Keld JÃrn Simonsen <keld@xxxxxxxxxx> writes: > On Wed, Jun 08, 2011 at 04:42:27PM +0900, Namhyung Kim wrote: >> 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. > Hi Keld, Thanks for the explanation. That means lower sectors reside on the outer tracks/cylinders in the disk, right? The 50% seems a huge improvement I couldn't stand against. Although my patch tried to choose closest-to-current-head disk if the disk head is in the lowest stripe - in the (similar) hope that it'd be on the outer tracks - I don't have the numbers, so I'll just give up on it. Besides, I just noticed that the rationale behind read_balance() pressumed that all components of the array are traditional disks. If we could detect all/some of them are not (i.e. SSD, etc.), it would be better off using some other criteria for the read balancing IMHO, something like nr_pending? -- Regards, Namhyung Kim -- 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
