Notes on shared storage

Shared storage is a Unix concept, which works on Unix on z/OS.

It allows you to share data between threads, and between different address spaces. Java Shared Classes exploit this. Java classes can be stored in the shared storage, and “Ahead Of Time” compilation can be stored in the shared cache. Next time a JVM starts, it can look in the shared cache, and use the already compiled class.

Each shared storage segment has a key which you use in the various shared memory API calls.

There are no real restrictions, in some of the documentation on the web, it suggests using 1234 as a key! This has its limitations. A better way, used by Java is based on the inode of a file

You can display information about InterProcess Communication Status (ipcs) using the ipcs command.

You can display shared memory, and shared semaphores.

ipcs command lists…

Shared Memory:                                                           
T         ID     KEY        MODE       OWNER    GROUP                    
m       8196 0x00000000 --rw-------   WEBSRV   WEBGRP                    
m      73733 0x6137142d --rw------- OMVSKERN     SYS1                    
m      73734 0x6137e82f --rw------- ZWESVUSR ZWEADMIN                    
m     139271 0x6100410f --rw------- ZWESVUSR ZWEADMIN                    
m       8200 0x610ea00f --rw------- OMVSKERN     SYS1                    

The Java shared class files on my system are

767 -rw-r--r--   1 ZWESVUSR ZWEADMIN  ... C290M17F1A64_semaphore_zoweGW_G43L00             
768 -rw-r--r--   1 ZWESVUSR ZWEADMIN  ... C290M17F1A64_memory_zoweGW_G43L00                

Inode 767 is 0x2ff, Inode 768 is 0x300.

0x61… inode…. 2d

061 because created with t = ftok(fn,idi );

Shared Memory:                                                          
T         ID     KEY        MODE       OWNER    GROUP                   
m       8196 0x00000000 --rw-------   WEBSRV   WEBGRP                   
m     139269 0x6137142d --rw------- OMVSKERN     SYS1                   
m      73734 0x6137e82f --rw------- ZWESVUSR ZWEADMIN                   
m     139271 0x6100410f --rw------- ZWESVUSR ZWEADMIN                   
m       8200 0x610ea00f --rw------- OMVSKERN     SYS1                   
Semaphores:                                                             
T         ID     KEY        MODE       OWNER    GROUP                   
s     135172 0x8137132d --ra------- OMVSKERN     SYS1                   
s      69637 0x8137e72f --ra------- ZWESVUSR ZWEADMIN                   
s     135174 0x8100400f --ra------- ZWESVUSR ZWEADMIN                   
s      69639 0x8c10840f --ra-ra-ra- ZWESVUSR ZWEADMIN                   
s     135178 0x810e9f0f --ra------- OMVSKERN     SYS1                   
s      69643 0x8c108a0f --ra-ra-ra- ZWESVUSR ZWEADMIN                   

More details

You can use

ipcs -m -a

to display more information including size, and number of users.