100 IBM System Storage N series Hardware Guide
7.4.4 HA pair configuration failover basic operations
When a failover occurs, the running partner node in the HA pair configuration takes over the
functions and disk drives of the failed node. It does so by creating an emulated storage
system that runs the following tasks:
Assumes the identity of the failed node.
Accesses the disks of the failed node and serves their data to clients.
The partner node maintains its own identity and its own primary functions, but also
handles the added function of the failed node through the emulated node.
7.4.5 Connectivity during failover
Front-end and back-end operations are affected during a failover. On the front end are the IP
addresses and the host name. On the back end, there is the connectivity and addressing to
the disk subsystem. The back-end and front-end interfaces must be configured correctly for a
successful failover.
Reasons for HA pair configuration failover
The conditions under which takeovers occur depend on how you configure the HA pair
configuration. Takeovers can be started when one of the following conditions occurs:
An HA pair node that is configured for immediate takeover on panic undergoes a software
or system failure that leads to a panic.
A node that is in an HA pair configuration undergoes a system failure (for example,
NVRAM failure) and cannot reboot.
There is a mismatch between the disks that one node can see and the disks that the other
node can see.
One or more network interfaces that are configured to support failover becomes
unavailable.
A node cannot send heartbeat messages to its partner. This situation might happen if the
node experienced a hardware failure or software failure that did not result in a panic, but
still prevents it from functioning correctly. An example is a failure in the interconnect cable.
You halt one of the HA pair nodes without using the -f flag. The -f flag applies only to
storage systems in an HA pair configuration. If you enter the halt -f command on an
N series, its partner does not take over.
You start a takeover manually.
Remember: When a failover occurs, existing CIFS sessions are ended. A graceful
shutdown of the CIFS sessions is not possible, and some data transfers might be
interrupted.
Restriction: If the storage for a node also loses power at the same time, a standard
takeover is not possible.
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