High Availability FailOver/iX Manual (32650-90911)

Product Description
Components
If any other error type occurs (such as a data transmission or device error), the
I/O subsystem manages the error and perform corrective action. HAFO remains
idle and not participate. In addition, HAFO remains idle when error types are
received from non-high availability devices.
CAUTION
The difference between a Hung device and an extremely slow device can be
very difficult to determine. It is possible that under extreme I/O loads a device
may become so slow in responding that a HAFO event is triggered. Before
adding HAFO to an existing system, Disk I/O subsystem performance should be
analyzed for possible performance bottlenecks, especially around “peak”
processing times. (Refer to Chapter 4, sections “Configuration Planning” and
“ADDCONF” for more information.)
Executing a Failover
When a trigger status is received, HAFO immediately begins the failover
sequence. This sequence activates the alternate data path and reroutes I/O to it.
Failover occurs on a per Ldev basis. Each device manager (the piece of the I/O
subsystem that manages a specific Ldev) detects a data path failure
independently as a result of an I/O to its Ldev. If the I/O rate is low, it is
possible that Ldevs on a failed path will not learn of a failure for a noticeable
period of time however, as I/O failures occur, each Ldev will failover. For
example, if three Ldevs on a fast-wide SCSI bus experience an array controller
failure, the associated three device managers will perform a HAFO failover
event independently. Figure 2-2 illustrates a failover event in a sample system.
Figure 2- 2 Failover
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High Availability Array
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