nStor NexStor 4200F Series, User Manual

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Chapter 2 - Topologies and Operating Modes
Alternate Path Software
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Alternate Path Software
This is a software tool that manages multiple paths between the host operating
system and LUNs. The software manages the multiple paths by detecting
duplicate disk objects that represent a single LUN. It then designates one disk
object as the primary disk object with a primary path, while the other is
designated the secondary disk object with an alternate path. If the primary path
becomes inaccessible, the software redirects the data to the secondary disk object
through the alternate path, preserving the LUN.
This redirection is known as path failover. The software continuously tries to
access the failed path by issuing a SCSI Test Unit Ready command. A good status
returned indicates the path is repaired and restored to operational status. The
software automatically redirects data back to the primary path and primary disk
object. This restoration of data transfer is known as path failback.
When using alternate path software the array must be configured to use the
multiport failover topology. The multiple target ID failover topology may
interfere with the operation of the alternate path driver.
When using alternate path software with the RAID Controller, the options must
be set to “Loop Preferred” for the multiport topology and “Loop Only” for the
multiple target ID topology.
Controller-Controller Nexus (C-C Nexus)
Controller-to-Controller Nexus is a connection between the two controllers of a
dual-active controller pair that enables each controller to copy write-back data to
its partner controller and determine if the partner controller is operating.
Fibre Channel Media Types
Optical transceivers are provided with the enclosure. Fibre optical transceivers
provide a more reliable media and supports distances up to 300 meters between
nodes.
4200F.book Page 36 Thursday, January 23, 2003 2:47 PM