Tuning storage

Running application servers on your z/OS® system often requires a high amount of virtual storage. Because virtual storage uses real storage as backup, real storage usage might also be high. Therefore make sure that you do not underestimate the amount of virtual storage that you allocate to running your application servers.

1. Allocate additional virtual storage.
   The setting of REGION on the JCL for the proc controls the amount of virtual storage available to a z/OS address space. The default values for the WebSphere® Application Server controller and servant are set to zero, which tells the operating system to allocate all the available region (close to 2GB). You can limit the amount of virtual storage allocated by setting the REGION parameter to a value other than zero. The size of the JVM heap is the most important factor when determining the setting of the REGION parameter. You should only need to set the REGION to something other than zero when the JVM heap size is very large. The z/OS operating system allocates user storage from the bottom of the address space, which is where the JVM heap is allocated, and system storage from the top. System abends can occur when the system tries to obtain virtual storage and none is available. A non-zero REGION parameter setting prevents this from occurring by preserving storage at the top of the address space for system use. In almost all cases running with the default REGION will be satisfactory.

   Note: For more information on REGION=0M and IEFUSI, please see Installing your application serving environment section of the documentation.
2. Convert application servers with high virtual storage usage to run in 64-bit mode.
   Running an application server in 64-bit mode allows you to specify larger JVM heap sizes.
3. Convert deployment mangers, that are managing cells in which very large applications are deployed, to run in 64-bit mode.
4. Allocate additional real storage.
   The total amount of real storage that your system requires depends on the number of servers you are running and the size of the JVM heaps for each server. You should allocate at least 512MB of real storage for a small configuration.

   Recommendation: Sometimes in a heavy use environment, 2GB of central storage is not enough to handle the real storage demands of a high volume Java™ application. In this situation, you might want to configure your servers to run in 64-bit mode. Running your servers in 64-bit mode gives you the ability to dedicate more central storage to the LPAR, and the ability to define more than 2GB of central storage. When you configure your servers to run in 64-bit mode, all of the storage is defined as central storage.

   The z/OS operating system running on a zSeries processor always runs in 64-bit mode. If you are using a non-zSeries processors, or are running your servers in 31-bit mode, you can minimize paging by defining more expanded storage.