The DEFAULT CLUSTER command indicates that Configuration is to assign AMPs to clusters automatically for the new configuration.
An AMP cluster is a collection of AMPs that are grouped together logically to provide fallback capability for each other for tables that are created with the FALLBACK option. Each AMP in a cluster contains a backup copy of the primary data for one of the other AMPs in the cluster. (Systems that have been upgraded from pre-16.10 versions of Teradata Database use a legacy fallback scheme whereby each AMP in a cluster contains a backup copy of a portion of the primary data for each of the other AMPs in the cluster, unless they have undergone a system reconfiguration.)
- the default cluster size.
- If you do not specify a cluster size, the default and recommended size is 2.
A cluster must have a minimum of two AMPs.
- For dedicated fallback, the cluster size can be increased up to eight AMPs, with each AMP having fallback on one other AMP.
- For legacy fallback, the usual recommended cluster size is two AMPs, but some clusters may need to have more than two if there are an odd number of AMPs or if adding AMPs over time causes the cluster size to increase.
Typically, you type the DEFAULT CLUSTER command at the end of a session during which AMPs were added to the system.
When the DEFAULT CLUSTER command is executed, AMPs are assigned to clusters automatically.
In a system with n AMPs, each cluster contains c AMPs and n/c clusters.
For legacy fallback, if nis not divisible by c, the last cluster will be at least two, and no smaller than 50% less and not larger than 50% more than any other cluster. The number of clusters is adjusted accordingly.
After the DEFAULT CLUSTER command is accepted, AMPs are assigned to clusters, and the system displays:
The cluster assignment is complete.
Execution of the DEFAULT CLUSTER command overrides all previous cluster assignments.
- For systems with a single cabinet containing all nodes, no two or more AMPs of a cluster reside in the same node.
- For systems with multiple node cabinets, no two or more AMPs of a cluster reside in the same cabinet.
- For systems with a single cabinet containing all disk arrays, no two or more AMPs of a cluster are serviced by the same disk array.
- For systems with multiple cabinets of disk arrays, no two or more AMPs of a cluster are serviced by the same disk array cabinet.
- For systems with more than one disk array controller, no two or more AMPs of a cluster are serviced by the same controller.
Examples: Specifying a default cluster size
Typing DEFAULT CLUSTER 4 for a system of 15 AMPs results in a cluster size of 4. Three clusters have four AMPs each, and a fourth cluster has three AMPs.
Typing DEFAULT CLUSTER 3 for a system of 16 AMPs results in cluster size of 3. Four clusters include 3 AMPs each, and 1 cluster includes 4 AMPs.
Typing DEFAULT CLUSTER 8 for a system of 16 AMPs results in a cluster size of 8. Two clusters include 8 AMPs each.