diff options
Diffstat (limited to 'nova/privsep')
| -rw-r--r-- | nova/privsep/qemu.py | 35 |
1 files changed, 21 insertions, 14 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: |