[CentOS] Re: Shrinking a volume group

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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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