On Dec 13, Igor had a very good posting regarding FS performance on RTP,
(see the archive at
http://qnx.iaware.org/pipermail/qdn.public.qnxrtp.os/2000-December/001403.html
but he made a common mistake by stating “you should not have non-ultra
devices on ultra bus”.
Here is part of Adaptec hardware FAQ:
http://www.adaptec.com/support/faqs/aha294x.html#38
Q: I was told that connecting non-Ultra devices to my card would make all my
devices slow down to the speed of the slowest device. If I attach my CD-ROM to
the same card I’m using for my Ultra Wide hard drive, will my performance
decrease?
A: No. This is a common misconception. Since SCSI is bus structured, the host
adapter can communicate with only one device at a time. It will communicate with
that device based on the ID (for priority) and the BIOS settings for transfer
speed. The transfer speed setting is the maximum speed the adapter will use for
I/O with that device. When doing I/O with any SCSI device on the bus, the
adapter will do transfer at "up to’ the setting in the BIOS. Most operating
systems are single tasking and will support one application at a time. If the
application calls for a copy operation, for example, from a CD-ROM to a hard
drive, the system will access the CD-ROM at it’s speed, download the data to
memory, then access the hard drive and transfer the data to the hard drive at
it’s sustained rate. Each device will operate at it’s sustained rate until the
entire I/O operation is completed.
I could swear I’ve read that recommendation somewhere on Adaptec’s site.
This FAQ also appears to be somewhat outdated, notice the statement
about most OS being single tasked.
Anyway, I’d be glad if that’s wrong. Just curious why HP put 2 SCSI
controllers into my HP Kayak (2940UW dedicated to hard drives and 53C825
for all other stuff)…
The (Adaptec) Easy CD Creator FAQ also recommends to have CD writers on
a separate bus.
Frank Liu wrote:
On Dec 13, Igor had a very good posting regarding FS performance on RTP,
(see the archive at
http://qnx.iaware.org/pipermail/qdn.public.qnxrtp.os/2000-December/001403.html
but he made a common mistake by stating “you should not have non-ultra
devices on ultra bus”.
Here is part of Adaptec hardware FAQ:
http://www.adaptec.com/support/faqs/aha294x.html#38
Q: I was told that connecting non-Ultra devices to my card would make all my
devices slow down to the speed of the slowest device. If I attach my CD-ROM to
the same card I’m using for my Ultra Wide hard drive, will my performance
decrease?
A: No. This is a common misconception. Since SCSI is bus structured, the host
adapter can communicate with only one device at a time. It will communicate with
that device based on the ID (for priority) and the BIOS settings for transfer
speed. The transfer speed setting is the maximum speed the adapter will use for
I/O with that device. When doing I/O with any SCSI device on the bus, the
adapter will do transfer at "up to’ the setting in the BIOS. Most operating
systems are single tasking and will support one application at a time. If the
application calls for a copy operation, for example, from a CD-ROM to a hard
drive, the system will access the CD-ROM at it’s speed, download the data to
memory, then access the hard drive and transfer the data to the hard drive at
it’s sustained rate. Each device will operate at it’s sustained rate until the
entire I/O operation is completed.
Previously, Igor Kovalenko wrote in qdn.public.qnxrtp.os:
I could swear I’ve read that recommendation somewhere on Adaptec’s site.
This FAQ also appears to be somewhat outdated, notice the statement
about most OS being single tasked.
Well there may be more to this. There’s a new electrical standard added
on, maybe to SCSI III. I think that it’s called LVD or LVP. As best
I can figure out, It’s Ultra SCSI differential, but if there are
non-differential devices on the bus, it steps down to non-differential.
Why would this affect the speed? Well in non-differential mode, with
longer cable lengths, you can’t do Ultra speed. I think the Adaptec
article I was reading mentioned some work around, but I think it
required that the drives use a special SCSI chip that they invented.
I’m not very sure of any of this, so pls flame on low burn.
