Groups > linux.kernel > #1203648

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

From	Ming Lei <ming.lei@canonical.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v9 6/6] block: loop: support DIO & AIO
Date	2015-08-09 16:30 +0200
Message-ID	<pVzLP-dy-3@gated-at.bofh.it> (permalink)
References	(2 earlier) <pUKzE-1p3-19@gated-at.bofh.it> <pULcm-2og-9@gated-at.bofh.it> <pVsAG-6A9-13@gated-at.bofh.it> <pVywp-6Xi-5@gated-at.bofh.it> <pVz97-7GV-3@gated-at.bofh.it>
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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.

>
>> For non-512 byte sector size of backing device, it is needed as
>> I described above.
>
> If the backing device sector size is smaller than that of the loop

I guess you mean bigger? Suppose the backing device sector
size is 256(smaller), direct I/O should be used without problem.

> device you should not allow direct I/O as you will be switching forth
> and back ue to the IO patterns all the time.


Thanks,
Ming
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Thread

[PATCH v9 0/6] block: loop: improve loop with AIO Ming Lei <ming.lei@canonical.com> - 2015-08-06 10:50 +0200
  [PATCH v9 6/6] block: loop: support DIO & AIO Ming Lei <ming.lei@canonical.com> - 2015-08-06 10:50 +0200
    Re: [PATCH v9 6/6] block: loop: support DIO & AIO Christoph Hellwig <hch@infradead.org> - 2015-08-07 09:50 +0200
      Re: [PATCH v9 6/6] block: loop: support DIO & AIO Ming Lei <ming.lei@canonical.com> - 2015-08-07 10:30 +0200
        Re: [PATCH v9 6/6] block: loop: support DIO & AIO Christoph Hellwig <hch@infradead.org> - 2015-08-09 08:50 +0200
          Re: [PATCH v9 6/6] block: loop: support DIO & AIO Ming Lei <ming.lei@canonical.com> - 2015-08-09 15:10 +0200
            Re: [PATCH v9 6/6] block: loop: support DIO & AIO Christoph Hellwig <hch@infradead.org> - 2015-08-09 15:50 +0200
              Re: [PATCH v9 6/6] block: loop: support DIO & AIO Ming Lei <ming.lei@canonical.com> - 2015-08-09 16:30 +0200
                Re: [PATCH v9 6/6] block: loop: support DIO & AIO Dave Chinner <david@fromorbit.com> - 2015-08-10 01:50 +0200
                Re: [PATCH v9 6/6] block: loop: support DIO & AIO Ming Lei <ming.lei@canonical.com> - 2015-08-10 05:00 +0200
                Re: [PATCH v9 6/6] block: loop: support DIO & AIO Christoph Hellwig <hch@infradead.org> - 2015-08-10 10:10 +0200
                Re: [PATCH v9 6/6] block: loop: support DIO & AIO Ming Lei <ming.lei@canonical.com> - 2015-08-10 16:40 +0200

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