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Date: Mon, 10 Aug 2015 09:41:26 +1000
From: Dave Chinner <david@fromorbit.com>
To: Ming Lei <ming.lei@canonical.com>
Cc: Christoph Hellwig <hch@infradead.org>, Jens Axboe <axboe@kernel.dk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dave Kleikamp <dave.kleikamp@oracle.com>, Zach Brown <zab@zabbo.net>,
        Maxim Patlasov <mpatlasov@parallels.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v9 6/6] block: loop: support DIO & AIO
Message-ID: <20150809234126.GA906@dastard>
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On Sun, Aug 09, 2015 at 10:27:14AM -0400, Ming Lei wrote:
> On Sun, Aug 9, 2015 at 9:44 AM, Christoph Hellwig <hch@infradead.org> wrote:
> > On Sun, Aug 09, 2015 at 09:04:32AM -0400, Ming Lei wrote:
> >> Now the logical block size of loop is 512byte, but the sector size of
> >> backing device may be 4K, so we can't do 512 byte algined direct
> >> I/O to the filesystem in this situation.
> >>
> >> With runtime switch to buffered I/O we can fix this problem, but
> >> most of times it won't fall in this case.
> >
> > The right way to handle this case is to not allow direct I/O at all,
> > you'll be constantly switching between direct and buffered I/O
> > otherwise, which is a bad idea.
> 
> In theory we should not allow dio for this case, but in reality,
> as Dave Chinner mentioned, all buffered I/O and normal dio
> are page aligned, so we can make most of sane applications
> benefited from this patch, and for few insane applications,
> frequent swich between buffered I/O and direct I/O shouldn't
> be very bad too since draining dio is just like batched sync I/O.

If Christoph thinks we should not allow this case, then go with what
Christoph says.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com