Here's how to get control over density and compression under OSF1.  I
tried this on a Alpha running 2.1, but the tables are the same in 3.0,
so I imagine the same change will work.

You need to know what your tape drive says in response to a SCSI
INQUIRY command.  That's the weird string of letters beginning EXB-
below.  I found it out using the /sbin/scu program.  I don't know if
all drives are the same.

Then you need to modify the file /sys/cam_data.c as follows:

Add this to the end of cam_devdesc_tab, before { 0 }

{ "EXABYTE EXB-85058SQANXR1", 24, DEV_TKZ09,
  (ALL_DTYPE_SEQUENTIAL << DTYPE_SHFT) | SZ_8MM_CLASS,
  (struct pt_info *)ccmn_null_sizes, SZ_NO_BLK, 0x3c000, 
  &exb8500c_dens, &tkz09_mod, SZ_NO_FLAGS,
  NO_OPT_CMDS, 240, SZ_NO_QUE, DD_REQSNS_VAL | DD_INQ_VAL,
  36, 64
},


And add this to DENSITY_TBL after tkz09_dens but before the }; that
ends the table.

#define	SEQ_54000_BPI_COMPRESS	0x90
#define	SEQ_45434_BPI_COMPRESS	0x8c

exb8500c_dens = {
    {
    { /* Minor 00 or rmtXl -- 8200 uncompressed */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_54000_BPI,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 01 or rmtXh -- 8500 compressed */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_45434_BPI_COMPRESS,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 02 or rmtXm -- 8500 uncompressed */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_45434_BPI,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 03 or rmtXa -- 8200 compressed */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_54000_BPI_COMPRESS,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 04 */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_45434_BPI,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 05 */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_45434_BPI,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 06 */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_45434_BPI,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    },
    { /* Minor 07 */
    /* Flags */ 
    DENS_VALID | DENS_BUF_VALID ,
    /* Density code	Compression code	Speed setting	*/
    SEQ_45434_BPI,		NULL,			NULL,
    /* Buffered setting	Blocking				*/
    1,			NULL
    }
    }

This will produce device designators:

Device	       format  compression 
/dev/rmt0l	8200	   no
/dev/rmt0a	8200	  yes
/dev/rmt0m	8500	   no
/dev/rmt0h	8500	  yes