----- Original Message ---- From: Peter Grandi <pg_lxra@xxxxxxxxxxxxxxxxxxx> Thank you for your insightful response Peter (Yahoo spam filter hid it from me until now). > Most 500GB drives can do 60-80MB/s on the outer tracks > (30-40MB/s on the inner ones), and 3 together can easily swamp > the PCI bus. While you see the write rates of two disks, the OS > is really writing to all three disks at the same time, and it > will do read-modify-write unless the writes are exactly stripe > aligned. When RMW happens write speed is lower than writing to a > single disk. I can understand that if a RMW happens it will effectively lower the write throughput substantially but I'm not sure entirely sure why this would happen while writing new content, I don't know enough about RAID internals. Would this be the case the majority of time? > The system time is because the Linux page cache etc. is CPU > bound (never mind RAID5 XOR computation, which is not that > big). The IO wait is because IO is taking place. http://www.sabi.co.uk/blog/anno05-4th.html#051114 > Almost all kernel developers of note have been hired by wealthy > corporations who sell to people buying large servers. Then the > typical system that these developers may have and also target > are high ends 2-4 CPU workstations and servers, with CPUs many > times faster than your PC, and on those system the CPU overhead > of the page cache at speeds like yours less than 5%. > My impression is that something that takes less than 5% on a > developers's system does not get looked at, even if it takes 50% > on your system. The Linux kernel was very efficient when most > developers were using old cheap PCs themselves. "scratch your > itch" rules. This is a rather unfortunate situation, it seems that some of the roots are forgotten, especially in a case like this where one would think running a file server on a modest CPU should be enough. I was waiting for Phenom and AM2+ motherboards to become available before relegating this X2 4600+ to file server duty, guess I'll need to stay with the slow performance for a few more months. > Anyhow, try to bypass the page cache with 'O_DIRECT' or test > with 'dd oflag=direct' and similar for an alterative code path. I'll give this a try, thanks. > Misaligned writes and page cache CPU time most likely. What influence would adding more harddrives to this RAID have? I know in terms of a Netapp filer they always talk about spindle count for performance. - 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 - 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
