Re: 2.6.4-mm2

[Jens Axboe]

On Fri, Mar 19 2004, Andrew Morton wrote:
> Jens Axboe <axboe@xxxxxxx> wrote:
> >
> > On Fri, Mar 19 2004, Andrew Morton wrote:
> > > Jens Axboe <axboe@xxxxxxx> wrote:
> > > >
> > > > > Is it not the case that two dm maps can refer to the same queue?  Say, one
> > > > > map uses /dev/hda1 and another map uses /dev/hda2?
> > > > > 
> > > > > If so, then when the /dev/hda queue is plugged we need to tell both the
> > > > > higher-level maps that this queue needs an unplug.  So blk_plug_device()
> > > > > and the various unplug functions need to perform upcalls to an arbitrary
> > > > > number of higher-level drivers, and those drivers need to keep track of the
> > > > > currently-plugged queues without adding data structures to the
> > > > > request_queue structure.
> > > > > 
> > > > > It can be done of course, but could get messy.
> > > > 
> > > > That would get nasty, it's much more natural to track it from the other
> > > > end. I view it as a dm (or whatever problem) that they need to track who
> > > > has pending io on their behalf, which is pretty easy to to from eg
> > > > __map_bio().
> > > 
> > > But dm doesn't know enough.  Suppose it is managing a map which includes
> > > /dev/hda1 and I do some I/O against /dev/hda2 which plugs the queue.  dm
> > > needs to know about that plug.
> > 
> > I don't follow at all. If dm initiates io against /dev/hda2, it's part
> > of that mapped device and it can trigger and note the unplug just fine.
> 
> Right.  But suppose I have a plain old ext2 fs mounted on /dev/hda1.  I do
> some I/O against /dev/hda1, and that causes the /dev/hda queue to be
> plugged.  Device mapper doesn't know that the queue is currently plugged,
> because it was some totally unrelated piece of code which caused the plug.

You are right, that breaks. No good.

-- 
Jens Axboe

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