The catalog entry field LAST-WRITE-BLOCK-COUNT is supplied with a block counter by MAREN. This indicates the number of blocks (not PAM pages) occupied on the tape.
When tape processing is terminated, the device’s BLOCK-ID is read and the number of blocks used is determined. This is not done when tapes are read.
The information on a tape’s occupancy level supplied by the block counter must be interpreted in view of a variety of factors such as the following:
- tape length;
- number of tracks;
- state of the MTC;
- state of the read/write head;
- number of files;
- number of gaps between blocks;
- size of the gaps between blocks;
- is the block size recorded;
- compression factor (determined by the device);
- entries in tape administration block 0 (e.g. fault administration by the device).
Each time a file located on a tape is closed, the block counter or LAST-WRITE-BLOCK-COUNT field is supplied with a value. If FSEQ > 0001, the catalog entry field contains the number of blocks of the relevant archive number including this file. If FSEQ = 0001, the catalog entry field contains the total number of blocks of the relevant archive number (not the number of blocks up to the end of the file for FSEQ =0001).
If a tape contains 10 files, for instance, and the file is extended with FSEQ=0002 or rewritten, the files with FSEQ=0003 to FSEQ=0010 can no longer be accessed although their catalog entries are still available. The file with FSEQ=0001 then has the same value in the LAST-WRITE-BLOCK-COUNT catalog entry field as the file with FSEQ=0002.