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Date: Thu, 25 Oct 2012 10:12:01 +0800 From: Ni zhan Chen <nizhan.chen@...il.com> To: YingHang Zhu <casualfisher@...il.com> CC: Dave Chinner <david@...morbit.com>, akpm@...ux-foundation.org, Fengguang Wu <fengguang.wu@...el.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state On 10/25/2012 10:04 AM, YingHang Zhu wrote: > On Thu, Oct 25, 2012 at 9:50 AM, Dave Chinner <david@...morbit.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@...morbit.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@...morbit.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@...morbit.com > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@...ck.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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