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@hagar ~]# cat /proc/mdstat
> Personalities : [raid1]
> md1 : active raid1 sdb2[1] sda2[0]
> 244035264 blocks [2/2] [UU]
> =====
>
> =====
> [root@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@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
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