Slow SCSI under linux-2.1.123-2.aic-pre10

[Doug Ledford]

Rune Frøysa wrote:
> 
> SCSI is slow under linux on a Dell Precision 410. /dev/sdb is a 6*9Gb
> Raid0 set (connected to the scsi port on the back), which
> theoretically should get somehing like 30-40MB/s. I get 9.61, which is
> slower than the local disk.
> 
> scsi0 : Adaptec AHA274x/284x/294x (EISA/VLB/PCI-Fast SCSI) 5.1.0pre10/3.2.4
>        <Adaptec AIC-7890/1 Ultra2 SCSI host adapter>
> scsi1 : Adaptec AHA274x/284x/294x (EISA/VLB/PCI-Fast SCSI) 5.1.0pre10/3.2.4
>        <Adaptec AIC-7880 Ultra SCSI host adapter>
> scsi : 2 hosts.
>   Vendor: QUANTUM   Model: VIKING II 9.1WLS  Rev: 3506
>   Type:   Direct-Access                      ANSI SCSI revision: 02
> Detected scsi disk sda at scsi0, channel 0, id 0, lun 0
> (scsi0:0:0:0) Synchronous at 80.0 Mbyte/sec, offset 31.
>   Vendor: CRD-5330  Model: UR53-600S         Rev: C1-2
>   Type:   Direct-Access                      ANSI SCSI revision: 02
> Detected scsi disk sdb at scsi1, channel 0, id 0, lun 0
> (scsi1:0:0:0) Synchronous at 40.0 Mbyte/sec, offset 8.
> 
> /local/oracle at myrddraal #hdparm -t /dev/sda
> 
> /dev/sda:
>  Timing buffered disk reads:  32 MB in  2.55 seconds =12.55 MB/sec
> 
> /local/oracle at myrddraal #hdparm -t /dev/sdb
> 
> /dev/sdb:
>  Timing buffered disk reads:  32 MB in  3.33 seconds = 9.61 MB/sec

The first timing you listed is proof enough that the driver is not limiting
things to 9.61MB/s.  Nor is the kernel.  In my experience, a software RAID0
array on those same 9GB disks without that hardware RAID thing in the middle
is often times very much faster.  You made the comment that this fake SCSI
device that is a hardware RAID controller should be able to get 30-40MB/s
because of the very fast disks it has.  This is entirely untrue.  You will
only get as much performance as the RAID controller itself can deliver, and
many times those little controllers have very slow, innefficient CPUs that
simply don't deliver the transfer speeds you are looking for.

-- 

 Doug Ledford  <dledford at dialnet.net>
  Opinions expressed are my own, but
     they should be everybody's.

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