Reliability and availability features
165
Data Integrity, Availability, and Protection
The following is an overview of the redundant components of the
Symmetrix DMX back-end disk subsystem.
Redundant disk
directors
A pair of directors is used to access each disk drive. One director is
connected to one physical path to the drive, and the other director is
connected to a second physical path to the drive. The directors are
responsible for moving data between the global memory and the
disks and, as such, are each connected to the global memory through
redundant internal paths, to eliminate any possible single points of
failure.
Redundant cable
paths
Each disk director is attached to its associated set of Fibre Channel
disks by a separate, independent cable assembly that carries the Fibre
Channel loop, control bus, and other protocol signals.
Redundant disk ports Each Fibre Channel disk drive has two fully independent Fibre
Channel ports that are designed to connect to two separate loops. In
the Symmetrix DMX architecture, these two ports are each connected
to a different link control card (LCC), which in turn is connected
through separate cables to two different disk directors. These two
pathways are completely independent of one another.
Fibre Channel
arbitrated loop
design
Symmetrix DMX systems employ an arbitrated loop design that
contains monitoring and control hardware and software to maximize
the performance and availability of each loop. The loop is connected
in a Star-Hub topology, with the hub ports gated with a bypass
switch that allows individual loop segments (that connect to a Fibre
Channel disk drive) to be dynamically inserted or removed. The loop
initiator is a Symmetrix Fibre Channel disk director, which feeds data
into and controls the hub. All of the monitoring and control logic is
contained on the link control card, which also contains the
bypass/hub logic for the loop. To ensure the highest level of
availability, the communications link to the monitoring and control
functions is not carried by the Fibre Channel loop—but, rather is
implemented through a separate path. If the Fibre Channel loop is not
operating for some reason, the director can access the LCC through
the control bus and reconfigure the loop into a working state.
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