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Update git submodules 

* Update nova from branch 'master'
  - Merge "libvirt: Use 'writeback' QEMU cache mode when 'none' is not viable"
  - libvirt: Use 'writeback' QEMU cache mode when 'none' is not viable
    
    When configuring QEMU cache modes for Nova instances, we use
    'writethrough' when 'none' is not available.  But that's not correct,
    because of our misunderstanding of how cache modes work.  E.g. the
    function disk_cachemode() in the libvirt driver assumes that
    'writethrough' and 'none' cache modes have the same behaviour with
    respect to host crash safety, which is not at all true.
    
    The misunderstanding and complexity stems from not realizing that each
    QEMU cache mode is a shorthand to toggle *three* booleans.  Refer to the
    convenient cache mode table in the code comment (in
    nova/virt/libvirt/driver.py).
    
    As Kevin Wolf (thanks!), QEMU Block Layer maintainer, explains (I made
    a couple of micro edits for clarity):
    
        The thing that 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=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 OSes do
        that.
    
        So if 'cache=none' is safe enough for you, then 'cache=writeback'
        should be safe enough for you, too -- because both of them have the
        boolean 'cache.writeback=on'.  The difference is only in
        'cache.direct', but 'cache.direct=on' only bypasses the host kernel
        page cache and data could still sit in other caches that could be
        present between QEMU and the disk (such as commonly a volatile write
        cache on the disk itself).
    
    So use 'writeback' mode instead of the debilitatingly slow
    'writethrough' for cases where the O_DIRECT-based 'none' is unsupported.
    
    Do the minimum required update to the `disk_cachemodes` config help
    text.  (In a future patch, rewrite the cache modes documentation to fix
    confusing fragments and outdated information.)
    
    Closes-Bug: #1818847
    Change-Id: Ibe236988af24a3b43508eec4efbe52a4ed05d45f
    Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
    Looks-good-to-me'd-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Zuul 2019-04-02 16:34:20 +00:00 committed by Gerrit Code Review
parent a838f2ab08
commit d191352f4f
1 changed files with 1 additions and 1 deletions
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2
nova

@ -1 +1 @@
Subproject commit 3f3937a64d4dece69ca5f8e5e2e8017a89226387
Subproject commit 2384c41b781a84de98d0932f44d4b3c544c3fe3d