Also, the DS8* hardware assigns space by Mod 1 groups, 1113 cylinders, or 53 groups of 21 cylinders.
On Wed, Apr 1, 2015 at 2:52 AM, Peter Hunkeler <[email protected]> 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? > > -- > 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 -- Mike A Schwab, Springfield IL USA Where do Forest Rangers go to get away from it all? ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
