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

From: Christoph Hellwig
Date: Sun Aug 09 2015 - 09:44:14 EST

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.

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