How to set optimal physical log size

Question

How to set an optimal physical log size in IDS11.5 versions.

Cause

Transactions are able to continue working during checkpoints. To avoid blocked transactions,the database server must have enough physical log space to contain all of the transaction activity that occurs during checkpoint processing. Checkpoints can be triggered when the physical log is 75 percent full and checkpoint processing must complete before the remaining 25 percent of the physical log is used. Transaction blocking occurs as soon as the system detects a potential for a physical log overflow, because every active transaction could generate physical log activity.

Answer

The optimal size of the physical log depends on two factors:

1.The rate at which transactions generate physical log activity.

[1]Buffer Pool Space less than 4G.
For systems with less that four gigabytes of buffer pool space, the physical log can
be sized at 110 percent of the combined size of all the buffer pools.

[2]For larger buffer pools (Commonly larger than 4G)
Start with four gigabytes of physical log space and then monitor checkpoint activity. If checkpoints occur too frequently and appear to impact performance, increase the physical log size,and look into the Trigger column of the onstat -g ckp output. If the trigger events occur frequently by PLOG, then increase the size of the physical log.

2.Whether or not you set the RTO_SERVER_RESTART configuration parameter.

If RTO_SERVER_RESTART is enabled, the physical log size must be less than all buffer pools to maintain the RTO_SERVER_RESTART policy.

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