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

["Keld Jørn Simonsen" <keld@keldix.com>]

On Wed, Jun 08, 2011 at 04:42:27PM +0900, Namhyung Kim wrote:
> NeilBrown <neilb@suse.de> writes:
> 
> > On Wed,  8 Jun 2011 16:00:45 +0900 Namhyung Kim <namhyung@gmail.com> wrote:
> >
> >> 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 :(

the nearest position was the case for the initial implementation of
raid10-far.  But this had bad performance for an array with disks of
varying specifications. And also it led to not using the faster
outer sectors. Using the closest-to-beginning gave a spped-up of about
50 % in some cases.

best regards
keld