RAID5 can't do read balancing. Any 1 piece of data is only on 1 drive. However, RAID5 does do read ahead, my speed is about 3.5 times as fast as a single disk. A single disk: 18 M/sec, my RAID5 array, 65 M/sec. Guy -----Original Message----- From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of Steven Ihde Sent: Monday, December 06, 2004 12:49 PM To: linux-raid@xxxxxxxxxxxxxxx Subject: Re: Looking for the cause of poor I/O performance On Sat, 04 Dec 2004 17:00:08 -0800, Steven Ihde wrote: [snip] > A possible clue is that when tested individually but in parallel, hda > and hdc both halve their bandwidth: > > /dev/hda: > Timing cached reads: 1552 MB in 2.00 seconds = 774.57 MB/sec > Timing buffered disk reads: 68 MB in 3.07 seconds = 22.15 MB/sec > /dev/hdc: > Timing cached reads: 784 MB in 2.00 seconds = 391.86 MB/sec > Timing buffered disk reads: 68 MB in 3.02 seconds = 22.54 MB/sec > /dev/sda: > Timing cached reads: 836 MB in 2.00 seconds = 417.65 MB/sec > Timing buffered disk reads: 120 MB in 3.00 seconds = 39.94 MB/sec > > Could there be contention for some shared resource in the on-board > PATA chipset between hda and hdc? Would moving one of them to a > separate IDE controller on a PCI card help? > > Am I unreasonable to think that I should be getting better than 37 > MB/sec on raid5 read performance, given that each disk alone seems > capable of 40 MB/sec? To answer my own question... I moved one of the PATA drives to a PCI PATA controller. This did enable me to move 40MB/sec simultaneously from all three drives. Guess there's some issue with the built-in PATA on the ICH5R southbridge. However, this didn't help raid5 performance -- it was still about 35-39MB/sec. I also have a raid1 array on the same physical disks, and observed the same thing there (same read performance as a single disk with hdparm -tT, about 40 MB/sec). So: 2.6.8 includes the raid1 read balancing fix which was mentioned previously on this list -- should this show up as substantially better hdparm -tT numbers for raid1 or is it more complicated than that? Does raid5 do read-balancing at all or am I just fantasizing? Thanks, Steve - 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
