Messages in this thread | | | Date | Mon, 30 Sep 2013 09:28:56 -0500 | From | Ric Wheeler <> | Subject | Re: [RFC] extending splice for copy offloading |
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On 09/30/2013 10:24 AM, Miklos Szeredi wrote: > On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler <rwheeler@redhat.com> wrote: >> On 09/30/2013 10:51 AM, Miklos Szeredi wrote: >>> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields <bfields@fieldses.org> >>> wrote: >>>>> My other worry is about interruptibility/restartability. Ideas? >>>>> >>>>> What happens on splice(from, to, 4G) and it's a non-reflink copy? >>>>> Can the page cache copy be made restartable? Or should splice() be >>>>> allowed to return a short count? What happens on (non-reflink) remote >>>>> copies and huge request sizes? >>>> If I were writing an application that required copies to be restartable, >>>> I'd probably use the largest possible range in the reflink case but >>>> break the copy into smaller chunks in the splice case. >>>> >>> The app really doesn't want to care about that. And it doesn't want >>> to care about restartability, etc.. It's something the *kernel* has >>> to care about. You just can't have uninterruptible syscalls that >>> sleep for a "long" time, otherwise first you'll just have annoyed >>> users pressing ^C in vain; then, if the sleep is even longer, warnings >>> about task sleeping too long. >>> >>> One idea is letting splice() return a short count, and so the app can >>> safely issue SIZE_MAX requests and the kernel can decide if it can >>> copy the whole file in one go or if it wants to do it in smaller >>> chunks. >>> >> You cannot rely on a short count. That implies that an offloaded copy starts >> at byte 0 and the short count first bytes are all valid. > Huh? > > - app calls splice(from, 0, to, 0, SIZE_MAX) > 1) VFS calls ->direct_splice(from, 0, to, 0, SIZE_MAX) > 1.a) fs reflinks the whole file in a jiffy and returns the size of the file > 1 b) fs does copy offload of, say, 64MB and returns 64M > 2) VFS does page copy of, say, 1MB and returns 1MB > - app calls splice(from, X, to, X, SIZE_MAX) where X is the new offset > ... > > The point is: the app is always doing the same (incrementing offset > with the return value from splice) and the kernel can decide what is > the best size it can service within a single uninterruptible syscall. > > Wouldn't that work? > > Thanks, > Miklos
No.
Keep in mind that the offload operation in (1) might fail partially. The target file (the copy) is allocated, the question is what ranges have valid data.
I don't see that (2) is interesting or really needed to be done in the kernel. If nothing else, it tends to confuse the discussion....
ric
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