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

To: Daniel Stodden <daniel.stodden@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: linux-next regression: IO errors in with ext4 and xen-blkfront
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 27 Oct 2010 06:23:09 -0400
Cc: Jens Axboe <axboe@xxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
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I'm really not interested in getting into this flamewar again.

If you want to make Xen blockdevices work reliably you need to implement
a cache flush primitive in the driver.  If your cache flush primitive
also enforced ordering that's fine for data integrity, but won't help
your performance.

Note that current the _driver_ does not implement the cache flushes
correctly which is what started this thread and the previous flamewar.
If you can fix it using the existing primitive with just driver changes
that's fine - but according to
http://mail-index.netbsd.org/port-xen/2010/09/24/msg006274.html at least
the NetBSD people didn't think so.

For details on the implementation refer to the
Documentation/block/writeback_cache_control.txt file in the kernel tree,
for reasons why we got rid of barriers with their syncronization
semantics refer to various threads on -fsdevel and lkml during the past
couple of month (search your favour archive for barriers).

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