3.7 Data set open/close performance  3.7.1 DFSMS changes in z/OS V2R2 Faster search for Access Method Block List Moving VSAM control blocks above the bar 3.7.2 Db2 for z/OS changes Chapter 3. Synergy with the IBM Z platform  121 Use z/OS Version 2 Release 2 or higher with Db2 12 to take advantage of these performance  enhancements. With the PTF for APAR PI73101, changes were also made in Db2 12 to reduce the storage  cushion to 10% of the region size and to reduce the amount of storage that is reserved for  MVS usage to allow for more usable storage below the bar. This enhancement is also  available in Db2 11 with APAR PI71764. Db2 12 uses this enhancement to allow more control blocks to be allocated above the bar.  The Media Manager interface for this improvement is not an externally documented interface.  Only authorized callers (such as Db2) can request to use this feature. In DFSMS V2R2, some of the control blocks for linear clusters (VSAM LDS) are moved above  the bar, if requested so by the Media Manager caller. This new feature can free up virtual  storage that is allocated below the bar and allows media manager interface users to open  more clusters than they did previously because the required storage is now allocated in 64-bit  addressable memory. Although the limit of open data sets in Db2 is 200,000, the practical number can be much  less, depending on the virtual storage that is available below the bar. Having many open  VSAM data sets (in the DBM1 address space) uses a large amount of virtual storage that  must be in 31-bit addressable private storage in z/OS V2R1. z/OS V2R2 addresses this issue with large AMBL chains and improved performance for Db2  linear clusters. VSAM now builds a binary tree to track the opens and closes for each cluster.  In a sense, it looks similar to the volume table of contents (VTOC) Index, which is a binary  tree that is pointing to VTOC. With this binary tree, a faster search can be done to determine  the last open processing. With many open data sets, a significant improvement can be  expected, though performance is not expected to improve when there are only a few open  clusters. The Access Method Block List (AMBL) is a VSAM internal control block that is associated  with opened clusters. Within VSAM, the AMBLs are organized in sequential chains. These  chains can be long when many clusters are opened. Then, going through the chain to locate  an AMBL (mainly during close processing) takes a long time and performance can be  affected. DFSMS was enhanced to allow more and faster data set open (and close) processing. Currently, a maximum of 200,000 open data sets is allowed per Db2 subsystem. Opening a  large number of data sets can be time consuming. z/OS release Version 2 Release 2 made  some improvements to data set open (and close) processing to enhance data set  allocation/deallocation time.