Data transfer limit calculations
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 private storage area for a single request.
The global transfer limit controls how much data a Data Collector will attempt to return to a remote Data Collector for a single data request from a remote user. This transfer limit controls the demand on Coupling Facility resources. Because the entire request for data from all Db2 subsystems must be satisfied from within the local transfer limit buffer, the local transfer limit must always be larger than the global limit.
Because concurrent users may be issuing simultaneous requests, each request can potentially use up to that amount of storage. If the local transfer limit is set too high, and your environment has many concurrent users, simultaneous requests can exceed the private virtual storage capacity of the Data Collector and cause it to fail.
Typical z/OS systems provide between 1300 MB and 1600 MB of available private storage, so 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 the limit is increased to accommodate a large user request, you must then 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 a large number of concurrent users, the size of the user requests should be reduced. You can reduce the actual size of the request, or consider requesting 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.
If you are changing the default limits, use the following considerations in your calculations:
- The combination of local transfer limits for all users and the global transfer limits for all Db2s should never exceed 1000 MB.
- The higher the local transfer limits, the lower the number of users that will be using those limits.
- The global transfer limit should always be lower than the local transfer limit.
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