Comment 2 for bug 1461740

Apparently this wasn't clear in the bug report that fallocate expects the file to exist,

http://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/sys-utils/fallocate.c#n321
http://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/sys-utils/fallocate.c#n371

This can be validated on the command line,

$ /usr/bin/fallocate -n -l 1 /tmp/test_file.fallocate

2015-06-29 02:58:39.425 DEBUG oslo_concurrency.processutils [req-6b576698-dfcd-4bd9-9166-0fcc22180794 None None] CMD "fallocate -n -l 1 /opt/openstack/nova/instances/80ebc3ae-f005-4c6c-9a78-33664df5c78f/disk.fallocate_test" returned: 0 in 0.036s