Lynn, I didn't get to work with the 3880-13, but with the 3880-23 I think I recall sequential pre-fetch was initially fetching three tracks, using a wrap-around buffer to keep track of the last block read and maintaining two tracks beyond the last track accessed in cache.
With 3990-3 I think this increased to five tracks, and I have no idea about 3990-6 and beyond. With HDS Arrays the sequential pre-fetch buffering is based on the stripe length of the underlying parity group and the data length of the start SSCH that triggers the sequential pre-fetch staging and monitoring. Ron > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:[email protected]] On > Behalf Of Anne & Lynn Wheeler > Sent: Wednesday, March 21, 2012 9:05 AM > To: [email protected] > Subject: Re: [IBM-MAIN] megabytes per second > > [email protected] (Ron Hawkins) writes: > > I'm afraid sequential pre-fetch kinda of makes your point invalid for > > sequential IO. > > when ibm first came out with full-track cache (3880-13/sheriff) ... it > advertised a 90% hit rate ... based on 3380 track, 10 records per track and > sequential read, where first sequently read on track would fetch the full > track ... and then the next 9 sequential reads would already be in cache. > > however, if the application when to full-track buffering ... the same exact > application and data would go from 90% hit ratio to zero percent hit ratio > (effectively each track would be read as a whole, streamed through the > 3880-13 cache right into processor memory ... and then would have any > additional cache reference for the track). double "full-track" > buffering would then overlap retrieval of the following track with the > processing of the records in the previous track (masking disk retrieval > latency, aka akin to instruction execution with prefetch &/or out-of-order > execution to mask processor cache miss latency). > > -- > virtualization experience starting Jan1968, online at home since Mar1970 > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, send email to > [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
