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

[Junichi Nomura]

On 05/30/15 01:54, Christoph Hellwig wrote:
> 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?

See below.

>> 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.

OK.

> But if we only do that once on
> path fail over it's not an actual issue.

But with your patch, the fail over happens after full completion
of the request. Is it ok?

-- 
Jun'ichi Nomura, NEC Corporation