From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: What are the semantics of BLKIF_OP_WRITE_BARRIER?
Date: Thu, 22 Jul 2010 13:41:25 -0700
Message-ID: <4C48ACF5.209@goop.org>
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To: Daniel Stodden <daniel.stodden@citrix.com>
Cc: "Xen-devel@lists.xensource.com" <Xen-devel@lists.xensource.com>, Jake Wires <Jake.Wires@citrix.com>
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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