Ok, now I am confused. I have a second Dell Precision Workstation 410: System A: CPUs 2 X 500 MHz RAM 4 X 128 Meg SDRAM Bus 100 MHz Timing buffer-cache reads: 128 MB in 0.87 seconds =147.13 MB/sec System B: CPUs 2 X 1000 MHz RAM 4 X 256 Meg Registered SDRAM Bus 100 MHz Timing buffer-cache reads: 524 MB in 2.00 seconds =262.00 MB/sec Why is system B almost twice as fast? Is registered RAM faster? I know the CPU speed is twice as fast, but the system bus is still 100 MHz. There are other differences I don't think would have an effect. Video cards, modem, SCSI cards, HW RAID card, USB mouse. Guy -----Original Message----- From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of Mark Hahn Sent: Saturday, December 04, 2004 6:51 PM To: Guy Cc: linux-raid@xxxxxxxxxxxxxxx Subject: RE: Looking for the cause of poor I/O performance > Timing buffer-cache reads: > 128 MB in 11.18 seconds = 11.45 MB/sec ... > > Timing buffered disk reads: > 64 MB in 9.42 seconds = 6.79 MB/sec ... > > This is from a single disk: > Timing buffer-cache reads: 128 MB in 0.87 seconds =147.13 MB/sec > Timing buffered disk reads: 64 MB in 3.51 seconds = 18.23 MB/sec excellent! this is really a great example of how a machine's limited internal bandwidth infringes on your raid performance. running hdparm -T shows that your machine can manage about 150 MB/s when simply doing a syscall, copying bytes to userspace, and returning. no involvement of any IO device. this number is typically about half the user-visible dram bandwidth as reported by the stream benchmark. when you try to do parallel IO (either with a bunch of hdparm -t's or with raid), each disk is desperately trying to write to dram at about 18 MB/s, ignoring other bottlenecks. alas, we already know that your available dram bandwidth is much lower than 14*18. for comparison, a fairly crappy SiS 735-based k7 system with 64b-wide PC2100 can deliver maybe 1.2 GB/s dram bandwidth. hdparm -T is about 500 MB/s, and would probably have trouble breaking 200 MB/s with raid0 even if it had enough buses. an older server of mine is e7500-based, dual xeon/2.4's, with 2xPC1600 ram. it sustains about 1.6 GB/s on Stream, and about 500 MB/s hdparm -T, and can sustain 250 MB/s through it's 6-disk raid without any problem. a newish server (dual-opteron, 2xPC2700) gives 1.4 GB/s under hdparm -T, and I expect it could hit 600 MB/s without much trouble, if given 10-12 disks and pcix (or better) controllers... regards, mark hahn. - 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
