The main solutions to DASD problems are to:
- Reduce the number of I/O operations
- Tune the remaining I/O operations
- Balance the I/O operations load.
The principal ways of reducing the number of I/O operations are to:
- Allocate VSAM Hiperspace™ buffers
- Allocate additional address space buffers
- Use data tables when appropriate
- Use or increase the use of main temporary storage
- Eliminate or minimize program compression
- Review and improve the design of applications run on CICS®
- Make use of a DASD controller cache, but only if data set placement tuning
has been done
- Minimize CI/CA splits by:
- Allocating ample free space (free space can be altered by key range during
load)
- Timely reorganizations of disk storage.
This can reduce service time. The principal ways of tuning the I/O operations
are to:
- Specify the correct CI size. This has an effect on:
- The space used on the volume
- Transfer time
- Storage requirements for buffers
- The type of processing (direct or sequential).
- Specify the location of the VTOC correctly.
- Take care over data set placement within the volume.
- Use an appropriately fast device type and, if necessary, use a cache memory
(but only if data set placement tuning has been done and if there are sufficient
channels to handle the device speed).
This can reduce queue time. The principal ways of balancing I/O operations
are to:
- Spread a high-use data set across multiple volumes.
- Minimize the use of shared DASD volumes between multiple processors.
- Place batch files and online files on separate volumes, especially:
- Spool files
- Sort files
- Assembler or compiler work files
- Page data sets.
- Place index and data on separate volumes (for VSAM KSDS files).
- Place concurrently used files on separate volumes. For example, a CICS journal should be the only data set in use on its volume.
Take the following figures as guidelines for best DASD response times for
online systems:
- Channel busy: less than 30% (with CHP ids this can be higher)
- Device busy: less than 35% for randomly accessed files
- Average response time: less than 20 milliseconds.
Aim for multiple paths to disk controllers because this allows dynamic
path selection to work.
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