Steve Bergman spake the following on 9/13/2006 8:21 AM: > OK. Now I'm a bit confused. Raid 1 read performance is not what I > expected. > > CentOS 4.4 > 2.6.9-42.0.2.ELsmp > > > ===== > [root at hagar ~]# cat /proc/mdstat > Personalities : [raid1] > md1 : active raid1 sdb2[1] sda2[0] > 244035264 blocks [2/2] [UU] > ===== > > ===== > [root at hagar scsi]# cat /proc/scsi/scsi > Attached devices: > Host: scsi0 Channel: 00 Id: 06 Lun: 00 > Vendor: SEAGATE Model: DAT DAT72-052 Rev: A16E > Type: Sequential-Access ANSI SCSI revision: 03 > Host: scsi2 Channel: 00 Id: 00 Lun: 00 > Vendor: ATA Model: Maxtor 7L250S0 Rev: BACE (This is /dev/sda) > Type: Direct-Access ANSI SCSI revision: 05 > Host: scsi3 Channel: 00 Id: 00 Lun: 00 > Vendor: ATA Model: Maxtor 7L250S0 Rev: BACE (This is /dev/sdb) > Type: Direct-Access ANSI SCSI revision: 05 > ===== > > ===== > [root at hagar ~]# hdparm -t /dev/sda2 /dev/sdb2 > > /dev/sda2: > Timing buffered disk reads: 154 MB in 3.01 seconds = 51.15 MB/sec > > /dev/sdb2: > Timing buffered disk reads: 162 MB in 3.03 seconds = 53.47 MB/sec > ===== > > ===== > > Then I run this script: > > ===== > # flush the cache > dd if=/dev/md1 bs=32M count=64 of=/dev/null > > # sync the data > sync > > # Run two read operations, on different parts of /dev/md1 simultaneously > # This reads a total of 1GB of data > time dd if=/dev/md1 bs=4k count=131072 of=/dev/null & > time dd if=/dev/md1 skip=262144 bs=4k count=131072 of=/dev/null & > ===== > > The results show about 58MB/sec transferred, which is about the same as > hdparm is showing for each drive individually. > > Running the same thing, but reading the whole 1GB using one dd process > in the foreground gives identical results. > > Why am I not seeing higher numbers? > > Thanks, > Steve Maybe the dat tape drive is slowing the bus down. I seem to remember that the smallest bus on the chain set the rest of the chain to the same speed. You could try and put it on another channel.YMMV -- MailScanner is like deodorant... You hope everybody uses it, and you notice quickly if they don't!!!!