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xen-devel
[Xen-devel] Re: [Qemu-devel] [PATCH] qemu and qemu-xen: support empty wr
I had the discussion with Jeremy in Boston before, but let's repeat it
here:
- is there actually any pre-existing xen backend that does properly
implement empty barries. Back then we couldn't find any.
- if this is a new concept to Xen please do not define an empty
barrier primitive, but a new flush cache primitive. That one
maps natively to the qemu I/O layer, and with recent Linux, NetBSD,
Windows, or Solaris guest will be a lot faster than a barrier
which drains the queue.
Note that what your patch implements actually is a rather inefficient
implementation of the latter. You do none of the queue draining which
the in-kernel blkback implementation does by submitting the old-style
barrier bio. While most filesystem do not care you introduce a quite
subtile chance of data corruption for reiserfs, or ext4 with
asynchronous journal commits on pre-2.6.37 kernels.
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