Gotcha. Please excuse the loose use of terminology on my part. But now I'm more convinced than ever that I should be getting better performance than I am. I'm getting 40MB/sec from each disk individually, I've shown with hdparm that I can pull 40MB/sec from all three disks simultaneously, but still my raid5 read performance (in a three-disk array) is slightly less than 40MB/sec. Any guesses what the issue could be? Is there a switch for read-ahead? -Steve On Mon, 06 Dec 2004 14:29:40 -0500, Guy wrote: > 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 - 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
