From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] xen block backend driver.
Date: Wed, 20 Apr 2011 23:40:16 -0400
Message-ID: <20110421034016.GB11501@infradead.org>
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> +#define WRITE_BARRIER	(REQ_WRITE | REQ_FLUSH | REQ_FUA)

Any in case you want to keep usingthis POS out of tree you really need
to fix this.  REQ_FLUSH | REQ_FUA is not equivalent to the old barrier
semantics that the Xen disk protocol exported to the guest.  You need
to drain the whole queue of outstanding I/Os as some old guest (at least
those using reiserfs, e.g. old SLES) rely on it.