IBM System Storage TS3100, Setup, Operator, And Service Manual

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The SCSI specification does not allow gaps in the SCSI element addressing. Special
handling is needed for empty drive slots to fulfill the specification. Also
temporarily removed drives need to have their addresses preserved to not confuse
the attached host and host application. Generally only drives which are currently
physically available or temporarily removed are reported. Empty (unused) slots
located at the bottom or the top should not be reported, with an exception in case
of a “removed” condition. A drive slot which does not contain a drive, and has a
position between used slots, needs to be reported as a SCSI element. To signal the
host application that this slot is not usable, its ACCESS bit will be disabled.
Note: When reducing the number of drives in your library, update the Logical
Library configuration. This will remove the Attention LED on the front
panel and the exclamation mark on the Home screen indicating that a drive
is missing.
Updating the Logical Library configuration will also update the drive element
addressing and drive numbering. Replacing LTO half high drives with full high
drives may require you to execute Restore Factory Defaults to correctly update the
drive element addressing and drive numbering. See “Configure Library:
Save/Restore” on page 5-68.
Using Persistent Binding to Ensure SCSI ID Assignment
When a server is booted, devices are discovered and assigned SCSI target and
LUN IDs. It is possible for these SCSI assignments to change between boots. Some
operating systems do not guarantee that devices will always be allocated the same
SCSI target ID after rebooting. Also, some software depends on this association, so
you do not want it to change. The issue of SCSI ID assignment is addressed by
persistent binding.
Persistent binding is an HBA function that allows a subset of discovered targets to
be bound between a server and device. Implemented by a World Wide Node
Name (WWNN) or World Wide Port Name (WWPN), persistent binding causes a
tape drive's WWNN to be bound to a specific SCSI target ID. After a configuration
has been set, it survives reboots and any hardware configuration changes because
the information is preserved. If a drive needs to be replaced, the new drive
assumes the WWNN of the old drive because the WWNN for the drive is
location-dependent within the library. Because the WWNN does not change,
persistent binding does not need to be changed thus preventing a potential outage.
Logical Unit Number (LUN) Scanning
The library uses a single SCSI or Loop ID per drive and dual LUNs to control the
tape drive (LUN 0) and library accessor (LUN 1). The library requires a Host Bus
Adapter (HBA) that supports LUN scanning. If it is not enabled, your host system
will not scan beyond LUN 0 and will fail to discover the library. It will only see
the tape drive.
Important: Some HBAs, such as RAID controllers, do not support LUN scanning.
Host Interfaces
The 2U library and the 4U library can be attached to servers using the following
interfaces:
v
SCSI (LVD Ultrium 160)
v Serial Attached SCSI (SAS)
Chapter 3. Installation Planning
3-7