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Considerations when setting data transfer limits

When setting DOMPLEX-level parameters you can specify both local and global limits for data transfers.

  • The local transfer limit controls how much local storage a user data request can occupy from a single request for data. This limit ensures that a single user cannot use too much of the Data Collector’s private storage area for a single request.
  • The global transfer limit controls how much data a Data Collector attempts to return to a remote Data Collector for a single data request from a remote user. This limit controls the demand on coupling-facility resources.

Related topic

Overview

For more information, see Specifying-DOMPLEX-level-parameters-for-option-sets.

Warning

Important

The entire request for data from all Db2 subsystems must be satisfied from within the local transfer limit buffer; consequently, the local transfer limit must always be higher than the global limit.

Because concurrent users can issue simultaneous requests, each request can potentially use up to the maximum amount of storage. If you have many concurrent users and a high local transfer limit, simultaneous requests can exceed the private virtual storage capacity of the Data Collector. Exceeding this capacity can cause the Data Collector to fail.

Typically, z/OS systems provide between 1300 MB and 1600 MB of available private storage. Therefore, a value of 1000 MB is a good working maximum for all concurrent user requests combined. Because this storage must be balanced between the number of active user requests and the size of those requests, 1000 MB can support a limit of 50 MB for 20 simultaneous requests. If you increase the limit to accommodate a large user request, you must decrease the number of users. For example, increasing the limit to 100 MB results in only 10 simultaneous user requests that obtain the maximum amount of data.

If you have many concurrent users, you should reduce the size of the user requests. Either reduce the actual size of the request, or request the data from a batch report request. Batch report requests that do not use the Data Collector as their source are not subject to these limitations.

Before changing the default limits, consider the following information:

  • The combination of local transfer limits for all users and global transfer limits for all Db2s should never exceed 1000 MB.
  • The higher the local transfer limits, the lower the number of users that use those limits.
  • The local transfer limit should always be higher than the global transfer limit.


 

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