[dm-devel] [PATCH for-4.2 2/3] block, dm: don't copy bios for request clones

[Christoph Hellwig]

On Thu, May 28, 2015 at 06:38:29AM +0000, Junichi Nomura wrote:
> I'm also worried about partial completion case.
> For successful partial completion, current code completes
> bio before fully completing the request.
> Your patch changes bios not completed until the request is
> fully completed.

Yes.  Why are you worried about this case?

> I'm not very sure how much impact does the removal of partial
> completion have in the real world.
> If partial completion is so negligible, I think it should be
> handled in such a way all the cases, instead of special casing
> REQ_CLONE.

It isn't negligible - under load it actually does matter that
we handle partial completions as arrays decide to just complete
a request partially, so if we don't handle them we might end
up in a loop not making progress.  But if we only do that once on
path fail over it's not an actual issue.