Can a SCSI device slow down the chain?

Can a SCSI device slow down the chain?

Post by Kevin at Parano » Sun, 07 Jul 1996 04:00:00



I'm not familiar with all the details of SCSI implementation, so I'd
like to hear something of an authoritative answer.  Is it possible for
a slow SCSI device to affect the performance of other faster devices
on the chain?  I have an "ancient" Sony-manufactured slow-as-hell SCSI
(revision 1 even!) CDROM that was the original Mac CDROM drive (taken
out of the case to mount internally).  I'm going to put it on a BT-946C
PCI Fast SCSI-2 card with 3 fairly quick Fast SCSI-2 drives in pretty
steady usage under Linux, but the CDROM would be used very infrequently.
If it might slow down the rest of the SCSI system, I'd rather just leave
it out.  Please no flames from those who aren't as SCSI ignorant. :)

Please copy any followups in email.. thanks in advance!

kevin
--

   <a href="http://www.paranoia.com/"> got nothing better to do? </a>
"The Internet interprets the US Congress as damage and routes around it"

 
 
 

Can a SCSI device slow down the chain?

Post by Rhia » Sun, 07 Jul 1996 04:00:00



Quote:> I'm not familiar with all the details of SCSI implementation, so I'd like
> to hear something of an authoritative answer.  Is it possible for a slow
> SCSI device to affect the performance of other faster devices on the
> chain?  I have an "ancient" Sony-manufactured slow-as-hell SCSI (revision
> 1 even!) CDROM that was the original Mac CDROM drive (taken out of the
> case to mount internally).  I'm going to put it on a BT-946C PCI Fast
> SCSI-2 card with 3 fairly quick Fast SCSI-2 drives in pretty steady usage
> under Linux, but the CDROM would be used very infrequently.  If it might
> slow down the rest of the SCSI system, I'd rather just leave it out.
> Please no flames from those who aren't as SCSI ignorant. :)

It depends on which adapter you're using...  In the case of BusLogic, the
answer is usually no.  All of their current cards allow you to configure
the devices separately, so that disallowing sync negotiation for one device
does not prevent the card from using sync negotiation with other devices.
Older cards did not have this feature - if you had to turn off sync, or
fast transfer negotiation, it was for the entire bus, and that would cause
a performance penalty with the faster devices.

If the device is SCSI-2 compliant, however, it should be able to decline a
negotiation which it doesn't support, so in most cases you shouldn't have
to explicitly set off that option.  But, as we've all discovered, not
everything responds correctly in all cases, despite claims of full
compliance.

Rhia

 
 
 

Can a SCSI device slow down the chain?

Post by Don Yunisk » Mon, 08 Jul 1996 04:00:00




Quote:>I'm not familiar with all the details of SCSI implementation, so I'd
>like to hear something of an authoritative answer.  Is it possible for
>a slow SCSI device to affect the performance of other faster devices
>on the chain?  I have an "ancient" Sony-manufactured slow-as-hell SCSI
>(revision 1 even!) CDROM that was the original Mac CDROM drive (taken
>out of the case to mount internally).  I'm going to put it on a BT-946C
>PCI Fast SCSI-2 card with 3 fairly quick Fast SCSI-2 drives in pretty
>steady usage under Linux, but the CDROM would be used very infrequently.
>If it might slow down the rest of the SCSI system, I'd rather just leave
>it out.  Please no flames from those who aren't as SCSI ignorant. :)

The CDROM device will slow down the SCSI bus whenever it is accessed.
Note that some OS's/applications (Win95, Windoze) tend to periodically
"poll" the CDROM to see if it has a CD installed, etc.  The time
involved, though, shouldn't be a real problem.

Note, however, that since the CDROM probably can't "disconnect" once
it is selected (a feature commonly used nowadays), once the transaction
with the CDROM is started, it can take considerable time (i.e. better
portion of a second to seek, etc.) to complete.

If you aren't big on using the CDROM, don't worry about it...  save
yourself a few bucks (i.e. *don't* buy a new one)

--don

 
 
 

Can a SCSI device slow down the chain?

Post by John B Dunnin » Tue, 16 Jul 1996 04:00:00


Just wondering if cabling order (relative to the controller) effects
performance for drives in a SCSI-II F/W chain.  All the drives are
identical.  Also - I really doubt this but I don't know for sure - does
SCSI ID effect performance?  Thanks for any input you might have :)

  |\______/|    John B Dunning - Network Systems Specialist
 /    /\    \             Wayne State College        
|  o_|  |_o  |            Home of the Wildcats  
|  (  \/  )  |        
 \__\|==|/__/   voice: (402) 375-7447    fax: (402) 375-7411

 
 
 

Can a SCSI device slow down the chain?

Post by Gary M. Wats » Thu, 18 Jul 1996 04:00:00



Quote:>Just wondering if cabling order (relative to the controller) effects
>performance for drives in a SCSI-II F/W chain.  All the drives are
>identical.  Also - I really doubt this but I don't know for sure - does
>SCSI ID effect performance?  Thanks for any input you might have :)

Yes, it does, but it's usually impossible to measure the difference
without specialized equipment.  In some, *extrememly* rare cases,
systems only work right in a particular order, but in no case
could anyone have predicted with certainty which order was going to
work.  And this is only found in isochronous things like video.

--

Trimm Technologies, A Division of Robroy Industries
Las Vegas, NV 89119                         Phone: (800) 423-2024 x2115
Disclaimer: the above stuff may or may not be anybody's opinion.

 
 
 

Can a SCSI device slow down the chain?

Post by Ed Scherna » Thu, 18 Jul 1996 04:00:00



> Just wondering if cabling order (relative to the controller) effects
> performance for drives in a SCSI-II F/W chain.

