7.6 HSM considerations

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There are two situations where HSM recall might impact your application performance:

When your application is connecting to the log stream whose data could not been previously offloaded. For this reason, current data are still in the interim storage and during connect process system logger attempts to offload the data which results in the allocation of the most recent offload data set.
After the application connects to a log stream and performs multiple IXGWRITE operations that result in hitting the highoffload threshold. System logger initiates an offload which result in the allocation of the most recent offload data set.

In these cases, if your installation is aggressively migrating offload data sets after a short period of time, there might be the possibility that the current offload data set might have been migrated while the application was not running. In these cases. the HSM recall might introduce a delay in your application.

To avoid undesired behavior, you can define whether offload processing is to recall the current offload data.

On the DEFINE LOGSTREAM keyword, you can specify the OFFLOADRECALL parameter to control this process. The values of the keywords are:

YES	Offload processing should recall the current offload data set.
NO	Offload processing should skip recalling the current offload data set and allocate a new one.

Note

When you chose not to recall the current offload data set, you should be aware that this might cause some wasted space on DASD once it is recalled since it was not a totally filled up data set. Care should be taken when using this option to size the data sets appropriately.

You can update this function at any time during normal operations. This keyword can be updated even when the log stream is actively connected. In this case the change will be marked as 'pending' and takes effect on the subsequent first connection to the log stream in the sysplex:

For a structure-based log stream, the change will also take effect during the next structure rebuild
For a DASD-only log stream, the change also will take effect upon the next offload data

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