[linux-lvm] Re: [PATCH] bdi_congestion_funp (was: Re: [PATCH] per process request limits (was Re: IO scheduler, queue depth, nr_requests))

[Jens Axboe]

On Fri, Feb 20 2004, Miquel van Smoorenburg wrote:
> > > Even if it isn't happening
> > > a lot, and something isn't bust it might be a good idea to
> > > do this.
> > 
> > Seems OK from a quick check.  pdflush will block in get_request_wait()
> > occasionally, but not at all often.  Perhaps we could move the
> > write_congested test into the mpage_writepages() inner loop but it hardly
> > seems worth the risk.
> > 
> > Maybe things are different on Miquel's clockwork controller.
> 
> I haven't tested it yet because of the "This patch isn't actually so good"
> comment, but I found another explanation.
> 
> >  drivers/block/ll_rw_blk.c |    2 ++
> >  fs/fs-writeback.c         |    2 ++
> >  2 files changed, 4 insertions(+)
> 
> *Lightbulb on* .. I just read fs-writeback.c. As I said, this happens
> with an LVM device. Could it be that because LVM and the actual device
> have different struct request_queue's things go awry ?
> 
> In fs-writeback.c, your're looking at the LVM device (and its
> request_queue, and its backing_dev_info). In__make_request, you're
> looking at the SCSI device.

In principle, the lvm/md queues themselves will never be congested. But
the underlying queues can be, of course.

Now this approach is _much_ better, imo. I don't particularly care very
much for how you solved it, though, I'd much rather just see both
setting and testing passed down (and kill the ->aux as well).

Regardless of the initial hw depth vs block depth (which is also a
generic device problem, not just dm related), this would be a good
addition to the congestion logic.

-- 
Jens Axboe