If you use any dynamic tape manager, such as the IBM Automatic Tape Allocation Manager, plan the introduction of SAA and DAA carefully. Some dynamic tape managers manage devices in an offline state. Because allocation assist functions assume online devices, issues can surface. 

Therefore, consider keeping some drives always online to a specific host, and leave only a subset of drives to the dynamic allocation manager. Alternatively, discontinue working with a dynamic tape allocation manager. 

Automatic tape switching (ATSSTAR), which is included with z/OS, works with online devices, and is compatible with DAA and SAA. 

## 3.3.13 25 GB logical volumes 

The TS7700 has traditionally supported 400 megabyte (MB), 800 MB, 1 gigabyte (GB), 2 GB, 4 GB, and 6 GB logical volumes. As of R3.2, 25 GB logical volumes are also supported. Using 25 GB logical volumes can have several advantages: 

GLYPH<SM590000> Fewer virtual volumes to insert and have managed by your TMS. 

GLYPH<SM590000> Migration from other tape libraries can be easier. 

GLYPH<SM590000> Large multi-volume workloads, such as a large database backup, can be stored with fewer logical tapes. 

Consider the following points if you choose to use 25 GB logical volumes: 

GLYPH<SM590000> 25 GB logical volumes that use RUN copy consistency points are viewed as Deferred consistency points. 

GLYPH<SM590000> If present only on physical tape, the entire volume must be recalled into disk cache before completing the logical mount. 

GLYPH<SM590000> Appending data to larger volumes requires a full replication to peers, and can result in larger inactive spaces on physical tape. 

GLYPH<SM590000> Many jobs running to 25 GB logical volumes can create a large increase in disk cache content, which can result in non-optimal performance. 

GLYPH<SM590000> Depending on the grid network performance, and the number of concurrently running copy tasks involving 25 GB volumes, consider increasing the Volume Copy Timeout  value from the default of 180 minutes to 240 minutes if copies are timing out. This can be done using the library request command Li Req SETTING COPY TIMEOUT <value>  either from MI or the Host Console. 

To avoid any performance effect, review your installation before you use the 25 GB volumes. 

## 3.3.14 Grid resiliency function: “Remote” fence 

Grid resiliency gives you the ability to automatically identify a “sick but  not dead” condition and take a predefined action. 

Unlike other IBM Z availability functions such as System failure management for z/OS LPARs or Hyperswap for disks, this feature does not react in seconds. The grid technology is designed not only for local implementations, but also for remote data placement, often thousands of miles away. Therefore, timeouts and retries must be much longer to cover temporary network issues. 

Although the function supports defining very small thresholds parameters, change the default only after you analyze the TS7700 grid environment to prevent false fence. 

