Persistent Binding - EMC Connectrix EC-1200 Planning Manual

Fibre Channel Switching Concepts
2

Persistent Binding

The upper levels of all device drivers used in a fabric environment
were designed for parallel SCSI. The parallel SCSI I/O model
assumed a small number of targets on a bus and a small number of
LUNs (logical unit numbers) on each target. The operating system
data structures for targets were often a 3-bit or 4-bit field that was
directly associated with the address signaling method used by the
SCSI hardware. In a fabric, the target level of the addressing
hierarchy is the 64-bit WWPN. This addressing level is indirectly
associated with the fabric hardware.
The association between the unique WWPN and the
Source/Destination IDs used to route frames through the fabric is
maintained by the Name Server in the fabric switch. Using a 64-bit
address with three-bit or four-bit device driver software architecture
has proved to be a challenge for the Fibre Channel fabric device
drivers.
The association of the operating system's controller-target value (the
c t in the c t d or the b t in the b t l ) and the WWPN of the
x y x y z x y x y z
Symmetrix is called binding. Binding takes place in the operating
system; the unique WWPN value is bound to an operating system
controller-target value. When this bind is persistent from boot to
boot, and across hardware changes and failures in the fabric, it is
called a persistent bind. To establish a persistent bind, the driver must
save this association in nonvolatile storage.
Binding is closely associated with the device discovery process.
Before any binding associations have been established, the driver
queries the Name Server for all ports that have logged in with an
FC-4 type of SCSI.
The Name Server delivers a list of DIDs and the driver logins into
each DID, then retrieves the WWPN associated with the DID and
processes this information against the CTD/BTL (controller, target,
devics/bus, target, LUN) value. (The exact mechanism and storage of
this information varies among HBA drivers.) If this association is not
saved, the list from the Name Server might be different the next time
the driver makes the association, resulting in a bind between the
WWPN and the HBA-target value that is inconsistent with the
original bind. In an operating system environment (depending on
consistent BTL or CTD associations from boot to boot) this results in
devices shifting in the device address space.
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Persistent Binding