fair (12/16/82)
Here's one for those of you interested in performance evaluation. I have looked at the drivers for various disks, for 2 BSD, 4 BSD, and v7, and noticed some funny things in the way the `iostat' statistics are calculated. There is a magic constant, on a per drive basis, which is based on the disk to memory byte transfer rate for each kind of drive. In all the drivers that I have looked at, the I/O byte count is shifted right 6 places right before being added into dk_wds, but the magic constants don't seem to take this into account... Does anyone know why the drivers do this? The only reason that I can think of is to postpone wrap around in dk_wds, but that doesn't quite make sense. The place to look for this is in the disk drive start routine of the driver. Erik E. Fair ucbvax!fair