cache_mode = 'writethrough' + cache_mode = 'writeback' cmd = ('qemu-img', 'convert', '-t', cache_mode, '-O', out_format) if in_format is not None: diff --git a/nova/tests/unit/virt/libvirt/test_driver.py b/nova/tests/unit/virt/libvirt/test_driver.py index d2cf52febc..a3208facf2 100644 --- a/nova/tests/unit/virt/libvirt/test_driver.py +++ b/nova/tests/unit/virt/libvirt/test_driver.py @@ -19099,7 +19099,7 @@ class LibvirtDriverTestCase(test.NoDBTestCase, TraitsComparisonMixin): mock_disk_op_sema.__enter__.assert_called_once() mock_direct_io.assert_called_once_with(CONF.instances_path) mock_execute.assert_has_calls([ - mock.call('qemu-img', 'convert', '-t', 'writethrough', + mock.call('qemu-img', 'convert', '-t', 'writeback', '-O', 'qcow2', '-f', 'raw', path, _path_qcow)]) mock_rename.assert_has_calls([ mock.call(_path_qcow, path)]) diff --git a/nova/tests/unit/virt/libvirt/test_utils.py b/nova/tests/unit/virt/libvirt/test_utils.py index 2debf92871..8aeb4af2e0 100644 --- a/nova/tests/unit/virt/libvirt/test_utils.py +++ b/nova/tests/unit/virt/libvirt/test_utils.py @@ -532,7 +532,7 @@ disk size: 4.4M libvirt_utils.extract_snapshot('/path/to/disk/image', src_format, '/extracted/snap', dest_format) - qemu_img_cmd = ('qemu-img', 'convert', '-t', 'writethrough', + qemu_img_cmd = ('qemu-img', 'convert', '-t', 'writeback', '-O', out_format, '-f', src_format, ) if CONF.libvirt.snapshot_compression and dest_format == "qcow2": qemu_img_cmd += ('-c',) diff --git a/nova/tests/unit/virt/test_images.py b/nova/tests/unit/virt/test_images.py index 227a223860..9dacc828ad 100644 --- a/nova/tests/unit/virt/test_images.py +++ b/nova/tests/unit/virt/test_images.py @@ -125,7 +125,7 @@ class QemuTestCase(test.NoDBTestCase): mock_disk_op_sema): images._convert_image('source', 'dest', 'in_format', 'out_format', run_as_root=False) - expected = ('qemu-img', 'convert', '-t', 'writethrough', + expected = ('qemu-img', 'convert', '-t', 'writeback', '-O', 'out_format', '-f', 'in_format', 'source', 'dest') mock_disk_op_sema.__enter__.assert_called_once() self.assertTupleEqual(expected, mock_execute.call_args[0]) diff --git a/releasenotes/notes/make-disk-image-conversion-faster-c4abe83ae702888b.yaml b/releasenotes/notes/make-disk-image-conversion-faster-c4abe83ae702888b.yaml new file mode 100644 index 0000000000..b6870c4755 --- /dev/null +++ b/releasenotes/notes/make-disk-image-conversion-faster-c4abe83ae702888b.yaml @@ -0,0 +1,8 @@ +--- +fixes: + - | + By using ``writeback`` QEMU cache mode, make Nova's disk image + conversion (e.g. from raw to QCOW2 or vice versa) dramatically + faster, without compromising data integrity. `Bug 1818847`_. + + .. _Bug 1818847: https://bugs.launchpad.net/nova/+bug/1818847 |