How CA Disk Determines Tape Utilization

Document ID : KB000020048
Last Modified Date : 14/02/2018
Show Technical Document Details

Question:

How does CA Disk determine tape utilization?

Answer:

Each tape and tape device have unique identifiers so that CA Disk can locate its capacity in a table.
 
With Sysparm CARTCALCY specified, CA Disk gets tape usage info from the micro-code of the device.
CA Disk checks the size of the NEXT data set to be processed and factors it by the value of Sysparm IDRCRATE
(average compression, default 35%) to determine if the data set will fit on the remaining tape.
 
If the calculation is that the data set will NOT fit on the current tape, the tape is closed and a new tape
is mounted and the data set is processed.
 
If a data set is NOT the first on the tape and the calculation is that the data set WILL fit, processing begins.
If the reflector is encountered while writing this "nth" data set (not first data set on a tape) that tape is closed,
a new tape is mounted and the entire data set is written to the new tape.
 
When a data set is the FIRST on the tape, writing continues until the reflector is hit and then new tape(s)
are mounted to complete the writing of that data set. When this data set is completely written to multiple tapes,
the last tape is closed and a new tape is mounted to begin processing the next data set.
 
This is fundamental processing logic in CA Disk that can not be altered by exits or parms.

Additional Information:

If the device and/or tape has not been added to the CA Disk table, the capacity will default to a basic value
from Sysparm CARTFEET which, even if set to "9999", will not fully utilize the tape.
 
If you believe CA Disk is not fully utilizing your tapes, open a Case to L1 and send in the output of a
simple CA Disk BACKUP of a single data set (small is okay) with these statements added:
 
//STEP1 EXEC DMS
//DIAGDSCL DD SYSOUT=*
//SYSPARMS DD *
  DIAGEXCPY
  SYSPARMSS
//SYSIN DD *
  FIND DSN=xxxxx [any data set name]
  BACKUP RETPD=2
/*
 
Provide info of the device make and model and the capacity of the tapes.
Then, L2 will review and advise if an update to the CA Disk internal table is needed.