Re: [dm-devel] REQUEST for new 'topology' metrics to be moved out of the 'queue' sysfs directory.

[Neil Brown <neilb@suse.de>]

On Tuesday June 30, adilger@sun.com wrote:
> On Jun 29, 2009  13:41 +0200, Jens Axboe wrote:
> > ... externally it just makes the API worse since tools then have to know
> > which device type they are talking to.
> > 
> > So I still see absolutely zero point in making such a change, quite the
> > opposite.
> 
> Exactly correct.  Changing these tunables just for the sake of giving
> them a slightly different name is madness.  Making all block devices
> appear more uniform to userspace (even if they don't strictly need all
> of the semantics) is very sensible.  The whole point of the kernel is
> to abstract away the underlying details so that userspace doesn't need
> to understand it all again.

Uniformity is certainly desirable.  But we shouldn't take it so far
as to make apples look like oranges.

We wouldn't want a SATA disk drive to have 'chunk_size' and 'raid_disks'.
Nor would we want a software RAID array to have a 'scheduler' or
'iosched' attributes.

> 
> In order to get good throughput on RAID arrays we need to tune the
> queue/max_* values to ensure the IO requests don't get split.
> 
> It would be great if the MD queue/max_* values would pass these tunings
> down to the underlying disk devices as well.  As it stands now, we have
> to follow the /sys/block/*/slaves tree to set all of these ourselves,
> and before "slaves/" was introduced it was nigh impossible to automatically
> tune these values.

I don't think that passing these values down is - in general - a well
defined problem.  This is (in part) because md/dm devices can be based on
partitions, and partitions don't have independent max_* values.

In your particular case, I don't expect that you use partitions, so it
makes perfect sense to do the tuning on a per-array basis.  But I
don't think that it is a concept that fits in the kernel.  As you say,
we have 'slaves/', which makes it practical to do this in user-space
and I would rather it stayed there.

NeilBrown