You're right. It spun at 70 revolutions per second. The 3380 spun at 60 rps, so its revolution took 16.67 ms.
The average latency of a disk drive was useful for calculating connect time when every I/O probably involved a real seek (disconnect time) and a real partial revolution for the search loop to find the correct record (which was all connect time). But with today's hardware, caching, RAID, channel speed, controller buffering, etc., the connect time component should consist almost totally of data transfer. 1/2 revolution's worth of data transfer indicates the average amount of data to be transferred per I/O is 1/2 of a full track. Since EXCP tells SMF to add one to its I/O counters not for every I/O request but rather for every block being transferred, then RMF's reported connect time for these I/Os should vary widely if BUFNO is varied widely, say from one to ten, while the EXCP counted reported by SMF would be constant. I don't doubt the validity of the IBM number at the time it was published (aeons ago). I doubt its validity for today's hardware. I am only trying to guess why IBM recommended that number aeons ago in the face of its obvious inapplicability today. Bill Fairchild Software Developer Rocket Software 275 Grove Street * Newton, MA 02466-2272 * USA Tel: +1.617.614.4503 * Mobile: +1.508.341.1715 Email: [email protected] Web: www.rocketsoftware.com -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Ron Hawkins Sent: Thursday, August 13, 2009 8:56 PM To: [email protected] Subject: Re: Degraded I/O performance in 1.10? Bill, My memory, and follow up calculation, says that a 3390 rotated every 14.2ms, not 16.67ms. Even so, it would hardly seem a good move to multiply or divide a metric based on transfer time by the avg latency of a disk drive. I don't see the relationship. Ron ---------------------------------------------------------------------- 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
