ISAM II Performance

ISAM II tuning is discussed in the CTOS ISAM II Programming Reference Manual 09-02385, this is just what I've found experimenting with ISAM II.

ISAM II performance can be changed....

1. Sector Block Size
2. Caching
3. Disk Speed/Processor Speed
4. Multi-Threaded processing on Resident Service.
5. If you use Cobol/2
6. Use ISAM III
 

1. Sector Block Size.
The size of the sector block can affect the access speed.
Consider the following test read/write figures on a large ISAM file (100+ MB).
Block Size   Result
   2         error 3138/3122 occurred.
   6         processing time was 45 mins.
  10         processing time was 86 mins.

The block size of 6 sectors closely matched the ISAM II record size used in the test.

I suspect that:
* ISAM II grabs the current requested record, and then grabs the next record as
  well for performance reasons.
* for 6 block size, 12 sectors are retrieved.  Where there is a block size of 10,
  ISAM II has to grab 20 sectors.  8 sectors are effectively wasted, as only
  12 sectors contain data.

Conclusion.
Sector Block Size must be closely matched the actual ISAM II dataset record size.
 

2. Caching.
Disk caching has the potential to double the performance of ISAM II.
Write Behind caching will make an even bigger difference.
 

3. Disk speed/Processor speed
As ISAM II is a disk intensive application, it would make sense that a faster hard disk will make ISAM II run faster.
Processor speed will allow ISAM, and other applications, to service requests quicker.
 

4. Multi-Threaded processing on Resident Service.
"Yes allows simultaneous processing of multiple requests..." - ISAM Manual

The default is NO.

This is an option with the ISAM Install command.
 

5. If you use Cobol/2
With Cobol/2 R1.2, a enhancement was made to improve ISAM access.  Compared to previous versions, R1.2 seems to be 3 -> 5 times faster.
 

6. Use ISAM III
When ISAM III was released, Unisys claimed a 30% performance increase over ISAM II, even when using ISAM II mode.
 

Summary
ISAM II tuning, like server performance tuning, requires effort.  Rewards can be found by implementing caching, and where possible, using faster disks & servers.
 
 

Disclaimer
This article is provided as is without any express or implied warranties.  While every effort has been taken to ensure accuracy of the information contained in this article, the author assumes no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein.
 

Dale 'Cat' Robinson - catfromdarwin@oocities.com
12 March 1999

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