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Host Server
SCSI / FCP
SCSI / FCP
adapter 0
adapter 1
ESS
Port 0
Port 1
Cluster 1
Cluster 2
LUN 0
LUN 1
LUN 2
LUN 3
S009000Q
Figure 7. Multipath connections between a host server and ESS LUNs
The IBM Subsystem Device Driver provides the following functions:
v Enhanced data availability
v Automatic path failover and recovery to an alternate path
v Dynamic load balancing of multiple paths
v Path selection policies for the AIX operating system
v Concurrent download of licensed internal code
In most cases, host servers are configured with multiple host adapters with SCSI or
fibre-channel connections to an ESS that, in turn, provides internal component
redundancy. With dual clusters and multiple host interface adapters, the ESS
provides more flexibility in the number of input-output (I/O) paths that are available.
When there is a failure, the IBM Subsystem Device Driver reroutes I/O operations
from the failed path to the remaining paths. This function eliminates the following
connections as single points of failure: a bus adapter on the host server, an external
SCSI cable, a fiber-connection cable, or a host interface adapter on the ESS.
In addition, multipath load balancing of data flow prevents a single path from
becoming overloaded with I/O operations.
The IBM Subsystem Device Driver path recovery algorithms now test Dead paths
that are not manually offline every 30 minutes.
Concurrent download of licensed internal code is the capability to download and
install licensed internal code on an ESS while applications continue to run. During
the time when new licensed internal code is being installed in an ESS, the upper
interface adapters inside the ESS may not respond to host I/O requests for
approximately 30 seconds. The IBM Subsystem Device Drivers makes this
transparent to the host through its path selection and retry algorithms.
13
Chapter 1. Introduction
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