Library Element Numbering Details

Library Element Numbering

Element numbering provides a unique identifier for important library elements like modules, cartridge slots, and drives. These numbers are used in library interfaces and software to locate a particular library element. Some library elements are components that can be replaced like drives and modules. Some elements are not individual components, like slot locations for cartridges.

The following library elements are numbered:

• Modules (base and expansion)
• Cartridge Slots (storage and I/O)
• Drives

Library controller cards and power supplies are not numbered separately. Their identifier is the module number in which they are located. Some modules might contain two power supplies, an upper one and a lower one. The firmware does not provide any identification to differentiate between an upper and lower power supply.

3 different numbering systems are used to identify elements in the library:

1. Logical Numbering - Referred to as "Logical ID" and abbreviated as "Log ID" or just "ID". This numbering system is the simplest and most commonly used system for identifying library elements in logs and library interfaces. Elements are numbered from the bottom up in sequential fashion, as they appear in the configured library. Numbering starts with 1, not zero.
2. Physical Numbering - Abbreviated as "Phy Num" or "PNUM". PNUMs are also assigned from the bottom up, starting with 1. Unlike Logical numbering, they are based on an element’s position in a maximum library configuration (one that has the maximum number of modules and drives installed). Elements in the base module have default PNUMs. Elements above the base module have higher PNUMs. Elements below the base module have lower PNUMs. Since 21U libraries and 48U libraries have different maximum configurations, their PNUMs are different.
   Note: PNUMs are for software internal use only. They are used in applications where each element needs a stable identification that does not change when components or modules are added to or taken out of the library. They are often listed with the Log ID as a diagnostic aid in library logs or interfaces
3. SCSI Element Addressing - Referred to as "Element Addresses". Element addresses are a standard numbering system that is defined in SCSI specifications that help software applications identify elements in the tape library. Element Addresses are derived from PNUM but are altered as defined by SCSI specifications. SCSI element addresses are assigned to tape drives, storage slots, and input/output (IO) mailbox slots.
   Starting element addresses are defined as follows for these 3 elements:

   • Drive element addresses start at 1 with the bottom drive that is installed in the library
   • Mail slot element addresses start at 101 with the bottom mail slot in the library
   • Cartridge Slot element addresses start with 1001 at the bottom slot in the library

   When a library is partitioned, each logical library is assigned drives, slots, and mail slots. Each element in the logical library is numbered beginning with the starting addresses that are shown earlier. PNUMs do not change when a library is partitioned, but SCSI Element addresses do.

Module Numbering

Modules have Logical IDs and Physical Numbers. They do not have SCSI Element addresses.

Module Logical ID

Module IDs are assigned during configuration in the library start-up process. Only modules that are connected in the stack are counted. Modules are numbered from bottom to top starting with '1'. Each module ID (including the base module) depends only on the position in the stack. In a 5-module library stack, for example, the bottom module ID is 1 and the top ID is 5. If a module is defective or was removed for repair, the numbering of the modules does not change until the library is started again. When the library is started again, it goes through a configuration process that includes renumbering of modules.

Module Physical Number (PNUM)

Each base and expansion module also has a physical number. The physical number is based on the position of the module relative to the base module. In a 21U Library, the base always has a PNUM of 4. In a 48U library, the base always has a PNUM of 9. Expansion modules above the base module have higher PNUMs. Expansion modules below the base module have lower PNUMs.

Magazine and Cartridge Slot Numbering

Slot numbering uses all 3 numbering systems to identify slots inside the library: Logical ID, Physical Number, and SCSI Element address.

Cartridge Slot Logical ID

The slot ID has the following format:

For example, ID 2.7 is Module 2, slot 7

The slot Logical ID designation is not dependent on mail slot configuration. Slot IDs are not renumbered if mail slots are enabled or disabled

Cartridge Slot Physical Number (PNUM)

Slot Physical Numbers are sequential numbers that are assigned relative to a full library stack. They start at 1 with the first possible slot at the bottom of a full library stack. They end with the last possible slot at the top of a full library stack. Default slot PNUMs in the base modules are different in a 21U versus a 48U Library. The PNUM of the first slot in a 48U base module is 321. The PNUM of the first slot in a 21U Library base module is 121. Slots in expansion modules above the base module have higher PNUMs. Slots in expansion modules below the base have lower PNUMs.

