KR editions 42 to 43

KR-42 SMS-I

We use datasets in mainframes everyday but rarely do we ponder over them; did you know that SMS is the application for managing datasets?

Did you know that SMS stands for System-managed storage? 
It is an IBM application for managing storage. It handles many aspects of datasets like migration, recall, security and also deletion of datasets. 
We come across the term DFSMS very frequently in mainframes. You would've seen When a dataset is being recalled you would see the message: 

ARC1020I DFSMSHSM IS RECALLING FROM TAPE DSN=TEST.DUMMY.FILE, YOU MAY CONTINUE THE RECALL IN THE BACKGROUND AND FREE YOUR TSO SESSION BY PRESSING THE ATTENTION KEY 

What is DFSMSHSM? 

The combination of system-managed storage and related hardware and software products is called the DFSMS* environment. DFSMSHSM (or DFSMShsm) is one part of this. This is responsible for migrating and recalling datasets. 

So, what does migration mean? 

KR-43 SMS-II

We'll continue with migration.....

Many a times we come across the terms MIGRAT1 and MIGRAT2. We know that it takes longer to recall a MIGRAT2 than a MIGRAT1 dataset. So, what is migration? Data migration is basically moving of data from user-accessible DASD volumes to DFSMShsm-owned migration volumes.

There are 3 levels of storage (and this is the path of migration):

Primary Storage
DASD volumes containing currently referenced data. DFSMShsm doesn't do anything in this case. 
Migration Level 1 Storage
These are DFSMShsm volumes containing data which have been moved from primary storage. These are also usually DASDs. 
Migration Level 2 Storage
Again DFSMShsm volumes (having data moved from migration level 1 or directly for primary storage). Level 2 volumes are usually on magnetic tapes. 

Primary storage will contain frequently accessed data - it can be retrieved quickly. When data isn't accessed for sometime, it will be moved to one of the migration levels (first to level 1 and then from 1 to 2). When a migrated dataset is referenced, DFSMShsm will recall the dataset into primary storage.

That is why you get the message:

ARC1020I DFSMSHSM IS RECALLING FROM TAPE DSN=TEST.DUMMY.FILE, YOU MAY CONTINUE THE RECALL IN THE BACKGROUND AND FREE YOUR TSO SESSION BY PRESSING THE ATTENTION KEY

In this case the dataset was in MIGRAT2 (which is a TAPE).

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