On Wed, Jun 08, 2011 at 04:42:27PM +0900, Namhyung Kim wrote: > NeilBrown <neilb@xxxxxxx> writes: > > > On Wed, 8 Jun 2011 16:00:45 +0900 Namhyung Kim <namhyung@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
