IEC messages associated with output tape abends may specify: "number" LOST BLOCKS. This refers to the number of blocks buffered in the hardware but not successfully written to the tape medium when the Buffered Tape Marks function is active.
An application activates the Buffered Tape Mark function by setting DCBE bit DCBESYNC_NONE in byte DCBEFLG3. With BTM active and when stacking multiple tape data sets by leaving the tape positioned at the end of each file created, multiple tape data sets might be buffered in the hardware prior to being written or synchronized to the tape medium. Therefore, the Lost Block number might include multiple tape data sets.
To determine which data sets were not successfully written to the tape medium, add the number of blocks externalized in IEC205I...TOTALBLOCKS associated with each immediately preceding data set, until the sum equals or exceeds the Lost Block number in the IEC message associated with the tape abend. The file sequence numbers in these associated IEC205I messages were either not completely written or never written to the tape medium.