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[Xen-devel] Re: linux-next regression: IO errors in with ext4 and xen-bl

To: Jens Axboe <axboe@xxxxxxxxx>
Subject: [Xen-devel] Re: linux-next regression: IO errors in with ext4 and xen-blkfront
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Fri, 22 Oct 2010 04:29:16 -0400
Cc: Theodore Ts'o <tytso@xxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, Andreas Dilger <adilger.kernel@xxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
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User-agent: Mutt/1.5.21 (2010-09-15)
In the barriers tree Xen claims to support flushes, but I doesn't.
It never handles REQ_FLUSH requests.  Try commenting out the

        blk_queue_flush(info->rq, info->feature_flush);

call and things should improve.  I still need to hear back from Xen
folks how to actually implement a cache flush - they only implement
a barrier write privilegue which could never implement an empty
cache flush.  Up to current kernels that meant it would implement
barrier writes with content correctly and silently ignore empty barriers
leading to very interesting data integrity bugs.  From 2.6.37 onwards
it simply won't work anymore at all, which is at least consistent
(modulo the bug of actually claiming to support flushes).

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