Coalesce - Combining fragments

Enhanced Definition

In the mainframe context, coalescing refers to the process of combining adjacent, smaller free storage blocks or fragments into a single, larger contiguous block. This action is performed by storage management systems to reduce fragmentation and improve the efficiency of space allocation and utilization within datasets, file systems, or databases.

Key Characteristics

- Storage Optimization: Primarily aims to optimize storage utilization by creating larger free extents, making it easier to allocate space for new or expanding data.
- Fragmentation Reduction: Directly addresses storage fragmentation, which occurs when free space is scattered in many small, non-contiguous blocks, potentially degrading performance.
- Automatic or Manual: Can be an automatic background process managed by the operating system or storage subsystem (e.g., VSAM, zFS) or triggered manually via utilities (e.g., database reorganization).
- Performance Impact: Reduces the need for multiple smaller allocations, potentially improving I/O performance and reducing overhead for future data insertions or expansions.
- Applicability: Relevant across various storage types, including VSAM datasets, z/OS UNIX file systems (HFS/zFS), and database management systems like DB2 and IMS.

Use Cases

- VSAM Space Management: When records are deleted from a VSAM KSDS (Key-Sequenced Data Set) or ESDS (Entry-Sequenced Data Set), free space is created. Coalescing combines these adjacent free CIs (Control Intervals) or CAs (Control Areas) to make larger contiguous free areas available for new record insertions or dataset expansion.
- z/OS UNIX File Systems (HFS/zFS): In HFS or zFS file systems, as files are deleted or truncated, free blocks become available. The file system can coalesce these blocks to form larger free extents, improving the efficiency of allocating space for new or growing files.
- DB2 Table Space Management: During REORG (reorganization) operations for DB2 table spaces, free pages left by deleted rows or moved data can be coalesced to create larger contiguous free extents, which improves future data insertion performance and reduces space overhead.
- IMS Database Space Management: Similar to DB2, IMS database utilities might perform coalescing actions during reorganization or space management to consolidate free space within data sets (e.g., HDAM, HIDAM) for better performance and utilization.

Related Concepts

Coalescing is fundamentally linked to storage fragmentation, which it seeks to mitigate, and storage allocation algorithms, which benefit from larger contiguous free blocks. It often works in conjunction with dataset utilities (like IDCAMS for VSAM), file system management (for z/OS UNIX), and database reorganization utilities (for DB2, IMS) that maintain the health and performance of storage structures. By reducing fragmentation, it directly impacts I/O performance by minimizing scattered allocations and potentially improving data locality.

Best Practices:

Regular Monitoring: Monitor storage fragmentation levels in critical VSAM datasets, z/OS UNIX file systems, and database table spaces to identify when coalescing or reorganization is beneficial.
Scheduled Reorganization: Implement a schedule for database REORG utilities (DB2, IMS) and VSAM REPRO/REORG processes to proactively coalesce free space and maintain optimal performance.
Appropriate Free Space Allocation: When defining VSAM datasets or database table spaces, specify appropriate FREESPACE percentages to allow for in-place updates and insertions, reducing the immediate need for extensive coalescing.
Utilize System Features: Leverage automatic coalescing features provided by z/OS components (e.g., VSAM, zFS) where available, ensuring they are properly configured and monitored.
Understand Impact: Be aware that extensive coalescing operations, especially manual ones like full dataset reorganizations, can be I/O intensive and should be scheduled during off-peak hours to minimize impact on online transactions.