On Wed, 2 Oct 2013, Jan Kara wrote:
On Tue 01-10-13 12:58:17, Zach Brown wrote:
- 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 :).
Yes. Also POSIX says that application must handle such case for read &
write. But in practice programmers are lazy.
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!
Return 4 KB once in a while to screw up buggy applications from the
start :-p
or at least have a debugging option early on that does this so people can use it
to find such buggy apps.
David Lang
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