Pete Clark on z/VSE: Virtual Storage Constraint Relief: Courtesy of VSE/VTAM

by Pete Clark
May 1, 2006

What would the average VSE systems programmer do to get 50K to 500K of free 24-bit system GETVIS (Get Virtual Storage)? Would you consider doing a little system maintenance work? I sure would. 

Many z/VSE systems programmers may know of a time when they literally would have committed some heinous act to free up any 24-bit system GETVIS. 

In the current VTAM implementation, only XDBUF (exchange identification buffers) and IOBUF (input output buffers) still use 24-bit storage if 31-bit storage is available. XDBUF uses 24-bit partition storage and is reasonably small with few buffers required, and therefore presents no problem. IOBUF uses 24-bit system GETVIS, a critical storage resource actively managed in many z/VSE installations and for some sites always in short supply. 

Well, the latest enhancement to the z/VSE system is available via PTF (Program Fix) and offers significant VSCR in 24-bit system GETVIS: You can order or download PTFs VSE/VTAM - APAR DY46471, PTF UD52964 and z/VSE - APAR DY46396, PTF UD52873 (AF Base) or UD52874 (Generation Feature). 

Installation of these two PTFs to a z/VSE system will enable 31-bit GETVIS support for IOBUFs by VSE/VTAM. Both PTFs are required for 31-bit IOBUF implementation; failure to install both means no 31-bit IOBUF support and error messages if you try to implement it. 

After installing the PTFs and doing an IPL to activate the support, add the VTAM parameter in Figure 1 to the VTAM start-up member. If you specified the YES option, 24-bit IOBUF storage is acquired from 31-bit storage.



Note: IOBUF31=YES and VTAM support of any Integrated Communications Adapter- (ICA-) attached devices (e.g., SDLC, BSC, X.25 and LAN) are mutually exclusive; i.e., when YES is specified, ICA resources can’t be activated. 

For z/VSE users who utilize the default as installed environment, the 10K to 100K savings of 24-bit system GETVIS may not be that dramatic. However, even 10K can be very important when needed. 

For z/VSE users with numerous active terminal and printer VTAM sessions, savings in excess of 500K-plus of 24-bit system GETVIS aren’t unusual. For z/VSE users with large active VTAM networks, this is a major VSCR enhancement and well worth the few minutes required to install the support, IPL, and change the VTAM start-up member. 

A new console display command is available to confirm the implementation. Simply enter the following command on the VSE console: 

D NET,VTAMOPTS,OPT=IOBUF31  

The z/VSE folks have published an informative document on the z/VSE Website about this new enhancement. You can download the document “z/VSE VTAM 31-bit I/O Buffer Support” from HTTP://www03.ibm.com/servers/eserver/zseries/zvse/news/.