HP XP Racks - Manual (Page 21)

3. Create the pair.
Planning external paths and path groups
The external path is the physical link from the local storage system port to the external storage
system port. You prepare the ports on the local and external systems and then set up the external
path prior to mapping your external volumes.
To prepare and set up ports, see the following sections:
•
“Setting port attributes on the local system” (page 30)
•
“Setting up ports on the external system” (page 33)
External paths
A path consists of cables and possibly switches. You configure your path according to bandwidth
considerations, which include distance, speed, and performance requirements.
Because workload can spike and cable or switch failures can occur, HP strongly recommends that
you set up redundant external paths. A maximum of eight paths can be used per mapped external
volume. Multiple paths—redundancy—allows you to perform I/O operations with external volumes
regardless of workload and/or path failure.
With multiple paths, the external storage system determines how they are used: some systems use
one primary path with alternates available as backups (Single path mode); other systems allow all
paths to be used at the same time, distributing I/O among them (Multi path mode). The path storage
system’s mode cannot be changed. With both modes, you place the paths in path groups and
prioritize each path.
Single path mode
For Single path mode, the external path with the highest priority (primary path) is used to execute
I/O to the external volume. If the primary path cannot be used, it is switched to an alternative
path, after a 3-minute period.
The following figure illustrates how failure is handled with redundant paths in Single path mode.
Planning external paths and path groups 21