Volumes can be moved to any configured import/export element using the Move Medium command; however, the elements cannot be used as a place to store a volume. This is consistent with the response given for the Device Capabilities page of the Mode Sense command. When a volume is moved to an import/export element, it is automatically unassigned from the logical library. From the SCSI interface perspective, after the move the volume no longer exists in the logical library, however it is still present in the physical library. Read Element Status for import/export elements always report that the element is empty. Therefore, logical library, import/export elements can be conceptually thought of as one way chutes where volumes can be sent through in order to eject them from the library. In order to provide compatibility with a physical SCSI library, the next command issued following a successful Move Medium command to an import/export element causes a unit attention condition with the following sense information:

• Sense Key 6h (Unit Attention)

• ASC / ASCQ 28h / 01h (Cap Element Accessed)

Presenting this unit attention tells the host that one or more import/export elements may have been accessed manually by an administrator. In this case, if the host elects to issue a Read Element Status for the import/export element that was just used in the previous Move Medium command, it will find that the element is empty. The host application can then assume that the volume has been removed. This is the same behavior a host would experience with a physical library if the real CAP is accessed. Note, however, that issuing the Read Element Status in response to the unit attention is not necessary. If the previous Move Medium had completed successfully, a host application can safely assume that the volume has been logically ejected from the library and recovery from the unit attention can be simply to retry the Move Medium command. Import/export elements never contain a volume.

As indicated in the Mode Sense Device Capabilities Page, movement from import/export cells is not supported. Any Move Medium command that uses and import/export element for the source address will fail with:

• Sense Key 5h (Illegal Request)

• ASC / ASCQ 21h / 01h (Invalid Element)

This means that import/export elements are never used to enter volumes into a logical library. They are only used as a way for SCSI host applications to programmatically eject volumes. Volumes can only be entered directly into a logical library to available storage elements through an assignment operation performed by an administrator using the GUI. This is described in more detail under Storage Elements below.

Whenever an administrator changes the number of import/export elements configured for a logical library using the GUI, the system raises a unit attention condition with the following sense information:

• Sense Key 6h (Unit Attention)

• ASC / ASCQ 2Ah / 01h (Mode Parameters Changed)

Host applications must issue Mode Sense to determine the new number of import/export elements available.

With a real SCSI library, the Prevent/Allow Medium Removal command allows a host to lock and unlock access to a physical CAP. Since a logical library does not have a physical CAP (it has logical import/export elements but they do not correspond to a physical CAP) this command is supported for compatibility purposes and will always succeed. It is essentially a NOOP.

When a logical library is created by the administrator using the GUI, all storage elements are initially empty. To populate volumes into the empty storage elements an administrator must perform an assignment operation using the GUI. Likewise, to remove volumes from occupied storage elements the administrator may perform an unassign operation. Volumes can also be logically removed via the Move Medium command when the destination is an import/export element. See Import/Export Elements.

Each time a volume is either assigned to or unassigned from a logical library, the system raises a unit attention condition on the interface for the next SCSI command received as follows:

• Sense Key 6h (Unit Attention)

• ASC / ASCQ 28h / 00h (Not Ready-to-Ready Transition)

This emulates the behavior of a physical library when an operator opens the door and either manually adds or removes volumes directly to/from storage elements. When the door is closed, the physical library re-inventories its contents and presents a Not-Ready-To-Ready Unit Attention when it has finished. For logical libraries, this happens instantly after assign or unassign operations. After receiving this unit attention a host application should issue Read Element Status for all storage elements to determine the current contents of the logical library.

When a volume is moved from a storage element (via the Move Medium command) to another location, the action carried out by the system depends on the destination element type as follows:

• Hand element - not allowed.

• Import/export element - the action carried out is as described under Import/Export Elements above.

• Drive element - the volume is moved from the associated physical library storage location to the physical drive in the physical library that corresponds to the SCSI element address specified in the command.

• Storage element - no robotic movement takes place, only the volumes associated SCSI Element address is changed.

Whenever an administrator successfully changes the number of storage elements configured for a logical library (using the GUI), and that change does not result in any volumes changing their SCSI element address, the system raises a unit attention condition on the interface for the next SCSI command received as follows:

• Sense Key 6h (Unit Attention)

• ASC / ASCQ 2Ah / 01h (Mode Parameters Changed)

Host applications must issue Mode Sense to determine the new number of storage elements available but can safely assume that the current volume inventory has not changed locations.

If the change to the number of configured storage elements does cause volumes to be logically moved to different storage elements (this can happen if the capacity is decreased causing volumes to be logically moved to other unused storage elements), the system raises two unit attention conditions for the next two commands received as follows:

• Sense Key 6h (Unit Attention)

• ASC / ASCQ 28h / 00h (Not Ready-To-Ready Transition)

• ASC / ASCQ 2Ah / 01h (Mode Parameters Changed)

In this case a client application should issue both a Mode Sense command (to determine the new number of storage elements available) and a Read Element Status to determine the contents of each storage element. If the client application ignores this unit attention and continues to use information about the library that it had obtained previously, subsequent commands may fail unexpectedly; therefore, it is important that a client application reconcile both the configuration and inventory of the library after receiving the Not Ready-to-Ready Transition and Mode Parameters Changed unit attentions.

Initially, the number of drive elements is set when the logical library is created. Subsequently, this number can be changed by the administrator (using the GUI). If this number is changed, the system raises a unit attention condition on the interface for the next SCSI command received as follows:

• Sense Key 6h (Unit Attention)

• ASC / ASCQ 2Ah / 01h (Mode Parameters Changed)

If the count is decreased and results in any association change (between drive element address and physical drive) then the system precedes the Mode Parameters Changed unit attention with:

• Sense Key 6h (Unit Attention)

• ASC/ASCQ 28h / 00h (Not Ready-To-Ready Transition)

If the host only receives the Mode Parameters Changed unit attention then it should issue Mode Sense to determine the new number of drive elements available. It can assume that no element address associations have changed. However, if the Not-Ready-To-Ready unit attention is received, the host should be sure to update its drive configuration with current information obtained via Read Element Status.

Physical drives can be added or removed from a logical library by the administrator using the GUI.

When a drive is added, it is automatically assigned to the next available drive element address starting with the lowest address. When a drive is removed, the system disassociates the physical drive from the drive element (the ED bit will be set to 1 and the Access bit set to 0 in a Read Element Status response) but the number of drive elements (as reported by Mode Sense) remains unchanged. No unit attention conditions are raised.

A drive failure condition (as perceived by the host application) will result from any one of the following actions:

• The drive is varied offline by the administrator.

• The drive is removed from the logical library by the administrator.

• The drive is physically powered off or disconnected.

• The drive is experiencing a physical hardware failure.

All of the above conditions result in the following:

The descriptor entry in the Read Element Status response has the Except bit (byte 1, bit 2) set to 1 and the Access bit set to 0 and the ASC/ASCQ fields (bytes 4 and 5) are set to 04h / 02h (Hardware Error, Tape Drive).

A Move Medium command that addresses the drive will fail with:

• Sense Key 4h (Hardware Failure)

• ASC/ASCQ 40h / 02h (Hardware Error, Tape Drive)