## 2.4.10 Virtual volumes and logical volumes 

No difference exist between multi-cluster grids and stand-alone cluster. The composite library, whether one distributed library or up to eight distributed libraries, is limited to 4,000,000 logical volumes. 

Note : Consider the following points: 

GLYPH<SM590000> A cluster is initially configured to support 1,000,000 logical volumes. Support for extra logical volumes can be added in increments of 200,000 volumes by using FC 5270. 

GLYPH<SM590000> All clusters in a grid must have the same quantity of installed instances of FC 5270 configured. If you configured a different number of FC 5270s in clusters that are combined to a grid, the cluster with the lowest number of virtual volumes constrains all of the other clusters. Only this number of virtual volumes is then available in the grid. 

## 2.4.11 Mounting a scratch volume through specific clusters 

Through MC, scratch allocation assist can be used to choose which clusters are candidates for scratch allocations, which use that MC. For more information, see “Scratch allocation assistance” on page 74. 

## 2.4.12 Mounting a specific virtual volume 

A mount for a specific volume can be sent to any device within any cluster in a grid configuration. With no other assistance, the mount uses the TVC I/O selection process to locate a valid version of the volume. 

The following scenarios are possible: 

GLYPH<SM590000> A valid copy exists in the TVC of the cluster where the mount is placed. In this case, the mount is signaled as complete and the host can access the data immediately. 

GLYPH<SM590000> No valid copy exists in the TVC of the cluster where the mount is placed. In this case, the following options are available: 

– Another cluster has a valid copy in cache. The virtual volume is read over the grid link from the remote cluster, which is called a  remote mount . No physical mount occurs. In this case, the mount is signaled as complete and the host can access the data immediately. However, the data is accessed through the grid network from a different cluster. 

– No clusters have a copy in disk cache. In this case, a TS7700T/TS7760C CP1 - CP7 is chosen to recall the volume from physical tape or cloud object store to disk cache. Mount completion is signaled to the host system only after the entire volume is available in the TVC. 

– No copy of the logical volume can be determined in an active cluster, in cache, or on a stacked volume or cloud. The mount fails. Clusters in service preparation mode or in service mode are considered inactive. 

To optimize your environment, DAA is used in IBM z/OS configurations automatically to choose the TS7700 that can provide the most ideal TVC selection. For more information, see “Device allocation assistance” on page 73. 

