On Sun, Aug 9, 2015 at 9:44 AM, Christoph Hellwig <h...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
