8464ch03_business requirements.fm Draft Document for Review February 7, 2020 3:15 pm 3.3.13 25 GB logical volumes  3.3.14 Grid resiliency function: “Remote” fence  125 Chapter 3. IBM TS7700 usage considerations  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. 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. 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. Consider the following points if you choose to use 25 GB logical volumes: To avoid any performance effect, review your installation before you use the 25 GB volumes. 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. 