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24 Device management
Discovery
Discovery is a feature that makes it easy to display attached FC and SCSI target devices and have them
mapped automatically on the host side for the connected bus/port.
There are two discovery methods available—Manual Discovery and Auto Discovery. Auto Discovery can
be set to occur after either reboot events (when the card 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 network storage
router to Manual Discovery Only. The default setting for FC Discovery is Manual Discovery.
For specific information on Discovery settings, see the Fibre Channel configuration and SCSI
configuration sections in ”N1200-320 4Gb Network Storage Router management” on page 27 and
”Discovery menu” on page 45.
Host bus adapter configuration
A host system using a Fibre Channel Host Bus Adapter (HBA) typically maps devices into the existing
device mapping scheme used by the host operating system. Refer to the HBA manual for the mapping
table.
Mapping usually involves pairing FC AL_PAs to SCSI target addresses. The HBA claims 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 network storage router corresponds to a
Bus:Target identifier, with the SCSI devices attached to the network storage router 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 network storage router 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 network storage router
address is 1. If the selection is 125, the network storage router address is 0xEF. Some FC HBAs configure
differently, so verify the AL_PA by reviewing the documentation for the HBA.
Logical unit management
Because SAN resources can be shared, multiple hosts can have access to the same devices on the SAN.
To prevent conflicts, the network storage router provides the means to restrict access and only allow hosts
to find and access selected devices. Simple LUN masking can restrict access, but many times this leaves
gaps in the list of LUNs presented to a host, since devices are always associated with fixed LUNs. The
network storage router, 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 network storage router. 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 network storage router. 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 network storage router. Each host accessing
the network storage router can be associated with a specific map.
For a host connected to an FC port, a map is a table of LUNs where each entry in the table is either empty
or contains device address information needed to route commands to the appropriate device.
The FC port on the network storage router has a set of maps which include user defined maps and a few
special predefined maps.
There are currently four special predefined maps: Indexed, Auto Assigned, SCC, and Port 0 device maps.
Until a user configures the network storage router otherwise, the default map setting is Indexed.