HP LTO Ultrium 4 drives technical reference manual, volume 2: software integration 51
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Controlling data compression
The data compression hardware in HP Ultrium drives can detect whether incoming data is already
compressed and will not attempt to compress it again. The drive can switch dynamically and
automatically between compressing and non-compressing modes, thereby optimizing both
compression ratio performance and data rate. As this mode of behavior is embedded in the Ultrium
format, there is no need for host application software to switch the drive’s data compression on and
off and HP strongly recommends to leave data compression at its default of 1 (on). This is set in the
Select Data Compression Algorithm (SDCA) field of the Device Configuration mode page (page
10h).
Host applications may read the Data Compression mode page (0Fh) to determine that the drive is
capable of data compression; the DCC bit is set to indicate this. HP recommends that the host does
not attempt to modify either the Data Compression Enable (DCE) or Data Decompression Enable
(DDE) bits.
Interpreting the current compression ratio and reporting it through applications is desirable for
customers so that HP can accurately support customer installations. Without knowing the
compression ratios achieved, it is difficult for support to suggest whether performance achieved is
good or merely tolerable for customers’ data. HP recommends reporting compression and includes
example source code for interpreting this functionality.
Other mode page information
Accessing Cartridge Memory without threading the tape
You can configure Ultrium drives so that when a cartridge is loaded, the Cartridge Memory can be
accessed without threading the tape. This functionality is set with the Autoload field (byte 5, bits
0–2) in the Control mode page (0Ah). Zero (default) allows the cartridge to load in a conventional
way. 001b or 010b allows the media to be loaded but not threaded, so the drive can read the
Cartridge Memory contents. All other values for this field are reserved.
Buffer size at EW-EOM
The Buffer Size at EW-EOM field in the Device Configuration Mode page (10h) is set to zero.
HP Ultrium tape drives automatically allow sufficient space between EW-EOM and “physical” EOT
to satisfy backup applications.
Synchronize at EW-EOM
The SEW field (byte 10, bit3) on the Device Configuration mode page (10h) specifies how the drive
behaves when reaching EW-EOM.
The parameter is fully supported. Default 0: the drive continues to buffer data objects after passing
EW-EOM. If you require behavior compatible with earlier drive generations, send a
MODE SELECT
command to set this bit to 1.
LTO 4 SAS drive “HP Ultrium 4-SCSI” (“SCSI” is not a typo!)
LTO 4 FC drive “HP Ultrium 4-SCSI” (“SCSI” is not a typo!)
