I think that most of the non-cylinder allocated dataset "problems" went
away with ECKD. I remember in the past that if a dataset was
non-cylinder allocated, then the CCW string used by ???? access method
would not be able to use the "search this cylinder for the record"
opcode, but would need to search each track individually because the
dataset __might__ not end on a cylinder boundry. IIRC, with ECKD, the
access method does a "define extent" to set up the range of tracks for
I/O and the control unit will abort any attempt to get outside that
extent. So the access method (just can't remember which one!) can feel
free to use multitrack searching.
Actually, not quite true. There is a flag in the DEFINE EXTENT CCW that
says that this chain was "converted from CKD". If set, multi-track
search chains still stop at the end of every cylinder. The flag is set
for chains uses by access methods such as SAM, PAM, DAM.
--
Bruce Black
Senior Software Developer
Innovation Data Processing
----------------------------------------------------------------------
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
