Well while we're on the subject ;-) I have a three-disk raid5 array. In summary, the raid5 performs slightly worse than any of the three disks alone. Memory bandwidth tested by hdparm seems more than adequate (1.6GB/sec). Shouldn't read-balancing give me some benefit here? Kernel is 2.6.8. The system is an i865PE (I think) chipset with a 2.4GHz P4. I believe the memory bandwidth is more than adequate and that the disks are performing up to spec when tested alone (Seagate Barracudas, hda & hdc are 80GB PATA, sda is 120GB SATA): /dev/hda: Timing cached reads: 3356 MB in 2.00 seconds = 1676.58 MB/sec Timing buffered disk reads: 122 MB in 3.03 seconds = 40.24 MB/sec /dev/hdc: Timing cached reads: 3316 MB in 2.00 seconds = 1657.42 MB/sec Timing buffered disk reads: 122 MB in 3.02 seconds = 40.34 MB/sec /dev/sda: Timing cached reads: 3344 MB in 2.00 seconds = 1673.09 MB/sec Timing buffered disk reads: 122 MB in 3.04 seconds = 40.19 MB/sec Now, the raid5 array: /dev/md1: Timing cached reads: 3408 MB in 2.00 seconds = 1704.26 MB/sec Timing buffered disk reads: 114 MB in 3.01 seconds = 37.83 MB/sec Slightly worse! Bonnie++ gives me an even lower number, about 30.9 MB/sec for sequential input from the raid5. hda and hdc are attached to the on-board PATA interfaces (one per channel, no slaves on either channel). sda is attached to the on-board SATA interface (the other on-board SATA is empty). 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? 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
