IBM Mainframe Forum

hi everyone,
I'm new here and new to mainframe.
Happy to join you.

I'm working on IO measurement for datasets used in my job.
To do that, I collect SMF30BLK and SMF30DCT data from EXCP section of type 30 record.
Meantime, I can have total EXCP info. from JESMSGLG.

In my imagination, since SMF contains data for all datasets, I can sum them up and
the sum should be equal to the value I get from JESMSGLG.
However, the EXCP info. from JESMSGLG is different from the sum of my collected SMF30BLK for all DD, either the sum of SMF30DCT.
Can someone please tell me what's wrong in my thought?
Are they different things in JESMSGLG and SMF type 30?

Thanks for your time and reply in advance.

Would I be correct in thinking that the JES amount is usually higher than the SMF amount.

Yes, the amount in JES is higher than SMF.
Can you tell me what causes the difference?