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Planning Transaction Logs  



The size and location of a transaction log can affect transactionperformance. Before you create a transaction log, decide the sizeand location of the transaction log.

Later, you can change the size of a transaction log, or moveit. However, careful planning at this stage reduces the need forfuture changes.

This section describes:

Task Section
Deciding thesize of a transaction log
Deciding the Size of a Transaction Log
Deciding the location ofa transaction log
Deciding the Location of a Transaction Log

Decidingthe Size of a Transaction Log  

When you create a transaction log, you can specify its size.The default size is 4000 blocks; this gives acceptable performanceon most systems.

If you know the expected rate of transactions, HP suggeststhe following formula to calculate the transaction log size:size= 40 × rate

where:

size
is the size of the transactionlog in blocks.
rate
is the average number of transactionsexecuted per second.

If you do not know the rate of transactions, accept the defaultsize of 4000 blocks.

Decidingthe Location of a Transaction Log  

If possible, choose a disk that is:

Fast
Achieve speed by using ahigh-performance disk, such as a solid-state disk, that is not heavilyused.
Highly available
Achieve high availabilityby having multiple access paths to the data.

In anOpenVMS Cluster, use a disk that can be accessed by the other nodesin the cluster. This ensures that if one node fails, transactionsrunning on other nodes are not blocked.
Reliable
Achieve reliability by keeping multiplecopies of the data.

Using a shadowed disk is more reliablethan using a nonshadowed disk, but may be slower because transactionlogs are almost exclusively write-only.

You may need to choose between speed and either availabilityor reliability. For example, if the node is a workstation, you maychoose to sacrifice speed for availability and reliability by puttingthe node's transaction log on a shadowed HSC-based disk, insteadof on a faster disk attached to the workstation.

In a cluster environment, try to distribute the transactionlogs across different disks. Having more than one transaction logon a disk can lead to poor transaction performance.


NoteMake sure that the disk has enough contiguous spaceto hold the transaction log. A discontiguous transaction log leadsto poor transaction performance.


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