SMRT Option 2 (below the line storage) specifies 'System Storage'.
What is it used for? Who owns this storage?
Component: TPX for Z/OS
'System Storage' identifies to TPX how many bytes TPX should leave/release for the operating system below the line to satisfy GETMAIN requests;
e.g. VSAM, the security package and others.
This storage is not owned by TPX.
Pieces of it can be requested from the operating system by programs using GETMAIN.
If this storage is exhausted you may experience S80A, S878 abends.
The TPX (recommended) region should be REGION=0M defined at startup, no storage limitations.
Once TPX starts it issues a conditional GETMAIN asking the operating system how much storage is available below the line.
Assume, z/OS returns: available storage is 8.5 MB available below the 16M line
If TPX does a GETMAIN for all 8.5 Megabytes there would be no storage left for other services, below the line.
Any GETMAIN from operating system components would fail with S80A, S878 abends.
Most likely TPX would not come up.
To avoid this scenario, TPX checks the 'System Storage' value and subtracts this from the 8.5 MB available.
Storage Allocated (below 16m line)
System Storage: 2048 K
Minimum Storage: 2048 K
( Highly recommend to set 'System Storage' to 2048K )
If 'System Storage' is set to 2048K, TPX does a GETMAIN only for 6.5 Megabytes for its modules, Slots and DSA below the line.
The 2 MB left remains under the control of the operating system to satisfy GETMAIN requests below the line.
TPX itself does not do any explicate GETMAIN here except when you issue VTADD commands.
For every VT added with VTADD, TPX will request round approximately 150 bytes through GETMAIN.
For TPX R5.4 and above, VTADD will use storage above the line.
If you are using VTADD adjust XA Storage, above the line - SMRT option 3, based on the number of VTs you add.