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

    When one node fails or becomes impaired and a takeover occurs, the partner node continues to serve the failed node’s data.

  • Nondisruptive software upgrades or hardware maintenance

    During hardware maintenance or upgrades, when you halt one node and a takeover occurs (automatically, unless you specify otherwise), the partner node continues to serve data for the halted node while you upgrade or perform maintenance on the node you halted.

    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.

    For more information about nondisruptive software upgrades, see the Data ONTAP Upgrade and Revert/Downgrade Guide for 7-Mode.

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
    The nodes use the interconnect to perform the following tasks:
    • 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.

    For more information about disk ownership, see the Data ONTAP Storage Management Guide for 7-Mode.

  • 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