GLYPH<SM590000> Recovery point objective, depending on the configuration 

The host data that is stored on a virtual CST or ECCST volume is displayed by the LI REQ commands and in the MI. Depending on the selected logical volume size (400 MB - 25 GB), the uncompressed size can vary. For example, 1200 MiB - 75,000 MiB when a compression ratio of 3:1 is achieved. 

## 2.2.5 Mounting a scratch virtual volume 

When a request for a scratch is sent to the TS7700, the request specifies one or more source categories. The TS7700 selects a virtual VOLSER from the candidate list of scratch volumes in the primary or alternative category. Whether one or two category options are provided depends on if the ACS routine’s chosen Data Class is assigned a specific media type (for example, MEDIA2 or MEDIA1). If a media type is not explicitly assigned to the ACS routine assigned Data Class, both MEDIA2 and MEDIA1 are provided. If both are provided, the TS7700 first chooses from MEDIA2 volumes and then MEDIA1 (if present). 

Scratch volumes at the mounting cluster are chosen by using the following priority order: 

1. All volumes in the source or alternative source category that are owned by the local cluster, not currently mounted, and do not have pending reconciliation changes against a peer cluster 

2. All volumes in the source or alternative source category that are owned by any available cluster, not currently mounted, and do not have pending reconciliation changes against a peer cluster 

3. All volumes in the source or alternative source category that are owned by any available cluster and not currently mounted 

4. All volumes in the source or alternative source category that can be taken over from an unavailable cluster that has an explicit or implied takeover mode enabled 

The first volumes that are chosen in the preceding steps are the volumes that were in the source category the longest. Volume serials are also toggled between odd and even serials for each volume selection. 

Note:  Step 1of how scratch volumes are chosen can be skipped by configuring the LI REQ PFRLOCO  option. Skipping step 1 can help with scratch usage rotation by viewing all volumes in the entire grid by age first (step 2) instead of only those locally owned, which might include only recently returned to scratch. 

For all scratch mounts, the volume is temporarily initialized as though the volume was initialized by using the EDGINERS  or IEHINITT  program. The volume includes an IBM-standard label that consists of a VOL1 record, HDR1 record, and tape mark. 

If the volume is modified, the temporary header information is applied to a file in the TVC. If the volume is not modified, the temporary header information is discarded and any previously written content (if it exists) is not modified. In addition to choosing a volume, TVC selection processing is used to choose which TVC acts as the input/output (I/O) TVC, as described in 2.4.4, “I/O TVC selection” on page 64. 

When the Fast Ready attribute is set or implied, no recall of content from physical tape or cloud Object Storage is required in a TS7700T or TS7760C. No mechanical operation is required to mount a logical scratch volume. In addition, the volume’s current consistency is ignored because a scratch mount requires a write from BOT and always emulates a simple VOL1/HDR1 label. 

