On Sun, Mar 30, 2008 at 11:34:20AM +0200, Keld Jørn Simonsen wrote: > On Sun, Mar 30, 2008 at 10:55:28AM +0200, Keld Jørn Simonsen wrote: > > On Sat, Mar 29, 2008 at 04:25:31PM -0400, Bill Davidsen wrote: > > > Christian Pernegger wrote: > > > > > > The md raid10,f2 generally has modest write performance, if U is a > > > single drive speed, write might range between 1.5U to (N-1)/2*U > > > depending on tuning. Read speed is almost always (N-1)*U, which is great > > > for many applications. Playing with chunk size, chunk buffers, etc, can > > > make a large difference in write performance. > > > > Hmm, I have other formulae for this. raid10,f2 write speed would rather > > be U*N/2, and read speed be U*N - possibly enhanced by also having > > bigger chunks than on a regular non-raid disk, and enhanced by lower > > access times. The formulae are both for sequential and random reads. > > And also faster transfer rates due to using the outer tracks of the > disk. This factor could amount to up to a factor of 2 when reading from > the high end of the array vs reading from the high end of the bare disk. The faster transfer rates for reading would amount to an improvement, both for sequential and random reading, in average of about 17 % for raid10,f2 - given that it confines its reading to the outer faster tracks of the disks. For N>2 (f3, etc) this does not get much better. The lover average seek times will also be geometrically determined (as for the transfer rates as noted in another mail) and thus most likely the speedup will be equal across disk sizes. The latency component would for raid10,f2 improve slightly more than to the double (less than half the average seek time). This will affect random reading. I did some calculations on the geometry of a CD - Which has its inner tracks at a radius of 15 mm, and its outer tracks at the radius of 59 mm (approximately). Because there is more data in the outer tracks, you need less tracks to make up for half the size of the disk, and the range of the head movements are thus less than half of the original head span. For a CD the head movements to cover half of the data on the outer part would only be 16 mm, compared to 44 mm to cover the whole CD size from inner to outer tracks. This is a littele less than a third, which should amount to the same reduction in latency time. Given that there is less data in the inner tracks, the improvement is somewhat offset by the reduced probability that inner tracks will be read. I am sure that this can be calculated on a strict geometric base, but I could not find the formulae. Anyway something like a 2-3 times improvement in latency would be expected. And then some disks do not follow the simple mapping between logical sectors and physical layout. Average access time will improve for N>2, eg raid10,f3 will have about 50 % improved latency compared to raid10,f2, and raid10,f4 will only have around half the latency of raid10,f2. 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
