Batch size (BATCHSZ)

This attribute is the maximum number of messages to be sent before a sync point is taken.

The batch size does not affect the way the channel transfers messages; messages are always transferred individually, but are committed or backed out as a batch.

To improve performance, we can set a batch size to define the maximum number of messages to be transferred between two sync points. The batch size to be used is negotiated when a channel starts, and the lower of the two channel definitions is taken. On some implementations, the batch size is calculated from the lowest of the two channel definitions and the two queue manager MAXUMSGS values. The actual size of a batch can be less; for example, a batch completes when there are no messages left on the transmission queue or the batch interval expires.

A large value for the batch size increases throughput, but recovery times are increased because there are more messages to back out and send again. The default BATCHSZ is 50, and you are advised to try that value first. You might choose a lower value for BATCHSZ if your communications are unreliable, making the need to recover more likely.

Sync point procedure needs a unique logical unit of work identifier to be exchanged across the link every time a sync point is taken, to coordinate batch commit procedures.

If the synchronized batch commit procedure is interrupted, an in-doubt situation might arise. In-doubt situations are resolved automatically when a message channel starts. If this resolution is not successful, manual intervention might be necessary, using the RESOLVE command.

Some considerations when choosing the number for batch size:

• If the number is too large, the amount of queue space taken up on both ends of the link becomes excessive. Messages take up queue space when they are not committed, and cannot be removed from queues until they are committed.
• If there is likely to be a steady flow of messages, we can improve the performance of a channel by increasing the batch size because fewer confirm flows are needed to transfer the same quantity of bytes.
• If message flow characteristics indicate that messages arrive intermittently, a batch size of 1 with a relatively large disconnect time interval might provide a better performance.
• The number can be in the range 1 through 9999. However, for data integrity reasons, channels connecting to any of the current platforms must specify a batch size greater than 1. A value of 1 is for use with Version 1 products, apart from IBM MQ for MVS.
• Even though nonpersistent messages on a fast channel do not wait for a sync point, they do contribute to the batch-size count.

This attribute is valid for channel types of:

• Sender
• Server
• Receiver
• Requester
• Cluster sender
• Cluster receiver