Re: barriers vs. reads

[Jens Axboe <axboe@suse.de>]

On Tue, Jun 22 2004, Werner Almesberger wrote:
> Jens Axboe wrote:
> > It can happen with direct io of any sort, the solution has to take this
> > into account. That's why we currently have handling for rbtree aliases
> > as well.
> 
> How well is this actually supposed to work ? When reading what
> as-iosched does, I was left with the impression that you could
> construct a set of partially overlapping requests that doesn't
> get sorted in FIFO order.

Overlapping requests are only detected if they start at the same
sector.

The mechanism is just there because of the data structure use, Linux has
never made any effort to guard against this outside of the page cache
context. If you issue direct io that overlaps, you are providing your
own rope.

> I haven't tried to feed as-iosched such a request mix, though,
> so maybe I'm wrong.
> 
> (For partially overlapping requests, it may actually be nice
> to be able to break them into multiple parts, and queue them
> separately. Particularly if they also come with distinct
> priorities.)

Bad idea, unless you have zero setup overhead for the hardware issued
commands. Linux will also attempt to remerge these requests when it
later discovers they are adjacent. You can block this by disallowing
merging of request with different priorities, but I really don't see why
you'd want to do that. It would be a net loss in the end anyways.

-- 
Jens Axboe