Ron, It's been so long that I had forgotten about RPS. My comments about a connected search loop became obsolete with the advent of RPS. Then the average value of 1/2 rotation was to compute the disconnect time waiting for RPS to cause a reconnect to the channel, assuming that the sector value had been computed correctly. Some more milliseconds of disconnect time were added in to account for average seek. After RPS' advent, connect time was 100% due to data transfer. Today it is different thanks to FICON and controller microcode. At one time, all the handshaking necessary to get the I/O started was lumped into pend time. With RPS, connect time was reliably used for calculating work done, and pend and disconnect time were attributed to queueing and thus non-repeatable. Another component of queueing that is not visible and usually ignored is I/O interrupt pending time, caused by not having a CPU available to field an interrupt as soon as the I/O ends. This component is sti! ll with us.
After 20+ years our memories have trouble recalling all the details. Like you, I would want to see the original quote, its context, and the year when it was published. 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: Friday, August 14, 2009 9:13 AM To: [email protected] Subject: Re: Degraded I/O performance in 1.10? Bill, With XA I don't think that RPS was ever included in connect time. I admit I only started working on XA in 1984, but everything I had from back then by Beretvas and Freisenborg uses disconnect time to estimate if there is a seek problem based on RPS being counted in Disconnect time. Of course this was focused on 3880 Controllers. I have no idea if it was different for earlier models that required reconnect for handling TIC. Ron > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of > Bill Fairchild > Sent: Friday, August 14, 2009 6:24 AM > To: [email protected] > Subject: Re: [IBM-MAIN] Degraded I/O performance in 1.10? > > 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 > ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- 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
