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Date: Sat, 8 Jun 1996 16:39:17 +0200 (MET DST)
From: Michael Beckmann <petzi@zit.th-darmstadt.de>
To: scsi@freebsd.org
Subject: Fast SCSI-2 = 6 MB/sec ?
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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 <ncr 53c810 scsi> rev 2 int a irq 10 on pci0:8
ncr0 waiting for scsi devices to settle
(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): Direct-Access
sd0(ncr0:0:0): FAST SCSI-2 100ns (10 Mb/sec) offset 8.
4303MB (8813870 512 byte sectors)
(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): Direct-Access
sd1(ncr0:2:0): FAST SCSI-2 100ns (10 Mb/sec) offset 8.
1003MB (2055035 512 byte sectors)
(ncr0:3:0): "PLEXTOR CD-ROM PX-4XCE 1.01" type 5 removable SCSI 2
cd0(ncr0:3:0): CD-ROM
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
can't get the size
 
(ncr0:5:0): "HP C1533A 9503" type 1 removable SCSI 2
st0(ncr0:5:0): Sequential-Access
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.


Michael