Yes. I did say it reads ahead! Guy -----Original Message----- From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of David Greaves Sent: Monday, December 06, 2004 4:10 PM To: Guy Cc: 'Steven Ihde'; linux-raid@xxxxxxxxxxxxxxx Subject: Re: Looking for the cause of poor I/O performance but aren't the next 'n' blocks of data on (about) n drives that can be read concurrently (if the read is big enough) 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 - 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
