Quorum disk configuration

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A quorum disk is an MDisk or a managed drive that contains a reserved area that is used exclusively for system management. A system automatically assigns quorum disk candidates. When you add new storage to a system or remove existing storage, however, it is a good practice to review the quorum disk assignments.

It is possible for a system to split into two groups where each group contains half the original number of nodes in the system. A quorum device determines which group of nodes stops operating and processing I/O requests. In this tie-break situation, the first group of nodes that accesses the quorum device is marked as the owner of the quorum device and as a result continues to operate as the system, handling all I/O requests. If the other group of nodes cannot access the quorum device or finds that the quorum device is owned by another group of nodes, it stops operating as the system and does not handle I/O requests.

A system can have only one active quorum device that is used for a tie-break situation. However, the system uses up to three quorum devices to record a backup of system configuration data to be used in the event of a disaster. The system automatically selects one quorum device to be the active quorum device. The active quorum device can be specified by using the chquorum command-line interface (CLI) command with the active parameter. To view the current quorum device status, use the lsquorum command.

The other quorum devices provide redundancy if the active quorum device fails before a system is partitioned. To avoid the possibility of losing all the quorum devices with a single failure, assign quorum disk candidates on multiple storage systems or run IP quorum applications on multiple servers.
Note: Mirrored volumes can be taken offline if no quorum disk is available. The synchronization status for mirrored volumes is recorded on the quorum disk.
When you change the managed disks that are assigned as quorum candidate disks, follow these general guidelines:
  • When possible, aim to distribute the quorum candidate disks so that each MDisk is provided by a different storage system. For information about which storage systems are supported for quorum disk use, refer to the supported hardware list.
  • Before you change a quorum candidate disk, ensure that the status of the managed disk that is being assigned as a quorum candidate disk is reported as online. Also, ensure that it has a capacity of 512 MB or larger.
  • Use smaller capacity MDisks or use drives as the quorum devices to significantly reduce the amount of time that is needed to run a recover system procedure (also known as Tier 3 or T3 recovery), if necessary.

Quorum MDisks or drives in HyperSwap or stretched system configurations

To provide protection against failures that affect an entire location (for example, a power failure), you can use volume mirroring with a configuration that splits a single system between two physical locations. For more information, see HyperSwap® configuration details or stretched system configuration details. For detailed guidance about HyperSwap and stretched system configuration for high-availability purposes, contact your service representative.

If you configure a HyperSwap or stretched system with the enhanced stretched system functions, the system automatically selects quorum disks that are placed in each of the three sites. If you are not using the enhanced stretched system configuration functions, then assign quorum disks manually as described here.

Generally, when the nodes in a system are split among sites, configure the system this way:
  • Site 1: Half of system nodes + one quorum device
  • Site 2: Half of system nodes + one quorum device
  • Site 3: Active quorum device
Typically the quorum devices at site 1 and site 2 are quorum disks and the quorum device at site 3 is an IP quorum application. However, the system can be configured to use either quorum disks or IP quorum applications at any site. This configuration ensures that a quorum device is always available, even after a single-site failure.

The following scenarios describe examples that result in changes to the active quorum disk:
  • Scenario 1:
    1. Site 3 is either powered off or connectivity to the site is broken.
    2. If topology is standard, the system selects a quorum disk candidate at site 2 to become the active quorum disk. If topology is HyperSwap or stretched, the system operates without any active quorum disk.
    3. Site 3 is either powered on or connectivity to the site is restored.
    4. Assuming that the system was correctly configured initially, the system automatically recovers the configuration when the power is restored.
  • Scenario 2:
    1. The storage system that is hosting the preferred quorum disk at site 3 is removed from the configuration.
    2. If possible, the system automatically configures a new quorum disk candidate.
    3. In HyperSwap or stretched topology, the system selects only a new quorum disk that is in site 3. In a standard topology, the system selects a quorum disk candidate at site 1 or 2 to become the active quorum disk.
    4. A new storage system is added to site 3.
    5. In standard topology, the administrator must reassign all three quorum disks to ensure that the active quorum disk is now at site 3 again. In HyperSwap or stretched topology, the system automatically assigns the new active quorum disk when the storage system is installed and the site setting is configured.

Fibre Channel over IP usage

Fibre Channel over IP (FCIP) routers can be used for quorum disk connections under the following circumstances:
  • The FCIP router device is supported for remote mirroring (Metro Mirror or Global Mirror).
  • The maximum round-trip delay must not exceed 80 ms, which means 40 ms each direction.
  • A minimum bandwidth of 2 megabytes per second is guaranteed for node-to-quorum traffic.
Note:
  1. To avoid fabric topology changes in case of IP errors, it is a good practice to configure FCIP links so that they do not carry ISLs.
  2. Connections that use iSCSI are not supported.

Usage of wavelength division multiplexing devices that do not require electrical power

Passive wavelength division multiplexing (WDM) devices can be used for quorum disk connections. These connections rely on SFP transceivers with different wavelengths (referred to as colored SFP transceivers) for fiber sharing. The following requirements apply when you use these type of connections:
  • The WDM vendor must support the colored SFP transceivers for usage in the WDM device.
  • The Fibre Channel switch vendor must support the colored SFP transceivers for ISL.
  • The WDM device for Metro Mirror, Global Mirror, or HyperSwap functions is supported.
  • The SFP transceivers must comply with the SFP/SFP+ power and heat specifications.
Note: To purchase colored SFP transceivers for passive WDM, contact your WDM vendor.