Merge "qemu: Make disk image conversion dramatically faster"
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0962bb3b2f
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@ -35,23 +35,30 @@ def convert_image(source, dest, in_format, out_format, instances_path,
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# NOTE(mikal): this method is deliberately not wrapped in a privsep entrypoint
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def unprivileged_convert_image(source, dest, in_format, out_format,
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instances_path, compress):
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# NOTE(mdbooth): qemu-img convert defaults to cache=unsafe, which means
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# that data is not synced to disk at completion. We explicitly use
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# cache=none here to (1) ensure that we don't interfere with other
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# applications using the host's io cache, and (2) ensure that the data is
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# on persistent storage when the command exits. Without (2), a host crash
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# may leave a corrupt image in the image cache, which Nova cannot recover
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# automatically.
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# NOTE(zigo): we cannot use -t none if the instances dir is mounted on a
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# filesystem that doesn't have support for O_DIRECT, which is the case
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# for example with tmpfs. This simply crashes "openstack server create"
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# in environments like live distributions. In such case, the best choice
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# is writethrough, which is power-failure safe, but still faster than
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# writeback.
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# NOTE(mdbooth, kchamart): `qemu-img convert` defaults to
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# 'cache=writeback' for the source image, and 'cache=unsafe' for the
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# target, which means that data is not synced to disk at completion.
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# We explicitly use 'cache=none' here, for the target image, to (1)
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# ensure that we don't interfere with other applications using the
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# host's I/O cache, and (2) ensure that the data is on persistent
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# storage when the command exits. Without (2), a host crash may
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# leave a corrupt image in the image cache, which Nova cannot
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# recover automatically.
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# NOTE(zigo, kchamart): We cannot use `qemu-img convert -t none` if
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# the 'instance_dir' is mounted on a filesystem that doesn't support
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# O_DIRECT, which is the case, for example, with 'tmpfs'. This
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# simply crashes `openstack server create` in environments like live
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# distributions. In such cases, the best choice is 'writeback',
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# which (a) makes the conversion multiple times faster; and (b) is
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# as safe as it can be, because at the end of the conversion it,
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# just like 'writethrough', calls fsync(2)|fdatasync(2), which
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# ensures to safely write the data to the physical disk.
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if nova.privsep.utils.supports_direct_io(instances_path):
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cache_mode = 'none'
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else:
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cache_mode = 'writethrough'
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cache_mode = 'writeback'
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cmd = ('qemu-img', 'convert', '-t', cache_mode, '-O', out_format)
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if in_format is not None:
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@ -19099,7 +19099,7 @@ class LibvirtDriverTestCase(test.NoDBTestCase, TraitsComparisonMixin):
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mock_disk_op_sema.__enter__.assert_called_once()
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mock_direct_io.assert_called_once_with(CONF.instances_path)
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mock_execute.assert_has_calls([
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mock.call('qemu-img', 'convert', '-t', 'writethrough',
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mock.call('qemu-img', 'convert', '-t', 'writeback',
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'-O', 'qcow2', '-f', 'raw', path, _path_qcow)])
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mock_rename.assert_has_calls([
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mock.call(_path_qcow, path)])
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@ -532,7 +532,7 @@ disk size: 4.4M
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libvirt_utils.extract_snapshot('/path/to/disk/image', src_format,
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'/extracted/snap', dest_format)
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qemu_img_cmd = ('qemu-img', 'convert', '-t', 'writethrough',
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qemu_img_cmd = ('qemu-img', 'convert', '-t', 'writeback',
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'-O', out_format, '-f', src_format, )
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if CONF.libvirt.snapshot_compression and dest_format == "qcow2":
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qemu_img_cmd += ('-c',)
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@ -125,7 +125,7 @@ class QemuTestCase(test.NoDBTestCase):
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mock_disk_op_sema):
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images._convert_image('source', 'dest', 'in_format', 'out_format',
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run_as_root=False)
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expected = ('qemu-img', 'convert', '-t', 'writethrough',
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expected = ('qemu-img', 'convert', '-t', 'writeback',
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'-O', 'out_format', '-f', 'in_format', 'source', 'dest')
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mock_disk_op_sema.__enter__.assert_called_once()
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self.assertTupleEqual(expected, mock_execute.call_args[0])
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@ -0,0 +1,8 @@
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---
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fixes:
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- |
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By using ``writeback`` QEMU cache mode, make Nova's disk image
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conversion (e.g. from raw to QCOW2 or vice versa) dramatically
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faster, without compromising data integrity. `Bug 1818847`_.
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.. _Bug 1818847: https://bugs.launchpad.net/nova/+bug/1818847
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