Cylinder

Enhanced Definition

A cylinder is a fundamental unit of disk storage organization on mainframe Direct Access Storage Devices (DASD), representing all tracks at the same radial position across all platters of a disk pack. It provides a logical grouping for data that can be accessed without moving the read/write heads, significantly improving I/O performance.

Key Characteristics

- Physical Alignment: A cylinder consists of a stack of tracks, one from each recording surface (platter side) of a disk drive, all aligned vertically.
- Fixed Head Position: All tracks within a cylinder can be accessed by the read/write heads without requiring any physical movement of the head assembly; only electronic switching between heads is needed.
- Performance Optimization: Reading or writing data within a cylinder is significantly faster than moving to a different cylinder because it eliminates seek time, which is the slowest component of disk I/O.
- Capacity: The capacity of a cylinder is the number of tracks per cylinder multiplied by the capacity of a single track. For example, a 3390-3 device has 15 tracks per cylinder, each track holding 56,664 bytes.
- Addressability: Data on DASD is addressed using a Channel-Cylinder-Head-Record (CCHR) or Cylinder-Head-Record (CHR) format, where cylinder identifies the radial position.

Use Cases

- Sequential Data Set Allocation: When allocating sequential data sets (e.g., PS or PO for libraries) using JCL, specifying SPACE=(CYL,(primary,secondary)) minimizes seek time for sequential reads/writes by attempting to place data contiguously within cylinders.
- VSAM KSDS Optimization: For VSAM Key-Sequenced Data Sets (KSDS), allocating space in cylinders helps optimize performance by keeping related Control Intervals (CIs) and Control Areas (CAs) physically close, reducing I/O latency.
- Database Storage: DB2 and IMS databases often allocate their underlying data sets (VSAM or OSAM) in cylinders to ensure efficient access to frequently used data and minimize head movement for critical tables or segments.
- Temporary Work Files: Large temporary work files, especially those used by sort utilities or compilers, are frequently allocated in cylinders to ensure fast I/O during their short lifespan.

Related Concepts

A cylinder is a higher-level organizational unit than a track but a lower-level unit than a volume (a single physical disk pack). It directly impacts seek time, a critical component of I/O performance on DASD. When allocating data sets using JCL (e.g., SPACE=(CYL,(primary,secondary))), specifying cylinder units ensures that data is laid out to minimize head movement and improve access speed, especially for sequential processing. Cylinders are fundamental to how Storage Management Subsystem (SMS) manages space allocation policies.

Best Practices:

Allocate in Cylinders for Performance: Whenever possible, allocate data sets that will be accessed sequentially or frequently in cylinders using SPACE=(CYL,...) in JCL to minimize seek time and improve throughput.
Monitor Cylinder Utilization: Regularly monitor DASD space utilization at the cylinder level to identify potential fragmentation or hot spots that could impact performance.
Consider Control Area (CA) Splits: For VSAM KSDS, understand how Control Area (CA) splits relate to cylinder boundaries; frequent CA splits can indicate poor allocation parameters or data distribution, leading to performance degradation.
Align Data with Application Access: For critical applications, try to allocate related data (e.g., an index and its data component) within the same cylinder or adjacent cylinders if possible, though this is often managed by the storage subsystem.
Leverage SMS for Automation: Utilize Storage Management Subsystem (SMS) to automate the allocation of data sets in cylinders based on defined performance and availability goals, reducing the need for manual JCL specifications.