How HA pairs support nondisruptive operations and fault tolerance

HA pairs provide fault tolerance and let you perform nondisruptive operations, including hardware and software upgrades, and hardware maintenance.

•	Fault tolerance

•	Nondisruptive software upgrades or hardware maintenance

During nondisruptive upgrades of Data ONTAP, the user manually enters the cf takeover command to take over the partner node to allow the software upgrade to occur. The takeover node continues to serve data for both nodes during this operation.

The HA pair supplies nondisruptive operation and fault tolerance due to the following aspects of its configuration:

•	The controllers in the HA pair are connected to each other either through an HA interconnect consisting of adapters and cables, or, in systems with two controllers in the same chassis, through an internal interconnect

•	Continually check if the other node is functioning

•	Mirror log data for each other’s NVRAM or NVMEM

•	The nodes use two or more disk shelf loops, or storage arrays, in which the following conditions apply:

•	Each node manages its own disks or array LUNs

•	In case of takeover, the surviving node provides read/write access to the partner's disks or array LUNs until the failed node becomes available again

Note: Disk ownership is established by Data ONTAP or the administrator; it is not based on which disk shelf the disk is attached to.

•	They own their spare disks, spare array LUNs, or both, and do not share them with the other node.

•	They each have mailbox disks or array LUNs on the root volume that perform the following tasks:

•	Maintain consistency between the pair

•	Continually check whether the other node is running or whether it has performed a takeover

•	Store configuration information