8464ch02_Architecture.fm Draft Document for Review February 7, 2020 3:15 pm 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. The Library ID is used to tie the host’s definition of the library to the hardware. 2.2.2 Tape Volume Cache 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  30  IBM TS7700 Release 5.0 Guide