Hein,
With all due respect, I will not disagree with you that 255 buffers almost never get used. The key word in the preceding is ALMOST.
While I have not run them in a while, I did run some interesting timing tests in an environment where head movement was a given (e.g., a single user disk environment where the application was doing one-for-one processing of a sizable sequential file). The number of buffers used, even for large block sizes was impressive.
The preceding caused that relatively show CPU, with comparable disks, to range from 10% utilization to saturation, with the only variable being the number of buffers and their size.
Admittedly, that environment did stabilize short of the maximum, but it was very sensitive to the performance of the different elements of the system. In that case, the speed match was between the disk and itself, allowing for fragmentation, window turns, and file extends.
Edgar has shared little with us on the precise configuration of the systems and network involved in this. Even with large extend sizes, BACKUP is impressive at generating a data stream. In my (admittedly limited) experiments using DECnet within a node (effectively an infinite speed network), I have certainly maxed out at more than a handful of buffers, although I do max out the CPU running BACKUP.
As is said with vehicle mileage stickers, "Your mileage may vary". If there is one thing I have learned in all of my years of performance tuning, for systems and applications, one must be prepared to be surprised by the unexpected.
- Bob Gezelter,
http://www.rlgsc.com
