Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait /notify + callback chains

From: Christoph Hellwig (
Date: Thu Feb 01 2001 - 12:02:37 EST

On Thu, Feb 01, 2001 at 04:49:58PM +0000, Stephen C. Tweedie wrote:
> > Enquiring minds would like to know if you are working towards this
> > revamp of the kiobuf structure at the moment, you have been very quiet
> > recently.
> I'm in the middle of some parts of it, and am actively soliciting
> feedback on what cleanups are required.

The real issue is that Linus dislikes the current kiobuf scheme.
I do not like everything he proposes, but lots of things makes sense.

> I've been merging all of the 2.2 fixes into a 2.4 kiobuf tree, and
> have started doing some of the cleanups needed --- removing the
> embedded page vector, and adding support for lightweight stacking of
> kiobufs for completion callback chains.

Ok, great.

> However, filesystem IO is almost *always* page aligned: O_DIRECT IO
> comes from VM pages, and internal filesystem IO comes from page cache
> pages. Buffer cache IOs are the only exception, and kiobufs only fail
> for such IOs once you have multiple buffer_heads being merged into
> single requests.
> So, what are the benefits in the disk IO stack of adding length/offset
> pairs to each page of the kiobuf?

I don't see any real advantage for disk IO. The real advantage is that
we can have a generic structure that is also usefull in e.g. networking
and can lead to a unified IO buffering scheme (a little like IO-Lite).


Of course it doesn't work. We've performed a software upgrade.
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