Dynamically growing a compatibility mode aggregate

An aggregate can be dynamically grown if it becomes full. The aggregate (that is, the VSAM linear data set) must have secondary allocation that is specified when it is defined and space must be available on the volume. The number of extensions that are allowed is based on VSAM rules set by DFSMS. For more information about the extension rules, see Extension to another DASD volume in z/OS DFSMS Using Data Sets. The aggregate is extended when an operation cannot complete because the aggregate is full. If the extension is successful, the operation is again transparently driven to the application.

An administrator can restrict aggregates from growing dynamically, either on an individual aggregate basis or globally. To restrict dynamic growing of a specific aggregate, use the NOAGGRGROW parameter on the MOUNT command. To globally restrict dynamic growing of all aggregates, specify the aggrgrow=off option of the IOEFSPRM configurations option file (see IOEFSPRM).

If all systems in the shared file system environment are running release z/OS V2R3 or later, the aggrgrow attribute of a mounted file system can be dynamically changed by using the zfsadm chaggr command. See zfsadm chaggr for more details about changing attributes of mounted file systems.

During the extension, a portion of the extension is formatted. Applications that cause new blocks to be allocated or that are reading a file that is being extended will wait. Other applications will not wait. Applications that must wait, will wait for the extension and the (portion) format. Look for HI-A-RBA, the size of the data set in bytes, and HI-U-RBA, how much of it is formatted in bytes. If the aggregate has previously been extended but not fully formatted (that is, the HI-U-RBA (or hi-used-RBA) is less than the HI-A-RBA (or hi-allocated-RBA)), zFS will format another portion of the existing extension to make more space available. You can determine the HI-U-RBA and HI-A-RBA by using the IDCAMS LISTCAT ALL utility against the zFS aggregate and looking for HI-U-RBA and HI-A-RBA in the job output. Dividing HI-A-RBA or HI-U-RBA by 8192 will convert them to the number of 8K blocks.

When a dynamic extension fails (for example, because of insufficient space), zFS sets an internal indicator to avoid attempting another dynamic extension. This indicator can be reset by a successful explicit grow (for example, by using the zfsadm grow command) or by an unmount and mount of the file system.