4 Transaction service settings - transaction log

When an application running on WebSphere accesses more than one resource, WebSphere stores transaction information to properly coordinate and manage the distributed transaction. In a higher transaction load, this persistence slows down performance of the appserver due to its dependency on the operating system and the underlying storage systems.

To achieve better performance, move the transaction log files to a storage device with more physical disk drives, or preferably RAID disk drives. When the log files are moved to the file systems on the raided disks, the task of writing data to the physical media is shared across the multiple disk drives. This allows more concurrent access to persist transaction information and faster access to that data from the logs. Depending upon the design of the application and storage subsystem, performance gains can range from 10% to 100%, or even more in some cases.

This change is applicable only to the configuration where the application uses distributed resources or XA transactions, for example multiple databases and resources are accessed within a single transaction. Consider setting this property when the appserver shows one or more of the following signs: CPU utilization remains low despite an increase in transactions. Transactions fail with several timeouts. Transaction rollbacks occur with "unable to enlist transaction" exception. Application server hangs in middle of a run and requires the server to be restarted. The disk on which an appserver is running shows higher utilization.

Use the WebSphere Administrative Console to change the location of the transaction logs. Select Servers -> Application Servers -> <AppServer name>. Then select Transaction Service from the Additional Properties pane. Enter the new path in the Transaction log directory field. By default, the transaction log has a size of 1 MB and is stored in the directory <WAS_HOME>/tranlog/<AppServer name>.

It is recommended that you create a file system with at least three to four disk drives raided together in a RAID-0 configuration. Then, create the transaction log on this file system with the default size. When the server is running under load, check the disk input and output. If disk input and output time is more than 5%, consider adding more physical disks to lower the value. If disk input and output is low, but the server is still high, consider increasing the size of the log files.

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