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Date:      Sun, 9 Jun 1996 15:06:56 +0200
From:      se@zpr.uni-koeln.de (Stefan Esser)
To:        Michael Beckmann <petzi@zit.th-darmstadt.de>
Cc:        scsi@freebsd.org
Subject:   Re: Fast SCSI-2 = 6 MB/sec ?
Message-ID:  <199606091306.AA29281@Sisyphos>
In-Reply-To: Michael Beckmann <petzi@zit.th-darmstadt.de> "Fast SCSI-2 = 6 MB/sec ?" (Jun  8, 16:39)

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On Jun 8, 16:39, Michael Beckmann wrote:
} Subject: Fast SCSI-2 = 6 MB/sec ?
} I have just installed a Plextor SCSI CD ROM drive in my machine. I had to 
} find out that it supports only 6 MB/sec. Here is the output from dmesg:

} (ncr0:0:0): 200ns (5 Mb/sec) offset 8.
} (ncr0:0:0): "IBM OEM DFHSS4F 4141" type 0 fixed SCSI 2
} sd0(ncr0:0:0): FAST SCSI-2 100ns (10 Mb/sec) offset 8.

} (ncr0:2:0): 200ns (5 Mb/sec) offset 8.
} (ncr0:2:0): "IBM OEM 0662S12 2 23" type 0 fixed SCSI 2
} sd1(ncr0:2:0): FAST SCSI-2 100ns (10 Mb/sec) offset 8.

} (ncr0:3:0): "PLEXTOR CD-ROM PX-4XCE 1.01" type 5 removable SCSI 2
} cd0(ncr0:3:0): FAST SCSI-2 175ns (6 Mb/sec) offset 8.
}                                  ^^^^^^^^^^ 
} cd0(ncr0:3:0): NOT READY asc:3a,0 Medium not present

} (ncr0:5:0): "HP C1533A 9503" type 1 removable SCSI 2
} st0(ncr0:5:0): FAST SCSI-2 100ns (10 Mb/sec) offset 8.
} density code 0x13, variable blocks, write-enabled

} I know that the CD ROM drive can never deliver a throughput of 6 MB/sec, 
} (actually, it should deliver around 680 kByte/sec.) but I wonder 
} if the CD ROM drive decreases the performance of the entire bus, thus 
} limiting the bus throughput to 6 MB/sec. Am I right ? Should I be 
} concerned about this and replace the CD drive ?
} I wonder if my bus is running at 10 MByte/sec at all, because the hard 
} drives report both 5 MB/sec and 10 MB/sec.

You don't need to worry, SCSI allows to talk to 
each device with its individual maximum speed.

And the 6MB/s are quite impressive for a CDROM.
They mean, that reading at 600KB/s will only need
10% of the available SCSI bus capacity, leaving 
90% for the hard disk drives and the tape.

The IBM disk drives have unique firmware features,
which can cause problems at times. But the fact,
that they first negotiate for 5MHz transfers when
they actually are capable of 10MHz does no harm.
(Well, it is an interaction with some code in the
driver, which is intended towork around with other
drive's bugs ... :)

Anyway, all your drives are working with their 
maximum supported SCSI transfer rate. No reason to
be worried ...

Regards, STefan
-- 
 Stefan Esser, Zentrum fuer Paralleles Rechnen		Tel:	+49 221 4706021
 Universitaet zu Koeln, Weyertal 80, 50931 Koeln	FAX:	+49 221 4705160
 ==============================================================================
 http://www.zpr.uni-koeln.de/~se			  <se@ZPR.Uni-Koeln.DE>



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