The tape operations during OPEN processing are not buffered. The same applies to EOV and CLOSE processing.
Once OPEN processing has been completed, the output mode is set to buffered processing unless TAPEWR=IMMEDIATE was specified in the FILE macro or in the FCB, or the operand TAPE-WRITE=*IMMEDIATE was specified in the ADD-FILE-LINK command.
As soon as EOV/EOF processing is initiated for output files, DMS executes a synchronization operation in order to ensure that all user data has been written to tape. Once the buffer is empty, the end labels are written in unbuffered mode.
If, during automatic synchronization at the start of EOV/EOF or CLOSE processing, an unrecoverable write error is detected for one of the blocks in the tape cartridge buffer, the EXLST exit CLOSER is activated, but the user can no longer access the data in the buffer. The user can avoid this problem by explicitly initiating synchronization with the SYNC command. In this case, unrecoverable write errors are signalled via the EXLST exit ERRADR. The user can rectify the problem since he/she can still access the data in the tape cartridge buffer.