How is compression controlled?
Compression can be turned on or off at any time when writing. In compliance with the SCSI specifications, there are basically five ways to do this: Mode Select, Density Code; Mode Select, Device Configuration page; Mode Select, Data Compression page; Mode Select, Vendor Unique page, and the front panel (which overrides any Mode setting). There is a hierarchy to these settings, changing the Data Compression page will not have an effect if the Device Configuration page contradicts it. The priority is: Front Panel, Density Code, Device Configuration page, Vendor Unique page, and then Data Compression page. When the Mode Select, Density Code is used, the Vendor Unique codes 80h through 83h can be used to specify the maximum density for the cartridge and drive, either compressed or uncompressed. These codes were adopted to allow Backup and HSM software providers to easily invoke the maximum capacity of the cartridge/drive combination. When a section of tape has compressed data, the front pa
