Granularity

Key Characteristics

• • Scope of Control: Defines the extent of an operation, such as locking a single row versus an entire table in a database, or managing a specific dataset versus an entire DASD volume.
  • Resource Unit Size: Determines the minimum allocable or manageable unit, like a 4KB page in DB2, a block on a DASD volume, or an individual record within a VSAM cluster.
  • Impact on Concurrency: Finer granularity (e.g., row-level locks) generally allows for higher concurrency in shared environments but can increase overhead. Coarser granularity (e.g., table-level locks) reduces overhead but limits concurrent access.
  • Performance Implications: The choice of granularity directly affects I/O operations, CPU usage, contention management, and the efficiency of data access and updates.
  • Data Detail Level: Pertains to how detailed data is stored or reported, ranging from individual transaction records to summarized aggregates for reporting or auditing.
  • Security Access Precision: Dictates how precisely security rules can be applied, e.g., at the dataset level, member level within a PDS/PDSE, or even field level within a database.

Use Cases

• • Database Locking (DB2, IMS): Specifying the lock size (e.g., ROW, PAGE, TABLESPACE in DB2, or SEGMENT, DBD in IMS) to balance concurrency and overhead for SQL or DL/I transactions.
  • Data Storage and Retrieval: Designing VSAM KSDS files where individual records are the units of access, or defining DB2 tables with specific ROWIDs for granular updates.
  • Resource Allocation: Managing memory in 4KB pages or allocating DASD space in tracks or cylinders for datasets.
  • Performance Monitoring: Collecting performance metrics at a very detailed level (e.g., individual CICS transaction response times, DB2 SQL statement execution details) versus aggregated hourly reports.
  • Security Access Control (RACF/ACF2/TSS): Defining access rules for specific datasets, members within a PDS/PDSE, or resources rather than broad categories, ensuring the principle of least privilege.
  • Backup and Recovery: Performing recovery operations at the level of a specific DB2 table, IMS database, or even a single VSAM cluster rather than an entire DASD volume.

Related Concepts