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Fix launching of guests where instances_path is on GlusterFS
The FUSE module does not (currentl) support O_DIRECT on files. This prevents QEMU from starting guests using 'cache=none' for their disks located on a GlusterFS filesystem. The same also applies for a handful of other filesystems (notably tmpfs, or any other FUSE filesystem). This patch introduces a startup check in Nova compute service which tries to create a file $instances_path/.direct_io.test using the O_DIRECT flag. If this succeeds, then cache=none will be used for all disks, otherwise it will fallback to using cache=writethrough. While the latter does not have performance which is as consistent as cache=none, it is still host-crash safe and preserves data integrity with migration, if the filesystem is cache coherant (cluster filesystems like GlusterFS are, NFS by constrast is not). By doing the dynamic check for O_DIRECT, we ensure that if future FUSE modules gain O_DIRECT support, Nova will automatically do the right thing. * nova/tests/test_libvirt.py: Stub out os.open in the _check_xml_and_disk_driver() to enable testing of both O_DIRECT and non-O_DIRECT code paths * nova/tests/test_virt_drivers.py: Set instances_path to the current directory * nova/virt/libvirt.xml.template: Replace hardcoded 'none' string with the '$cachemode' variable for all disks. Add missing 'cache' attribute for the config disk * nova/virt/libvirt/connection.py: Check whether O_DIRECT is supported on the "FLAGS.instances_path" directory and use 'none' for cachemode if it is, 'writethrough' otherwise Bug: 959637 Change-Id: I60cbff1c3ad8299fe2aa37099390f9235f6724d0 Signed-off-by: Daniel P. Berrange <firstname.lastname@example.org>
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