> In this particular case, a file was preallocated in JCL to use N extents starting on a specific cylinder. Fixed size.
Sounds like a z/VSE system (formerly known as VSE/ESA, VSE/SP, DOS/VSE, DOS/VS, DOS/360). In DOS JCL (which is a different syntax to z/OS / OS/390 / MVS / OS/VS2 / OS/360 JCL), you manually allocate files to disk locations using the EXTENT statement. By contrast, in z/OS the operating system decides where on disk to locate your file (or dataset, to use mainframe terminology). (You don't have to manually allocate files any more in z/VSE – you can use VSAM, or store your files in libraries, and in both cases the OS decides on disk locations for you – but, originally, neither VSAM nor libraries existed, so you had to manually assign locations to all the files on disk.) It is very primitive, but remember it was designed in the 1960s to run on machines with only 16KB of memory–plus, humans could design a disk layout to maximise performance, by placing frequently used files on faster areas of the disk. Nowadays, the OS can do a better job of locating files on disks than humans can do, but this capability is kept for backward compatibility.
Thanks for this! I had a number of interactions over the years with the S/3x0 world, and I wasn't always sure what was under the hood. I was aware that there was a bewildering slew of xxAM access methods, but had no chance to look into them.
Sounds like a z/VSE system (formerly known as VSE/ESA, VSE/SP, DOS/VSE, DOS/VS, DOS/360). In DOS JCL (which is a different syntax to z/OS / OS/390 / MVS / OS/VS2 / OS/360 JCL), you manually allocate files to disk locations using the EXTENT statement. By contrast, in z/OS the operating system decides where on disk to locate your file (or dataset, to use mainframe terminology). (You don't have to manually allocate files any more in z/VSE – you can use VSAM, or store your files in libraries, and in both cases the OS decides on disk locations for you – but, originally, neither VSAM nor libraries existed, so you had to manually assign locations to all the files on disk.) It is very primitive, but remember it was designed in the 1960s to run on machines with only 16KB of memory–plus, humans could design a disk layout to maximise performance, by placing frequently used files on faster areas of the disk. Nowadays, the OS can do a better job of locating files on disks than humans can do, but this capability is kept for backward compatibility.