Rules for Mirroring

Rules for Mirroring

All data nodes in a sharded cluster must be mirrored, or all unmirrored. This requirement is reflected in the following ICM topology validation rules.

When the Mirror field is set to false in the defaults file (the default), mirroring is never configured, and provisioning fails if more than one DM node is specified in the definitions file.

When the Mirror field is set to true, mirroring is configured where possible, and the mirror roles of the DATA, DS, or DM nodes (primary, backup, or DR async) are determined by the value of the MirrorMap field (see General Parameters) in the node definition, as follows:

• If MirrorMap is not included in the relevant node definition, the nodes are configured as mirror failover pairs using the default MirrorMap value, primary,backup:

  • If an even number of DATA or DS nodes is defined, they are all configured as failover pairs; for example, specifying six DATA nodes deploys three data node mirrors containing failover pairs and no DR asyncs. If an odd number of DATA or DS nodes is defined, provisioning fails.

  • If two DM nodes are defined, they are configured as a failover pair; if any other number is defined, provisioning fails.

• If MirrorMap is included in the node definition, the nodes are configured according to its value, as follows:

  • The number of DATA or DS nodes must be a multiple of the number of roles specified in the MirrorMap value or fewer. For example, suppose the MirrorMap value, is primary,backup,async, as shown:

    "Role": "DATA",
    "Count": "",
    "MirrorMap": "primary,backup,async"
    
    

    

    In this case, DATA or DS nodes would be configured as follows:

    Value of Count	Result
    3 or multiples of 3	One or more mirrors containing a failover pair and a DR async
    2	A single mirror containing a failover pair
    1, 4 or more but not multiples of 3	Provisioning fails

  • The number of DM nodes must be the same as the number of roles specified in the MirrorMap value or fewer; if a single DM node is specified, provisioning fails.

• If more than one AR (arbiter) node is specified, provisioning fails. (While a best practice, use of an arbiter is optional, so an AR node need not be included in a mirrored configuration.)

All asyncs deployed by ICM are DR asyncs; reporting asyncs are not supported. Up to 14 asyncs can be included in a mirror. For information on mirror members and possible configurations, see Mirror Components in the High Availability Guide.

There is no relationship between the order in which DATA, DS, or DM nodes are provisioned or configured and their roles in a mirror. Following provisioning, you can determine which member of each pair is the intended primary failover member and which the backup using the icm inventory command. To see the mirror member status of each node in a deployed configuration when mirroring is enabled, use the icm ps command.