Where's Shai Hess in our time of need? Tony's iPhone (with toy keyboard) is responsible for this Email. Please do not snicker.....
On Aug 25, 2013, at 8:25 PM, "Joel C. Ewing" <[email protected]> wrote: > Yes, physical I/O internally within the DASD subsystem would likely be > at least full track I/O or even multi-track. However, physical I/O over > the channel will still be based on the z/OS perception that the devices > behind the controller are CKD DASD, with device-specific track and > cylinder structure, and channel I/O would still be at the single z/OS > logical-block, C-K-D-record level if from an application that considers > that the appropriate granularity of access. > > As long as z/OS perceives these as CKD devices with tracks and > cylinders, one cannot presume that those emulated track and cylinder > boundaries have no significance. z/OS still has to make decisions on > how to map data to the device and how to structure channel programs for > the device based on those perceived boundaries. Those decisions can > still impact the amount of useful data stored per track and the overhead > involved with transferring useful data over the physical channel. > > That said, sizing a PDS directory to end or not end at a track boundary > is probably irrelevant to performance; unless one encounters a > boundary-condition bug in an application that is somehow sensitive to > perceived track boundaries. > Joel C. Ewing > > On 08/25/2013 03:55 PM, Mike Schwab wrote: >> On emulated DASD, I/O is almost always an entire track at a time. >> >> On Sun, Aug 25, 2013 at 3:25 PM, John Gilmore <[email protected]> wrote: >>> Gerhard, >>> >>> Your arguments would be/are persuasive for a real, spinning DASD. On >>> an emulated one they are not. What, for example, does 'track >>> oriented' mean when the underlying architecture is FBA? When the >>> underlying architecture is random-access storage? >>> >>> John Gilmore, Ashland, MA 01721 - USA > > > > -- > Joel C. Ewing, Bentonville, AR [email protected] > > ---------------------------------------------------------------------- > 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
