SMS (Storage Management Subsystem) has been an integrated part of MVS for almost 15 years. During a lot of these years it has been possible to use the SMS compression facility. For about four years ago I discovered this facility.

SMS compression is for ordinary flat datasets what DB2 compression is for DB2 tablespaces. You can save disc space on the expense of CPU time. Nowadays disc space becomes a bottleneck because of ever increasing amounts of data, so using more CPU time for less disc space can be a good deal. On top of all this SMS compression is a real X37 abend killer. The sad thing about SMS compression is that so few people working with mainframes knows about how to use it.

SMS compression is acheived by using a SMS dataclass that supports SMS compression. When a SMS managed dataset is catalogued it must have a SMS dataclass assigned. This is acheived either by specifying a DATACLAS= parameter on the JCL DD card or by letting SMS assign a dataclass for you. If you let SMS assign the dataclass, the dataclass will be assigned according to the specifications in the socalled ACS routines. Fortunately, when you specify DATACLAS= this parameter overrides the ACS routine setting. The dataclass can also be specified using TSO ALLOC and ISPF option 3.2.

Which dataclass supports SMS compression? This is a very good question, because it depends on the way that your installation has configured the dataclasses. You are able to determine which dataclasses that your installation has defined and which one of them support SMS compression. You need access to either ISMF (IBM standard tool for SMS) or MXI. Typically TSO ISMF will work for you otherwise you have to contact your local storage administrator. He can tell you how to enter ISMF. The first panel in ISMF is a menu and one of the choices are dataclasses. Select this choice. On the next panel you can filter out which dataclasses you want to look at. Use a * to see them all. On the next panel there is a line for every dataclass. There is a column called COMPACTION which is not likely to appear on the panel. You have to scroll right until the COMPACTION column appears on the panel. Dataclasses where COMPACTION is YES supports SMS compression.

Now all you have to do is to use one of the dataclasses with COMPACTION=YES. Be aware that a dataclass also determines all kinds of DCB informations, but the values in the dataclass will never override the DCB information supplied by you. In other words: if you explicitly specify LRECL, RECFM and/or BLKSIZE, these values are used. When a dataset is created using data compression the column DSORG in ISPF 3.4 (DSLIST) will have the value PS-E. In ISPF option 3.2 there is a field called SMS compression and it will appear as either YES or NO. This field can be NO even though you have created the dataset with SMS compression. SMS compression will only be activated if you write enough records to create a socalled compression dictionary just like with DB2 compressed tablespaces. You have to try and find a suitable limit for when SMS compression will be activated at your installation.