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HP OpenVMS Systemsask the wizard |
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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.
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