Autoloader Modes of Operation 

The HP SureStore DAT24x6 and DAT40x6 may be operated in either stacker or random mode. Mode is initially determined by what happens after you insert a magazine:

• If the host sends a command (MOVE MEDIUM) to move a cartridge from the magazine to the drive, the autoloader goes into random mode.
  
• If you select a cartridge by pressing the Select button and then load it by pressing the Load Cartridge button, the autoloader goes into stacker mode.

Stacker Mode

In stacker mode (also known as sequential mode), the autoloader simply uses the cartridges sequentially from the magazine. When one cartridge is unloaded (following a SCSI UNLOAD command), the autoloader automatically loads the cartridge from the next highest numbered magazine slot.

You can select the first cartridge you want the autoloader to load by using the Select and Load Cartridge buttons on the front panel.

Random Mode

In random (or changer) mode, the host computer controls the movement of cartridges between magazine and drive. For example, suppose the cartridge from slot 3 is currently loaded and you want to restore a particular file from a particular backup. The software knows that the file is on tape 1, so the host instructs the autoloader to replace cartridge 3 in the magazine and then loads cartridge 1 so that it can restore the file. As a user, you may not even be aware that the cartridge has been changed.

Use of random mode requires two things:

• The host computer must be able to support multiple LUNs (Logical Unit Numbers), by which it identifies different devices at a single SCSI address, so that it can send commands separately to the changer mechanism and the DAT drive in the autoloader.
  
• The software must be able to provide the necessary functionality.

Changing Modes

The flowchart below shows how the autoloader moves between four states:

• Empty, when there is no magazine in the autoloader.
  
• Neutral, when a magazine has been inserted but no cartridges have been moved to the DAT drive and no commands have been received to move a cartridge. It is while the autoloader is in the neutral state that you can select a cartridge for loading using the Select button.
  
• Stacker mode
  
• Random mode

The diagram is simplified. In particular, it does not show how the situation is altered if the host computer cannot address multiple LUNs at a single SCSI address.