Important:  A composite library ID must be defined for a multi-cluster grid and  a stand-alone cluster. For a stand-alone cluster, the composite library ID must not be the same as the distributed library ID. For a multi-cluster configuration, the composite library ID must differ from any of the unique distributed library IDs. The composite library ID and distributed library ID are five-digit hexadecimal strings. 

## 2.2.2 Tape Volume Cache 

The Library ID is used to tie the host’s definition of the library to the hardware. 

The TS7700 TVC is a disk buffer that receives all emulated tape write data. It also provides all emulated tape read data. 

The host operating system (OS) sees the TVC as virtual IBM 3490E Tape Drives, and the 3490 tape volumes are represented by storage space in a fault-tolerant disk subsystem. The host never writes directly to any optional physical tape drives or cloud object stores that are attached to the TS7700. 

Originally, the TS7760 was delivered with 4 TB disk drive support. Since August 2017, only 8 TB disk drives are available. You can mix 4 TB or 8 TB drives within a frame, but you cannot mix them within a drawer or enclosure. 

The TS7760 CSA/XSA and TS7770 CSB/XSB cache systems use Dynamic Disk Pooling (DDP) or Distributed RAID techniques to RAID6 protect all logical volume data. These DDP or Distributed RAID configurations provide the resiliency of RAID6 protection, but with greatly accelerated rebuild speeds when a disk failure occurs. 

DDP on the TS7760 is built up from one or two drawers. In a single drawer DDP, the data can be re-created when up to two disks in a DDP becomes unavailable. In a two-drawer DDP configuration, up to four disks can become unavailable and the data can still be re-created, but only two disks can be rebuilt at the same time. Whether a DDP is built up from a single drawer or from two drawers depends on your configuration. Even numbers are always bounded to a two-drawer DDP. If an uneven number of drawers is installed, the last drawer in the frame is configured as a single drawer DDP. 

DDP on the TS7770 is at single drawer granularity. Up to two disks per physical drawer can be lost without losing access to data. TS7770 CSB/XSB drawers must always be added in physical pairs. In addition, the physical available capacity is then licensed by using the TS7700 Cache On-demand features. All physically installed spindles are used independent of whether all available capacity is licensed. 

DDP does not use a global spare concept but instead reserves free space in each DDP pool. If a DDM failure occurs, the parity generated data is read from all remaining DDMs and written to the reserved free space of the remaining DDMs. Because all DDMs receive rebuilt data, the speed of the rebuild is not governed by a single spare. This procedure is called reconstruction . 

If a second DDM fails during the reconstruction, the drawer pauses the first reconstruction, and starts a “Critical reconstruction” of any RAID6 stripes that depend on both failed disks. This process allows much faster rebuild times to return the disk cache to at least RAID5 protection. After the Critical reconstruction is finished, the paused reconstruction is restarted. 

After the reconstruction completes and new DDMs are inserted into the disk cache drawers, the content in the reserved space is redistributed back to all DDMs. Though similar to 

