Extended Common Storage

Key Characteristics

• • Location: Resides in virtual storage above the 16MB line, specifically encompassing the Extended System Queue Area (ESQA) and Extended Common Service Area (ECSA). This allows it to utilize the larger address space available in 31-bit and 64-bit addressing modes.
  • Shared Access: Data stored in ECS is accessible by any program running in any address space within the z/OS system, making it ideal for inter-address space communication and global system information.
  • System-Wide Scope: Unlike private storage, ECS is part of the common area, meaning its contents are not unique to a single address space but are shared across the entire z/OS instance.
  • Key-Controlled Protection: Access to ECS is controlled by storage keys. Programs must have the appropriate storage key (typically 0-7 for system keys) to read or write data, preventing unauthorized modification and ensuring system integrity.
  • Finite Resource: Although much larger than its 24-bit counterparts (CSA/SQA), ECS is still a finite system resource. Excessive or unmanaged allocation can lead to storage exhaustion and system instability.
  • Dynamic Allocation: Programs can dynamically allocate and free storage within ECS using macros like GETMAIN and FREEMAIN with specific parameters (e.g., LOC=EXTENDED or LOC=ANY for 31-bit).

Use Cases

• • System Control Blocks: Storing critical z/OS control blocks and tables that must be globally accessible to all system components and authorized applications, such as those related to resource management or dispatching.
  • Inter-Address Space Communication: Facilitating communication and data sharing between different address spaces, where one program writes data to ECS and another program in a different address space reads it.
  • Global Resource Serialization (GRS): Holding control blocks and queues used by GRS to manage access to shared resources across the entire sysplex, ensuring data integrity and preventing concurrent updates.
  • System-Wide Configuration Data: Storing system-wide configuration parameters, status indicators, or shared buffers that need to be consistently available to all running programs.
  • Application Shared Data: Authorized applications may allocate ECS to store data structures that need to be shared among multiple instances of the application running in different address spaces or for communication with system services.

Related Concepts