Mitchell Schoenbrun --------- maschoen@pobox.com
“Mitchell Schoenbrun” <maschoen@pobox.com> wrote in message
news:Voyager.001221123912.7107E@schoenbrun.com…
Previously, Igor Kovalenko wrote in qdn.public.qnxrtp.os:
I could swear I’ve read that recommendation somewhere on Adaptec’s site.
This FAQ also appears to be somewhat outdated, notice the statement
about most OS being single tasked.
Well there may be more to this. There’s a new electrical standard added
on, maybe to SCSI III. I think that it’s called LVD or LVP. As best
I can figure out, It’s Ultra SCSI differential, but if there are
non-differential devices on the bus, it steps down to non-differential.
Why would this affect the speed? Well in non-differential mode, with
longer cable lengths, you can’t do Ultra speed. I think the Adaptec
article I was reading mentioned some work around, but I think it
required that the drives use a special SCSI chip that they invented.
I’m not very sure of any of this, so pls flame on low burn.
It is LVD (Low Voltage Differential). LVD drives usually have larger buffers
and generally better specs. However there are non-LVD Ultra drives. I also
tried having both LVD and non-LVD drives on single bus and did not find any
difference, not using my tests anyway. Would be interesting to figure this
one issue too…
With a quick review of at Adaptec’s site I found that LVD is the same
as Ultra160, which uses “double-transition clocking” to double
bandwidth over Ultra2. I couldn’t find the article on compatibility
issues, but I know it’s out there.
It is LVD (Low Voltage Differential). LVD drives usually have larger buffers
and generally better specs. However there are non-LVD Ultra drives. I also
tried having both LVD and non-LVD drives on single bus and did not find any
difference, not using my tests anyway. Would be interesting to figure this
one issue too…
–
Mitchell Schoenbrun --------- maschoen@pobox.com
Interesting. My adapter was 2940UW which is not Ultra160. It is not even
Ultra2 so it probably simply worked in non-LVD mode for both drives (LVD
drive still was faster, due to bigger buffer I guess). No wonder I did
not see difference without non-LVD drive.
I also believe that 2940U2W supports LVD but that’s Ultra2, not
Ultra160. May be it still needs LVD since it is dual bus controller and
it would need double bandwidth. Does anyone know if there are dual
Ultra160 boards? How do they double already doubled bandwidth then? 
Mitchell Schoenbrun wrote:
With a quick review of at Adaptec’s site I found that LVD is the same
as Ultra160, which uses “double-transition clocking” to double
bandwidth over Ultra2. I couldn’t find the article on compatibility
issues, but I know it’s out there.
It is LVD (Low Voltage Differential). LVD drives usually have larger buffers
and generally better specs. However there are non-LVD Ultra drives. I also
tried having both LVD and non-LVD drives on single bus and did not find any
difference, not using my tests anyway. Would be interesting to figure this
one issue too…
–
Mitchell Schoenbrun --------- > maschoen@pobox.com
Previously, Igor Kovalenko wrote in qdn.public.qnxrtp.os:
It is LVD (Low Voltage Differential). LVD drives usually have larger buffers
and generally better specs. However there are non-LVD Ultra drives. I also
tried having both LVD and non-LVD drives on single bus and did not find any
difference, not using my tests anyway. Would be interesting to figure this
one issue too…
It should also be noted that if you have a slow, say 5 MB/s, device on the bus and you want to transfer 5 megs of data to or from that device, over the next N time period all other devices on the bus will be blocked for a total of slightly over 1 second. With some devices now running at 160 or now 320 MB/s, that’s a lot of missed data.
Also, older devices, along with causing de-lvd-negotiation, might not have a signalling profile good enough to let faster data signals get by on the bus unmutilated. This is especially the case if older devices are contained in older cases with older cabling.
Of course, if you want to avoid the whole mess right from the start, you can go for FibreChannel SCSI. Nothing on FibreChannel runs slower than 100 MB/s. 
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