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Redundancy built-in to a single FCA
With a single frame configuration (or a configuration with no FCA control system),
redundant power is provided for each BPC. If one BPC fails, the other BPC in the
frame picks up the load for the failed BPC.
Redundancy provided by two or more frames with an FCA
(control system).
For two or more frames that have an FCA control system, failure or repair of a
particular part does not cause the entire system to fail. Nor does it require
scheduled downtime for maintenance. For example, the replacement of any single
failed power supply can be accomplished without affecting the operation of the
library. The two redundant power supplies are fed directly by independent AC
power cords that support only 220 V AC (110 V ACis not supported).
Note: The AC power cords do not come standard on models L25, D25, L55, and
D55. You must specify what type of AC power cord is to be shipped with the
TS4500 frame control assembly.
Control path failover, data path failover, and load balancing
The path failover feature of the TS4500 tape library ensures the use of a redundant
communication path when the primary path fails.
Command failures and time outs are costly. You want your library to run smoothly
and efficiently. To ensure continued processing, libraries that are equipped with
Fibre Channel LTO and 3592 tape drive offer path failover and load balancing
capabilities. These capabilities allow the IBM device driver to resend a command to
an alternate path. The alternate path can include another host bus adapter (HBA),
Storage Area Network (SAN), or library control path drive. The device driver
initiates error recovery and continues the operation on the alternate path without
interrupting the application. Path failover and load balancing are built-in features
that are enabled by using a purchased license key.
Two types of path failover capabilities exist: control path failover (CPF) and data path
failover (DPF). Control refers to the command set that controls the library (the SCSI
Medium Changer command set on LUN 1 of the tape drives). Data refers to the
command set that carries the customer data to and from the tape drives (the
SCSI-3 Stream Commands (SSC) device on LUN 0 of the tape drives). Path failover
means the same thing in both. Path failover is where there is redundancy in the
path from the application to the intended target (the library accessor or the drive
mechanism, respectively), the device driver transparently fails over to another path
in response to a break in the active path.
Both types of failover include host-side failover when configured with multiple
HBA ports into a switch. But CPF includes target-side failover through the control
paths that are enabled on more than one tape drive. DPF includes target-side
failover for the dual-ported tape drives that are supported by the TS4500 tape
library.
DPF includes load balancing of the HBAs because the channel is a data-intensive
path (the control path carries very little data, so load balancing is not an issue).
The dynamic load balancing support optimizes resources for devices that have
physical connections to multiple HBAs in the same machine. When an application
opens a device that has multiple HBA paths configured, the device driver
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Chapter 1. Overview
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