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

[Ni zhan Chen <nizhan.chen@gmail.com>]

On 10/25/2012 10:04 AM, YingHang Zhu wrote:
> 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

But how can bdi related ra_pages reflect different files' readahead 
window? Maybe these different files are sequential read, random read and 
so on.

> 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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