Enabling High Availability
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leaf_ global variable defines the placement of replica partitions among leaves in a cluster.
leaf_ is set to
paired mode, the auto-rebalance operation only affects the newly attached leaves and their pairs.
load_ mode, the auto-rebalance operation runs a full rebalance of all the databases on all the leaves in a cluster.
For small clusters, such as a cluster with four leaf nodes, the
paired mode is preferable over the
load_ mode on a very small cluster, losing two nodes in different availability groups (AGs) will always take the cluster offline.
Consider the following example:
AG1 has nodes A and B.
Node A has primary partitions 1,2,3,4.
Node B has primary partitions 5,6,7,8.
AG2 has nodes C and D.
Node C has primary partitions 9,10,11,12.
Node D has primary partitions 13,14,15,16.
load_ mode, the secondary/backup partitions are balanced as follows:
Node A has backups of 9,10 (from Node C) and 13,14 (from Node D).
Node B has backups of 11,12 (from Node C) and 15,16 (from Node D).
Node C has backups of 1,2 (from Node A) and 5,6 (from Node B).
Node D has backups of 3,4 (from Node A) and 7,8 (from Node B).
If node A goes down, the cluster now requires both nodes C and D to continue serving its partitions, so removal of either of those nodes will take the cluster offline.
If you were in the
paired mode, you will instead have something like the following:
Node A has backups of all of Node C's partitions.
Node B has backups of all of Node D's partitions.
Node C has backups of of Node A's partitions.
Node D has backups of all of Node B's partitions.
In this case, if node A goes down, then you can also lose node D (but not Node C) in the other availability group without causing an outage.
paired mode, you should also be able to lose just A and B.
This is less of a concern on larger clusters, in which the backup partitions are still load balanced across a different AG but don't require every single node in it as in the case of small clusters of just four leaf nodes.
paired modes - the former balances the backup partition load more evenly, but also makes it more fragile in the event of an outage.
The following sections discuss how to enable high availability for each of these modes.
Last modified: October 31, 2023