Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

[YingHang Zhu <casualfisher@gmail.com>]

On Thu, Oct 25, 2012 at 9:50 AM, Dave Chinner <david@fromorbit.com> wrote:
> On Thu, Oct 25, 2012 at 08:17:05AM +0800, YingHang Zhu wrote:
>> On Thu, Oct 25, 2012 at 4:19 AM, Dave Chinner <david@fromorbit.com> wrote:
>> > On Wed, Oct 24, 2012 at 07:53:59AM +0800, YingHang Zhu wrote:
>> >> Hi Dave,
>> >> On Wed, Oct 24, 2012 at 6:47 AM, Dave Chinner <david@fromorbit.com> wrote:
>> >> > On Tue, Oct 23, 2012 at 08:46:51PM +0800, Ying Zhu wrote:
>> >> >> Hi,
>> >> >>   Recently we ran into the bug that an opened file's ra_pages does not
>> >> >> synchronize with it's backing device's when the latter is changed
>> >> >> with blockdev --setra, the application needs to reopen the file
>> >> >> to know the change,
>> >> >
>> >> > or simply call fadvise(fd, POSIX_FADV_NORMAL) to reset the readhead
>> >> > window to the (new) bdi default.
>> >> >
>> >> >> which is inappropriate under our circumstances.
>> >> >
>> >> > Which are? We don't know your circumstances, so you need to tell us
>> >> > why you need this and why existing methods of handling such changes
>> >> > are insufficient...
>> >> >
>> >> > Optimal readahead windows tend to be a physical property of the
>> >> > storage and that does not tend to change dynamically. Hence block
>> >> > device readahead should only need to be set up once, and generally
>> >> > that can be done before the filesystem is mounted and files are
>> >> > opened (e.g. via udev rules). Hence you need to explain why you need
>> >> > to change the default block device readahead on the fly, and why
>> >> > fadvise(POSIX_FADV_NORMAL) is "inappropriate" to set readahead
>> >> > windows to the new defaults.
>> >> Our system is a fuse-based file system, fuse creates a
>> >> pseudo backing device for the user space file systems, the default readahead
>> >> size is 128KB and it can't fully utilize the backing storage's read ability,
>> >> so we should tune it.
>> >
>> > Sure, but that doesn't tell me anything about why you can't do this
>> > at mount time before the application opens any files. i.e.  you've
>> > simply stated the reason why readahead is tunable, not why you need
>> > to be fully dynamic.....
>> We store our file system's data on different disks so we need to change ra_pages
>> dynamically according to where the data resides, it can't be fixed at mount time
>> or when we open files.
>
> That doesn't make a whole lot of sense to me. let me try to get this
> straight.
>
> There is data that resides on two devices (A + B), and a fuse
> filesystem to access that data. There is a single file in the fuse
> fs has data on both devices. An app has the file open, and when the
> data it is accessing is on device A you need to set the readahead to
> what is best for device A? And when the app tries to access data for
> that file that is on device B, you need to set the readahead to what
> is best for device B? And you are changing the fuse BDI readahead
> settings according to where the data in the back end lies?
>
> It seems to me that you should be setting the fuse readahead to the
> maximum of the readahead windows the data devices have configured at
> mount time and leaving it at that....
Then it may not fully utilize some device's read IO bandwidth and put too much
burden on other devices.
>
>> The abstract bdi of fuse and btrfs provides some dynamically changing
>> bdi.ra_pages
>> based on the real backing device. IMHO this should not be ignored.
>
> btrfs simply takes into account the number of disks it has for a
> given storage pool when setting up the default bdi ra_pages during
> mount.  This is basically doing what I suggested above.  Same with
> the generic fuse code - it's simply setting a sensible default value
> for the given fuse configuration.
>
> Neither are dynamic in the sense you are talking about, though.
Actually I've talked about it with Fengguang, he advised we should unify the
ra_pages in struct bdi and file_ra_state and leave the issue that
spreading data
across disks as it is.
Fengguang, what's you opinion about this?

Thanks,
         Ying Zhu
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com

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