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

[Ni zhan Chen]

On 10/25/2012 08:17 AM, YingHang Zhu wrote:
> On Thu, Oct 25, 2012 at 4:19 AM, Dave Chinner <david@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.
> 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.

And how to tune ra_pages if one big file distribution in different 
disks, I think Fengguang Wu can answer these questions,

Hi Fengguang,

> > > The above third-party application using our file system maintains
> > > some long-opened files, we does not have any chances
> > > to force them to call fadvise(POSIX_FADV_NORMAL). :(
> > So raise a bug/feature request with the third party.  Modifying
> > kernel code because you can't directly modify the application isn't
> > the best solution for anyone. This really is an application problem
> > - the kernel already provides the mechanisms to solve this
> > problem...  :/
> Thanks for advice, I will consult the above application's developers
> for more information.
> Now from the code itself should we merge the gap between the real
> device's ra_pages and the file's?
> Obviously the ra_pages is duplicated, otherwise each time we run into this
> problem, someone will do the same work as I have done here.
>
> Thanks,
>           Ying Zhu
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@xxxxxxxxxxxxx
> --
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