While migrating less frequently accessed data sets to tape improves MVS storage utilization, retrieving migrated data sets can impact the performance of z/OS NFS. z/OS NFS provides three processing attributes to specify how migrated data sets are to be handled by the server: retrieve(wait), retrieve(nowait), and noretrieve.

The retrieve(wait) attribute instructs the server to wait for a migrated data set to be recalled to a direct access storage device. The NFS client user or application process will not receive a response from z/OS NFS until the data set has been recalled. If the migrated file is relatively large, or migrated to tape, this can cause the NFS client to retransmit the request. The NFS client will retransmit the request a fixed number of times as specified by the retrans parameter of the mount command. If the recall of a migrated data set takes longer than the product of the timeo value on the mount command and one plus the retrans value for the mount, timeo * (retrans +1), the initial command will most likely need to be executed again. Using the default values for the AIX mount command of seven-tenths of a second for timeo and three attempts for retrans, a recall would need to take less than 3 seconds. An alternative approach is to use the retrieve(nowait) processing attribute.

The retrieve(nowait) attribute instructs the server to recall a migrated data set and immediately return a "device not available" message to the client without waiting for the recall to complete. With this option, users attempting to access the file can continue to use their session for other activity rather than waiting an indeterminate time for the recall of a file. They can attempt to access the file again after allowing some period of time for the recall to have completed.

If it is critical to the user that a file be recalled before further work can continue, the retrieve(wait) processing attribute can be specified more selectively as part of the NFS client user command syntax along with the file name. While doing this provides the user more control over command execution, the user command may still timeout and have to be executed again.

Specifying the retrieve(wait) processing attribute as the system default may also impact the availability of z/OS NFS subtasks to process such requests. The last parameter of the nfstasks site attribute determines the number of z/OS NFS subtasks available to process such recall operations. See Subtasking for more information about the nfstasks site attribute.

Another method of avoiding the unnecessary recall of migrated data sets is to specify the noretrieve processing attribute. With the noretrieve attribute, the server does not recall a migrated data set and a "device not available" message is returned to the client. This processing attribute is particularly useful when listing a directory with unknown contents.