[1933] in linux-scsi channel archive
Re: Internal to External SCSI Connections
daemon@ATHENA.MIT.EDU (Larry M. Augustin)
Sat May 31 23:36:51 1997
Date: Sat, 31 May 1997 20:28:35 -0700
From: "Larry M. Augustin" <lma@varesearch.com>
To: lma@varesearch.com, rjohanne@piper.hamline.edu
Cc: edwards@ioa.com, linux-scsi@vger.rutgers.edu
>From: Robert Johannes <rjohanne@piper.hamline.edu>
>
>> Narrow devices like CD ROM drives should never terminate the SCSI bus
>> since they typically use passive termination.
>
>Larry, you say cdroms should never terminate the scsi bus: any scsi bus
>or just the wide scsi bus? I'm asking because I've an ibm ultra scsi
>drive and a cd-writer on the same adaptec 2940 ultra scsi adapter. The
>cd-writer terminates the bus, is there a problem with this setup?
There are 2 possible problems:
1. The CD Writer probably uses passive termination. Check your
documentation or look for resistor packs near the SCSI connector.
This is more likely to result in SCSI bus errors at higher transfer
rates. The Ultra SCSI drive would be a better terminating device.
2. If you are terminating a wide bus with a narrow device, the wide
to narrow adapter must terminate the unused lines correctly.
>Also my ibm drive is supposed to transfer at a minimum sustained rate of
>5.1 megs/s; I've used adaptec cd-creator to test the transfer rate, and
>it always reports 3.72 megs/sec average rate for files that are 6 megs
>or more; am I getting wrong readings? Could there be something wrong
>with my scsi bus, could it be the cd-writer termination that is causing
>a slower transfer rate than the specified minimum?
5MB/sec isn't a great sustained transfer rate. I would expect a
narrow ultra scsi drive to get better than that as measured by iozone.
What's the drive's internal transfer rate? On an older, but good,
narrow fast (not ultra) Quantum drive we get 4.3MB read, 6.1MB write.
1. We benchmarked a couple of the IBM Ultra SCSI drives and got poor
performance also. I'm not sure why, but we speculated that the drives
had little or poorly managed cache. What model drive do you have?
2. The driver for the adaptec 2940 ultra scsi adapter has many
performance enhancing features disabled by default. Edit the
aic7xxx.c file and change the #define's according to the comments to
enable these. You can also get a 2.0.27 kernel with a new (still
experimental, but seems more stable than the current driver) version
from ftp.varesearch.com:/pub/kernel/linux2.0.27-gcc2.7.2. This
version has all the performance enhancing options turned on. (Thanks
to Dan Eischen for his work on this driver.)
3. The Adaptec card has always performed worse than other Ultra Wide
adapters on our benchmarks. The new driver I mentioned may fix this,
but we haven't benchmarked it yet.
4. Ultra SCSI transfers are often disabled in the Adaptec BIOS. Check
your BIOS for this. Perhaps you're not really running at Ultra SCSI
speeds.
5. The Adaptec cd-creator test may have other bottlenecks than your
drive. If that's a test of copying from the drive to the cd-writer,
perhaps that's the rate as limited by the cd writer?
Larry
