On Sat, Jan 14, 2006 at 09:19:41AM +0800, Max Waterman wrote: > I still wonder where all the theoretical numbers went though. > > The scsi channel should be able to handle 320MB/s, and we should have > enough disks to push that (each disk is 147-320MB/s and we have 4 of > them) - theoretically. LOL, they went where all theoretical performance numbers go. Whereever that is, and, lemme tell you it's not anywhere near me ::-). While your disks claim 147-320MB/sec I'd bet a whole lot they aren't breaking 100MB/sec. I don't think I've ever seen a single disk beat 80-90MB/sec of raw throughput. The maximum read throughput listed on storagereview.com is 97.4MB/sec: http://www.storagereview.com/php/benchmark/bench_sort.php On top of that, disk seeks are going to make that go way down. 80MB/sec was on an extended read. Seeking around costs time, which affects your throughput. > Why does the bandwidth seem to plateau with two disks - adding more into > the raid0 doesn't seem to improve performance at all? Lets say you read an 8MB file off a disk that runs at 40MB/sec. That means it takes 0.2 seconds to stream that data. If you stripe that disk, and in theory double read performance, you'll complete in 0.1 seconds instead. But if you read 8GB, that'll take you about 200 seconds. Stripe it, and in theory you're down to 100 seconds. Throw a third disk, you've dropped it to 66 seconds - a smaller payoff than the first disk. If you add a fourth, you can in theory read it in 50 seconds. So the second disk you added cut 100 seconds off the read time, but the fourth only cut off 16. If we go back to back to the 8MB case, your second disk saved 0.1 seconds. If you added a third, it saved 0.04 seconds. This is probably what you're seeing. And I'll bet you're close to the 8MB end of the scale than the 8GB end. > Why do I get better numbers using the file for the while device (is > there a better name for it), rather than for a partition (ie /dev/sdb is > faster than /dev/sdb1 - by a lot)? That's a bit weird and I don't have a good explanation. I'd go to linux-kernel@xxxxxxxxxxxxxxx with that information, some test cases, and I'll bet it's a bug. Was this true across kernel versions? > Can you explain why raid1 would be faster than raid0? I don't see why > that would be... Though reading is the same in theory, I like RAID1 better ::-). If I were you, I'd test all applicable configurations. But of course we haven't even gotten into write speed... -- Ross Vandegrift ross@xxxxxxxxxxxx "The good Christian should beware of mathematicians, and all those who make empty prophecies. The danger already exists that the mathematicians have made a covenant with the devil to darken the spirit and to confine man in the bonds of Hell." --St. Augustine, De Genesi ad Litteram, Book II, xviii, 37 - 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
