1. z/OS is an awesome multitasker
Back in the days, when z/OS was developed, computers were extremely expensive. Hence the developers built z/OS in such a way that different people could use z/OS to run multiple tasks at the same time. Every task that runs on z/OS runs in an address space and each task thinks that it’s the only one running. However, behind the screen, z/OS shares resources like memory and CPU across all tasks and there could be numerous tasks running at the same time sharing these resources.
2. z/OS handles large workloads
It sounds quite obvious that z/OS can handle large workloads. But there must be a way to decide what tasks to run first and what can wait. In other words, how do we prioritize? z/OS uses a feature called WorkLoad Manager which helps in prioritizing system, production and non-production tasks. This helps z/OS in handling large workloads making 100% use of the CPU.
3. It’s unbelievably hard to crash z/OS
Everyone expects z/OS to keep running and running and running. So anytime when a task crashes, we can not let that crash any other part of the system. Now remember we mentioned that every task runs in its own address space. So IBM has invested a lot of time and money in making sure that when a task crashes in one address space, it does not interfere with the normal functionality of the entire system, in other words making it robust and also clean up completely where the crash occurred so we can be up and running like new. But z/OS records any crash that occurs due to a hardware or software failure, so we can get into investigation later.
4. z/OS has the best recording
z/OS comes with its own inbuilt scribe and we call it the System Management Facility(SMF). SMF records quite a lot of information like when was the system started, what task is currently running, when was a file created, when was it last edited, who deleted it, etc,. Security audits, capacity planning and performance monitoring are some of the uses of all this information that is recorded using SMF. IBM can proudly claim that z/OS has the most extensive recording of any operating systems.
5. You need a job entry subsystem for z/OS
z/OS can run a series of jobs at the same time without any human intervention and they are called batch jobs. Users use a language called Job Control Language (JCL) to specify instructions for a specific job or a batch job to run. Once z/OS receives instructions from a user in the form of JCL, the role of a Job Entry Subsystem (JES) is to convert the JCL instructions into machine readable form and then queues them for processing, create address spaces, make sure all the input files for the job are ready, specify where the output has to be written and any other information required to run that job. Overall, JES acts like a manager, making sure, everyone has their resources to do their job and everyone does their job diligently.