Re: [PATCH v9 6/6] block: loop: support DIO & AIO

From: Dave Chinner
Date: Sun Aug 09 2015 - 19:41:56 EST

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@xxxxxxxxxxxxx> 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.


Dave Chinner
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