DASD Cache
DASD Cache, or disk cache memory, is a high-speed memory component integrated within a Direct Access Storage Device (DASD) controller or storage subsystem. Its primary purpose in the mainframe environment is to significantly improve I/O performance by storing copies of frequently accessed data blocks, thereby reducing the need for physical disk access.
Key Characteristics
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- Location: Resides within the storage controller (e.g., IBM DS8000 series) rather than the host CPU, acting as an intermediary between the z/OS system and the physical disk platters.
- Operation: Intercepts I/O requests, serving data directly from cache (a "cache hit") if available, or staging data into cache from disk on a "cache miss" for subsequent access.
- Types: Typically includes read cache (for data being read) and write cache (for data being written), often backed by Non-Volatile Storage (NVS) for data integrity.
- Algorithms: Employs sophisticated algorithms like Least Recently Used (LRU) or Most Recently Used (MRU) to manage data within the cache, ensuring the most relevant data is retained.
- Performance Impact: Dramatically reduces I/O response times and increases I/O throughput, which is critical for high-volume transaction processing and batch environments.
- Non-Volatile Storage (NVS): Essential for write caching, NVS (e.g., battery-backed RAM or flash) ensures that data committed to cache but not yet written to disk is preserved across power failures or system outages.
Use Cases
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- Online Transaction Processing (OLTP): Critical for high-performance systems like CICS, IMS, and DB2, where rapid access to shared data and databases is paramount for transaction response times.
- Batch Processing Acceleration: Can significantly speed up batch jobs that repeatedly access the same input or output datasets, reducing overall job run times.
- Paging and Swapping: Improves the performance of z/OS system paging and swapping datasets (
PAGE,SWAP), which directly impacts overall system responsiveness and virtual storage management. - System Catalogs and Libraries: Caching frequently accessed system datasets such as
SYSRES,SYSCTLG,LPALIB, and various program libraries enhances system startup, program loading, and general system operations. - Data Sharing Environments: In sysplex environments with shared DASD, cache can reduce contention and improve performance across multiple LPARs accessing the same data.
Related Concepts
DASD Cache is an integral component of the overall z/OS I/O subsystem, working in conjunction with channels, control units, and the physical DASD devices. It directly supports the performance goals defined by the Workload Manager (WLM) by ensuring that critical workloads experience optimal I/O response times. Its effectiveness is closely tied to the principles of data locality, as it aims to keep frequently accessed data "closer" to the CPU, even though it resides in the storage controller.
- Monitor Cache Performance: Regularly analyze cache hit ratios, I/O response times, and queue depths using tools like
RMFandSMFto identify bottlenecks and ensure optimal cache utilization. - Workload Characterization: Understand the I/O profile of your applications (read-intensive vs. write-intensive, sequential vs. random) to properly size and configure cache resources for different storage groups or volumes.
- Strategic Cache Allocation: Prioritize caching for critical, high-activity datasets or volumes (e.g., DB2 logs, CICS files, system paging datasets) to maximize performance benefits.
- NVS Configuration: Ensure adequate NVS capacity and proper configuration for write caching to maintain data integrity and avoid performance degradation during peak write operations or potential failures.
- Vendor Recommendations: Adhere to IBM and storage vendor recommendations for cache sizing, tuning parameters, and firmware levels to leverage the latest performance enhancements and reliability features.