Ron Hawkins wrote:
Being 1988 the DASD would have been 3380 which spun at 3600 RPM, which is an average latency of 8 1/3 msec. 3390s spun at 4200 RPM. 8.32 msec does divide evenly by 128 microseconds (128 * 6500 = 832000). I would be surprised if the proximity to average latency is anything more than a coincidence though.
I would be surprised if it isn't, although I'd use a value a little smidgen higher. The typical I/O requires some setup to get IOs to handle the request, it needs to be queued, wait until the device is available, position the heads, search or seek the record, transfer the data, and clean up. Processors in the late eighties were fast enough so that only the search or seek processing took any significant time compared to processing time. If the disks were favorably positioned at the time of request, there would be no overhead, vs. maximum overhead if it just passed the requested record. So half the latency represents an average; I'd also add a correction factor for arm positioning, but if you're the only one running, after the first I/O that also becomes negligible.
Gerhard Postpischil Bradford, VT ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
