Xen devel - Jul 2010 - What are the semantics of BLKIF_OP_WRITE_BARRIER?

What does BLKIF_OP_WRITE_BARRIER mean precisely?  Presumably it prevents
writes from being reordered past it.  But does it also imply a flush, in
that all previous writes are on stable storage?  If not, is there
another operation which does guarantee that?  If the backend doesn''t
support barriers, is there any other way for a guest to force writes to
stable storage?

How does blktap(2) deal with devices with volatile write caches?  Does
it generate its own barriers/flushes/FUA requests to push things out to
stable storage?

Thanks,
    J

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