Chapter 1 Overview of NEC Storage PathManager
NEC Storage PathManager software multiplexes paths between a server and a storage unit in a system with Express5800
and the NEC Storage series Disk Array SubSystem. This software automatically switches paths to be used in case of
failures in paths currently in use. Also, this software uses multiple paths simultaneously to distribute loads to the paths.
This document will provide basic information such as an overview, hardware configuration, and software configuration
of NEC Storage PathManager. This document will also illustrate features of the software.
In this document, "NEC Storage series Disk Array SubSystem" will be referred to as "NEC
Storage" hereinafter.
1.1 About NEC St
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Availability is one of the important points when constructing a SAN system and needless to say this
availability largely relies on the storage subsystem. This is because a storage subsystem is one of the most
common places in a system to experience failures.
A variety of methods have been found and used so far in order to reduce failure occurrence in storage units
and smoothly recover from failures. Examples include RAIDs, hot spares, and replications. These functions
are also provided in NEC Storage to achieve high availability.
Unfortunately, these solutions are only effective for media errors. If failures occur in host bus adapters,
controllers, or physical paths (such as fibre channel cables and switches) between a server and a storage unit,
spindle redundancy alone will not be sufficient to continue operating the system. This is because there will
be no means for accessing data in the storage unit even though the data themselves are intact. Even a heavily
protected storage unit will lose all its power once a path failure occurs. This also applies to NEC Storage.
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Handling path failures
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NEC Storage PathManager provides an infrastructure to multiplex paths and eliminate the abovementioned
vulnerability present between a server and a storage unit in a system.
When a failure occurs in a path and a storage unit can no longer be accessed during system operation, NEC
Storage PathManager will detect the event. NEC Storage PathManager then switches the path to be used
from the failed path to a normal path so that access to the storage unit can continue. No I/O will be lost
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