zOS mainframe Storage

z/OS is the operating system used by IBM mainframes and it was previously called OS390 and MVS. z/OS storage is radically different from other operating systems, in part because of the way it stores data in CKD (Count Key Data) format. This allows data to be stored in more or less any blocksize, within upper and lower limits. Compare this with Open Systems, where a blocksize is fixed once a disk is formatted.

z/OS has other unique components, for example the way files or datasets are allocated. File names are split into 'qualifiers' and every file name is unique. Files are recorded in Catalogues, which means there is no requirement to know the path to a file to find it. In fact, there is no path concept in z/OS

VSAM files provide a particular way of storing data records so they can be indexed and easily located by programs

Logical z/OS disks are relatively small and are managed by DFSMS. DFSMS determines where a file is stored and how long it is kept. For this to work, DFSMS needs a data management product like FDRABR or DFHSM both of which move data between storage tiers as it ages, and deletes data when its retention period expires.


Take a look at The Mainframe Playground to see a free education initiative for people working with IBM Mainframes and z/OS systems. It is maintained by Jan Sadek and contains some very useful information.

z/OS Storage and Datasets

Lascon updTES

I retired 2 years ago, and so I'm out of touch with the latest in the data storage world. The Lascon site has not been updated since July 2021, and probably will not get updated very much again. The site hosting is paid up until early 2023 when it will almost certainly disappear.
Lascon Storage was conceived in 2000, and technology has changed massively over those 22 years. It's been fun, but I guess it's time to call it a day. Thanks to all my readers in that time. I hope you managed to find something useful in there.
All the best

back to top