---
fixes:
  - |
    Update the way QEMU cache mode is configured for Nova guests: If the
    file system hosting the directory with Nova instances is capable of
    Linux's O_DIRECT, use ``none``; otherwise fallback to ``writeback``
    cache mode.  This improves performance without compromising data
    integrity.  `Bug 1818847`_.

    Context: What makes ``writethrough`` so safe against host crashes is
    that it never keeps data in a "write cache", but it calls fsync()
    after *every* write.  This is also what makes it horribly slow.  But
    cache mode ``none`` doesn't do this and therefore doesn't provide
    this kind of safety.  The guest OS must explicitly flush the cache
    in the right places to make sure data is safe on the disk; and all
    modern OSes flush data as needed.  So if cache mode ``none`` is safe
    enough for you, then ``writeback`` should be safe enough too.

    .. _Bug 1818847: https://bugs.launchpad.net/nova/+bug/1818847
