quoted

1. If the pdflush issue is fixed, we should go back to bdi congestion method,
as block layer is more appropriate and accurate to tell when the device is
congested. Device q depth 256 is very generic.
2. Consider RAID1 devices at different speed (SSD and iscsi LUN) not too sure
if this approach would lead to the FS layer IO performance  throttle at the
speed of the lowest ? wonder how to reliably test it.

The referenced commits are from 2008, there have been many changes in
the queue flushing etc, so we might need to revisit the current
behaviour completely. Using the congestion API is desired, but we also
need to keep the IO behaviour (or make it better of course). In such
case I'd suggest small steps so we can possibly catch the regressions.

  Ok. Will try. Its still confusing to me.

Thanks, Anand