Understanding CF space usage

The amount that you define will be allocated, but MVS determines how much is actually acquired when the PSS component is initialized. When approximately 75 percent of the acquired CF resource has been used, XBM invokes MVS IXLALTER expansion services. At this point, the structure attempts to expand a set amount and processing continues until the 75 percent threshold is again reached. XBM repeats this process until the acquired resource can no longer expand due to the defined SIZE parameter. Because XBM does not acquire all of the defined CF cache until the cache is actually needed, defining a larger SIZE should not be a major concern. Coordinate with the person responsible for MVS/CF resource configuration at your site to determine a suitable starting SIZE for XBM in your environment.

Use the XBM CF monitor to verify actual resource usage, as described in Displaying-XBM-subsystem-information-and-PSS-performance-statistics. Additionally, CF statistics are displayed in the XBM SYSPRINT when one of the following events occur:

• You issue the DISPLAY COMPONENT PSS or DISPLAY COMPONENT * command

  The following figure shows an example of the CF information contained in SYSPRINT when you issue one of these commands.

  BMC73055I  09.24.36 XOSM PSS component active for Snapshot
  BMC73059I  09.24.36 XOSM Snapshot support for component PSS enabled
  BMC73800I* 09.24.36 XOSM Joined XBM group XBMQA as member BMCXBM@XOSM on
             system SYSM at 09:23:53 on Jul 15,2011
  BMC73833I* 09.24.36 XOSM CF Structure XBMQA_CACHE1
             storage: used = 2 K, HWM = 2 K, total allocation = 4932 K

• You issue the DISPLAY COMPONENT XBM command

  The following figure shows an example of the CF information contained in SYSPRINT when you issue the DISPLAY COMPONENT XBM command.

  BMC73150I* 09.24.49 XOSM XBM Version 6.1.00 initialization complete
             at 09:23:59...
  BMC73059I  09.24.49 XOSM Snapshot support for component PSS enabled
  BMC73800I* 09.24.49 XOSM Joined XBM group XBMQA as member BMCXBM@XOSM on
             system
  BMC73833I* 09.24.49 XOSM CF Structure XBMQA_CACHE1
             storage: used = 2 K, HWM = 2 K, total allocation = 4932 K
• You stop the PSS component
• XBM shuts down

If CF resources are exhausted, snapshot jobs can fail. Initial workload monitoring and tailoring can prevent an inadequate CF resource definition in your CFRM policy from causing snapshot failures.

To change CF size (if you allocated too much or too little initially), you must first stop the PSS component on all XBM subsystems in the data sharing group to deallocate the CF structures. Make the appropriate policy change and then restart the PSS component on each XBM subsystem.

If your coupling facility does not have the storage space that the examples indicate, try specifying structures with a smaller maximum size. To check available coupling facility storage space from the MVS console, enter the following command:

The following figure shows an example of the display. The FREE SPACE statistic shows the current storage available in each partition of the coupling facility.