Re: [RFC] extending splice for copy offloading

Zach Brown
2013-10-01T19:58:59Z

> - 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

(It's not SIZE_MAX. It's MAX_RW_COUNT. INT_MAX with some
PAGE_CACHE_SIZE rounding noise. For fear of weird corners of fs code
paths that still use int, one assumes.)

> 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?

It seems like it should, if people are willing to allow splice() to
return partial counts. Quite a lot of IO syscalls technically do return
partial counts today if you try to write > MAX_RW_COUNT :).

But returning partial counts on the order of a handful of megs that the
file systems make up as the point of diminishing returns is another
thing entirely. I can imagine people being anxious about that.

I guess we'll find out!

- z