HP OpenVMS Systems

VAX I/O bandwidth?

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The Question is:

 
Hello Mr. Wizard,
 
  I'm trying to find out which configuration is
best for both my needs and my budget... So,
might you tell me what would be the maximum I/O
speed of a QBus on a VAX 4000-705A (I'm using
a CI-QBus adapter card) ? And what would be the
maximum I/O speed for communications over CI
or SCSI on a Vax 6000 or Vax 7000 ?
 
Thank you very much,
 
Hubert Quarantel-Colombani
 
 

The Answer is :

 
  Please contact Compaq presales technical support for assistance -- the
  potential range of configuration possibilities for this situation are
  far beyond the scope of Ask The Wizard.  (The specific VAX systems
  available for sale will also limit the choice of systems.)
 
  Of course, the OpenVMS Wizard would initially recommend use of OpenVMS
  Alpha systems, and not an OpenVMS VAX system.  The Wizard will assume
  you have a specific (unstated) requirement for the use of OpenVMS VAX.
 
  The Q-bus is excruciating slow by present I/O bus standards.  (The
  theoretical peak transfer for the Q-bus is circa 3.5 megabytes per
  second, though at most only 1.0 to 1.5 megabytes per second should
  be expected.)  I/O performance is limited to the aggregate activity
  across all Q-bus adapters present in the system, up to the expected
  Q-bus bandwidth value.
 
  There are no native SCSI connections for VAX 6000 and VAX 7000 XMI
  systems, nor is there support for direct connection of SCSI disks on
  the Q-bus.  (eg: no magnetic disks are permitted on the KZQSA.)  The
  usual approach has involved HSD-series controllers to convert DSSI to
  SCSI, or the use of CI and HSJ controllers.
 
  The performance of SCSI adapters varies, and there are multiple
  different SCSI specifications -- slow SCSI, fast SCSI, fast-wide
  SCSI, UltraSCSI, etc. all have differing performance characteristics.
 
  Modern PCI and XMI CI controllers will receive a substantial portion
  of the circa 14 megabytes per second throughput of the CI.  The CIXCD
  XMI to CI adapter performance is circa 5.8 megabytes per second.  There
  are faster PCI CI adapters available.
 
  All that said, the fastest I/O is no I/O.  This means the use of memory
  and processor should be considered, particularly around the use of
  sufficient memory and processing to maintain appropriate I/O caches.

  

     

     
answer written or last revised on ( 14-APR-1999 )
     
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