Nope.

Quote:>  All the drives are
> identical.  Also - I really doubt this but I don't know for sure - does
> SCSI ID effect performance?  

Yes it does, with 0 being the highest priority.

>Thanks for any input you might have :)

>   |\______/|    John B Dunning - Network Systems Specialist
>  /    /\    \             Wayne State College
> |  o_|  |_o  |            Home of the Wildcats
> |  (  \/  )  |
>  \__\|==|/__/   voice: (402) 375-7447    fax: (402) 375-7411


Ed
--
Many are my names in many countries. Mithrandir among the Elves, Tharkun
to the Dwarves; Olorin I was in my youth in the West that is forgotten,
in the South Incanus, in the North Gandalf; to the East I go not.
http://www.ids.net/~schernau/home.html
 
 
 

Can a SCSI device slow down the chain?

Post by Cris Simps » Thu, 18 Jul 1996 04:00:00




>>  All the drives are
>> identical.  Also - I really doubt this but I don't know for sure - does
>> SCSI ID effect performance?  

>Yes it does, with 0 being the highest priority.

  Yes it does, with 7 being the highest priority.  (See SCSI-2 Sec 5.6).
Some adapters boot from low IDs before high IDs, but this is not related to
arbitration priority.

cris

 
 
 

Can a SCSI device slow down the chain?

Post by Dave Pla » Thu, 18 Jul 1996 04:00:00


Quote:>Just wondering if cabling order (relative to the controller) effects
>performance for drives in a SCSI-II F/W chain.

Very slightly, if you're using asynchronous transfers.  Devices
further from the controller take longer to respond to signals (figure
roughly 4 nanoseconds of delay per handshake per foot of added cable).
Synchronous transfers (both normal and fast) are less subject to this delay.

Quote:>                                            All the drives are
>identical.  Also - I really doubt this but I don't know for sure - does
>SCSI ID effect performance?

Again, slightly.  If your controller and driver support
disconnect/reselect, and you have I/O in progress for two drives, and
both drives disconnect, and then both drives attempt to reselect, the
device with the lower SCSI ID wins the reselection.

So, as a general rule, you'd want to place the most critical disk (the
one with the highest I/O demand or most rigorous real-time
requirements) right next to the controller on the bus, and give it a
low SCSI ID number.

These effects would be quite slight in most situations, and usually
are not worth worrying about.

 
 
 

Can a SCSI device slow down the chain?

Post by Ralf Jo » Thu, 18 Jul 1996 04:00:00



: Just wondering if cabling order (relative to the controller) effects
: performance for drives in a SCSI-II F/W chain.  All the drives are
: identical.

The short answer is: no.
However, at 10 Mhz the scsi bus actually is a delay line, so the device
closest to the host adapter should have a higher performance. This may
be quite significant when using asynchronous data transfer. OTOH with
synchronous operation the difference should be negligible. (BTW: with
a 25 meter F/W/D bus you actually have a "copper fifo" that holds several
bytes of data during synchronous transfers - check the FAQ for details.)

: Also - I really doubt this but I don't know for sure - does
: SCSI ID effect performance?  Thanks for any input you might have :)

Again, the short answer is: no.
However, the SCSI ID also sets the priority for the bus arbitration phase.
When multiple devices compete for the bus, access is granted in this order:
7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8. So, if your OS (and SCSI driver)
supports multi-threaded SCSI operation and more than one device is accessed
concurrently - you might see different performance from "identical" drives.
(Usually it is recommended to put devices that can't tolerate delays - like
floppy disks or tape drives - on the high-priority SCSI IDs - AFAIK it's
also in the FAQ.)

--
Ralf.

 
 
 

Can a SCSI device slow down the chain?

Post by Ludo VanHemelry » Fri, 19 Jul 1996 04:00:00



>Just wondering if cabling order (relative to the controller) effects
>performance for drives in a SCSI-II F/W chain.  All the drives are
>identical.  Also - I really doubt this but I don't know for sure - does
>SCSI ID effect performance?  Thanks for any input you might have :)

>  |\______/|    John B Dunning - Network Systems Specialist
> /    /\    \             Wayne State College        
>|  o_|  |_o  |            Home of the Wildcats  
>|  (  \/  )  |        
> \__\|==|/__/   voice: (402) 375-7447    fax: (402) 375-7411


 An argument can be made, in favor of having a high-performance device
terminate a cable: besides establishing a standing-wave node at this
device, such a device is also likely to provide a more effective termination.
I did notice an increase in reliability after switching a Quantum LPS525S
and a CDR-510 CD-Rom drive, both supposedly Scsi-II.

 Ludo...

 
 
 

1. does scsi tape slow chain when not in use?

I am considering put a tape drive in an enclosure w/other disk drives, but
I am concerned about it slowling everything down.

If I put a DDS2 tape drive on a fast scsi chain, does it only
slow down the scsi chain while writes are being done to the tape, or
does it always slow the chain down to it's speed?

Thanks for any info,

john

2. Cogent EM110 - How did you do it?

3. Summary: Booting from another device in SCSI Chain

4. Restore

5. 1 scsi device in chain not detected, cannot install linux, some help please.

6. Dosemu serial irq config ??

7. daisy-chaining scsi devices to an rs/6000

8. token ring and linux

9. NFS: When one server goes down, entire network slows down to a crawl.

10. Speeding Up or Slowing Down an Ultra 10 with SCSI Drive and CD-ROM?

11. Slowing down the scsi bus

12. will DVD/CD-ROM on internal Ultra 60 SCSI bus slow down disks?

13. Kernel 2.1.129 slows down SCSI drive