Draft Document for Review February 7, 2020 3:15 pm 8464ch02_Architecture.fm 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. Chapter 2. Architecture, components, and functional characteristics  33