Cartridge Slot SCSI element address

SCSI element addresses are separately assigned to mail slots (import export elements) and storage slots (storage elements) for each library partition. SCSI initiators can change SCSI element addresses by using the element assignment mode page. SCSI element addresses are counted in the same order as the Slot PNUM. Slot PNUM and element addresses might not be contiguous if mail slots are enabled in the middle of a partition.

Drive Numbering

Drive numbering uses all 3 numbering systems to identify drives inside the library: Logical ID, Physical Number, and SCSI Element address.

Drive Logical ID

Drives are numbered from bottom to top, starting with 1. Only drives, which are present in the stack are counted. Adding or removing drives can result in a change of drive logical numbers. See the section on drive renumbering for details.

Drive Physical Number (PNUM)

Drive PNUMs start with the lowest drive bay (drive sled connector) in a full module stack. The numbering proceeds upward to the highest drive bay. Installed drives have the PNUM of the drive bay in which they are installed. Full High tape drives take up two bays but they are numbered by using the lower bay where they plug into the library. The PNUM of the upper bay behind a full high drive is never assigned to a drive. Since 21U and 48U Libraries have different full module stack heights, the PNUMs are assigned differently. The lowest drive bay of the base module in a 48U library has PNUM 25. The lowest drive bay of the base module in a 21U library has PNUM 10. Drives above the base module are numbered higher. Drives below the base module are numbered lower. Assignment of WWI's and serial number spoofing is based on the physical drive number.

Drive SCSI element address

SCSI element addresses are separately assigned for each library partition. SCSI initiators can change element addresses by using the element assignment mode page. SCSI element addresses are numbered in the same order as the Drive IDs. Gaps are not allowed in SCSI element addresses.

Drive renumbering considerations:

When drives in the library are changed, the renumbering of the drives may not be immediate or automatic. Some drive numbers can remain in the library configuration after they are removed. Persistence of a drive number after removal is done to maintain connectivity with software that is configured to the drives. It is expected that the original drive or a new drive might be inserted again. The renumbering of drives during drive addition or removal is described in the following sections.

Drive numbering during drive addition:

• Logical and Physical Numbering is updated automatically when the drive is inserted, and cartridges can be loaded or unloaded from the added drives.
• SCSI Element addressing does not change until a partitioning wizard is run to move the added drive into a partition.

Drive numbering during drive removal:

When a drive is physically removed, the drive remains in the library configuration, but the drive is described as "removed" and is not accessible to the host. The removed drive maintains the same Logical Numbering, Physical Numbering, and SCSI element addressing in the library configuration until:

1. Another drive is inserted in the same position as the removed drive.
2. A partitioning wizard is used to remove the drive from its current partition.
3. The "Reset the List of Known Drives and Modules" (Settings > Library > Advanced) function is used. Resetting the "list of known drives and modules" causes the library to rediscover only the drives and modules that are physically present. This operation alters the list of SCSI element addresses reported to hosts and renumbers the drives and modules. This operation cannot be undone. After the operation completes, use one of the logical library wizards to modify logical libraries as needed.

NOTES:

• If a removed drive is reinstalled, the library removes indicators that it is removed and sets the access bit so the host can use it again.
• The library reports a removed drive to the SCSI host if it is still in the library configuration, but the access bit is not set.
• Power down or reset does not change drive configuration, but factory reset changes it through the partitioning wizard that runs when it starts up again.

The following chart shows the Logical IDs, PNUMs, and Element Addresses for a partial 21U Library. It has a mix of full high and half high drives and is partitioned into two logical libraries.