Control unit cache section (SMF 42.2) statistics appear strange and unloved. Should I use them? No!

Why should I not use them?

The SMF 42 subtype 2 control unit cache records seem to have strange data in them – and seem to add little value.  The RMF 74 subtype 5 – Cache Subsystem Device Activity include most of the data in the 74.2,  and contains additional data.  The 74.5 record layout has good documentation.  The documentation for 42.2 does not.

Why do I think the record is unloved?

Some of the record formats are non standard. 

  1. Many SMF records use a time since midnight in hundred’s of a second.  SMF 42.2 does it in seconds (so I had to change my formatting routine – sigh).
  2. SMF dates tend to be packed decimal.   SMF 42.2 a date is documented as “Year, in the form 0cyy, where c is 0 for 19xx and 1 for 20xx, and yy”.  It is really just the year – 1900.  The value x79 is decimal 121 so 1900 + 121 is 2021 – this year.
  3. It has two data sections “Statistics gathered from last update period” and Statistics gathered from current update period.  The current is 90 seconds after the “last” ( should that be previous interval?).  I dont know what data these sections contain. Is it for the whole duration between the SMF records, or just the last 90 seconds of each interval.
  4. It has a field “Fast write bypasses per minute (an integer).” for each of the intervals.  I don’t know what it means especially when you consider the interval between the two sections is 90 seconds.
  5. The SMF records were produced every 30 minutes.  I dont know how to get the data for the other 27 minutes.
  6. There are sections for each SMS managed volume.   The data basically says “Is Cache and Fast write” enabled for this volume.  Yes/Yes. So what?

As this has a subtype of 2 I expect this record was produced 30+ years ago when people did not really know what they wanted in the SMF records.  I expect Development quietly ignored this subtype, and created other subtypes with useful data.  This is why I think you should use other records instead of this 42.2 records.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s