On Wed, 1 Apr 2015 09:52:16 +0200, Peter Hunkeler wrote: >I wonder why the boundary between track managed and cylinder mamanged space is >between cylinders 65519 snd 65520. And why is the space unit 21 cylinders when >it comes to the cylinder mamange space? >I understand there there are two things that are different between those two >areas (see also the excerpt from a DFSMS manual below): >a) For track managed space, allocation information is kept as x'cccchhhh', so >a half word for the cylinder and track (head) numbers, each. For cylinder >managed space it is x'ccccCCCh', i.e. 28bits for the cylinder and only 8 bits >for the head address. >b) For track managed space, allocation is in units of one track, for cylinder >managed it is in units of 21 cylinders. > > >65535 / 21 is 3120.73120 * 21 is 655203121 * 21 is 65541 > >I guess from the above calculations that 65520 was chose so that no space is >"lost" between the maximum tracked mananged address (theoretically 65535) and >the first cylinder managed address 65541. This assumes that there are no >non-EAV volumes out there which have 65535 cylinder (I think this yould be >possible under VM, is it?). > >I don't have an idea why "21 cylinders" was chosen as the cylinder managed >unit. Anyone? > >Any comments? > You might want to look at the IBM Redbook "DFSMS V1.10 and EAV Technical Guide"
http://www.redbooks.ibm.com/abstracts/sg247617.html (or maybe there is a more recent book than that) In section 18.3 it says "The reason that a multicylinder unit is 21 cylinders is derived from being the smallest unit that can map out the largest possible Extended Address Volume and stay within the architecture of the VTOC index (with a block size of 8192 bytes)." There is more said about the 21 cylinder unit after that. Bill ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
