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

Dave Chinner
2015-08-09T23:41:32Z

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