Virtual TapeServer SCSI-to-FC Adapter User Manual

4-3
Chapter 4: Addressing and Logical Unit Management
reboots) or link-up events (for instance, when cables are attached or a hub is rebooted). Discovery
can also be turned off by setting the VT5900-SA to Manual Discovery Only.
For specific information on Discovery settings, see the Fibre Channel Configuration and SCSI
configuration sections in “
Chapter 5, VT5900-SA Management” and “Chapter 6, Using Visual
Manager.”
Host Device Configuration
A host system using a Fibre Channel Host Bus Adapter (HBA) will typically map devices into the
existing device mapping scheme used by that operating system. Refer to the HBA manual for the
mapping table.
Mapping usually involves pairing FC AL_PAs to SCSI target addresses. The HBA will claim
enough SCSI bus entries to allow for 125 FC targets to map to SCSI Bus:Target entries. This is
usually done by a fixed mapping of AL_PA to Bus:Target. In such a configuration, the VT5900-
SA corresponds to a Bus:Target identifier, with the SCSI devices attached to the VT5900-SA
appearing as logical units (LUNs). Operating systems can extend the available SCSI limit of 15
targets per bus. Although this is not an issue for the operating system or most applications, there
are cases where older applications can have expectations about what are valid SCSI IDs, and not
correctly handle certain mappings. In particular, applications have been seen to exhibit difficulties
addressing target IDs greater than 15 (e.g. 16 and up). This problem can be resolved by
configuring the VT5900-SA to use hard addressing, and setting the AL_PA used by the unit to a
value that the HBA will map to an ID with a value less than 16.
For example, depending on the FC HBA, if the hard AL_PA selection is 1, the address is 1. If the
selection is 125, the AL_PA address is 0xEF. Some FC HBAs will configure differently, so verify
the AL_PA by reviewing the documentation for the HBA.
Logical Unit Management
Because SAN resources can be shared, it is possible for multiple hosts to have access to the same
devices on the SAN. To prevent conflicts, the VT5900-SA provides the means to restrict access
and only allow hosts to find and access selected devices. Simple LUN masking can restrict access
but may leave gaps in the list of LUNs presented to a host, since devices are always associated
with fixed LUNs. The VT5900-SA, however, provides controlled access to devices by use of LUN
management which goes beyond simple LUN masking.
LUN Management is the ability to present different hosts with different views of the devices
accessed through the VT5900-SA. For example, one FC host may see three disk LUNs and a tape
LUN at LUNs 0 to 3 when it performs discovery on the VT5900-SA. Another FC host may only
discover a tape LUN at LUN 0. Not only can the administrator control which devices a host may
access but also which LUNs are used to access these devices.
LUN Management is accomplished by allowing the administrator to configure multiple maps each
of which may present a different view of the devices behind the VT5900-SA. Each host accessing
the VT5900-SA can be associated with a specific map.