The SET MIRROR OPTION statement supports two options that can be used to assign new copy nodes to a parent in the tree so that the work of distributing transaction log pages is balanced among the nodes:
auto_add_server Specifies the name of a database server that acts as the top-most node of the automatic assignment tree.
auto_add_fan_out Specifies the maximum number of copy nodes that each branch in the tree should have. The minimum value you can specify is 2, and the default is 10. New nodes that are created as a result of one of the following actions have their parent assigned automatically:
connecting to the primary server when the child_creation option is set to automatic
executing CREATE MIRROR SERVER...USING AUTO PARENT on the copy node when the child_creation option is set to manual
executing CREATE MIRROR SERVER...USING AUTO PARENT on the root database server
When the auto_add_server option is specified, the database server specified by that option is used as the top-most node. When the add_auto_server option is not specified, then the root database server is used as the parent for all copy nodes using automatic assignment and the auto_fan_out option is ignored.
The root database server determines which node in the tree should be assigned as the parent for the new copy node, based on the number of children for each node and its status as reported to the root database server by periodic status updates. The copy node sees the assignment of its new parent as it applies transaction log operations received from the root database server, so the copy node changes its parent connection when it receives the transaction log operation.
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