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? -- Peter Hunkeler ------ Excerpt from "z/OS DFSMS Using Data Sets" ----- Chanpter "Extended Address Volumes" For an extended address volume, the extended addressing space (EAS) is cylinders whose addresses are equal to or greater than 65,536. The ccc portion is non-zero for the cylinders of EAS. These cylinder addresses are represented by 28-bit cylinder numbers. For compatibility with older programs, the ccc portion is hexadecimal 000 for tracks in cylinders whose addresses are below 65,536. These cylinder addresses are represented by 16-bit cylinder numbers. This is the base addressing space on an extended address volume. A multi-cylinder unit is a fixed unit of disk space that is larger than a cylinder. Currently, on an EAV, a multicylinder unit is 21 cylinders and the number of the first cylinder in each multi-cylinder unit is a multiple of 21. The cylinder-managed space is space on the volume that is managed only in multicylinder units. Cylinder-managed space begins at cylinder address 65,520. Each data set occupies an integral multiple of multicylinder units. Space requests targeted for the cylinder-managed space are rounded up to the next multicylinder unit. The cylinder-managed space only exists on EAV volumes. The track-managed space is space on a volume that is managed in tracks and cylinders. Track-managed space ends at cylinder address 65,519. Each data set occupies an integral multiple of tracks. Track-managed space also exists on all volumes. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
