Re: [PATCH/RFC] md/raid10: optimize read_balance() for 'far copies' arrays

NeilBrown [off-list ref] writes:

On Wed,  8 Jun 2011 16:00:45 +0900 Namhyung Kim [off-list ref] wrote:

quoted

If @conf->far_offset > 0, there is only 1 stripe so that we can treat
the array same as 'near' arrays. Furthermore we could calculate new
distance from the previous position even for the real 'far' array
cases if the position of given disk is already in the lowest stripe.

I agree that it still make sense to to balancing if far_offset != 0.
However  there is absolutely no point in your change to the calculation of
new_distance.
You only wont new_distance to contain a distance from head position if we
want to choose the device with the 'closest' head.  But we don't.  We want to
choose the device were the data is closest to the start of the device.  So
the current value for new_distance is correct.

Still can't understand why we choose the closest-to-the-start disk in
case we could have possible sequencial access on other disk. Probably
because of the lack of my understanding how md/disk works :(

If you would like to resubmit with just the first change I'll happily apply
the patch.

OK. Will do that right soon.

If you have performed some tests and can demonstrate some cases where this
makes something faster, and can show us the results of those tests, I would
be even more happy!!!

I wish I could. :) However, unfortunately, I don't have such a real system
to test on.

Thanks.