Multiple Serial Replication Slaves with MariaDB Xpand
Xpand provides sophisticated and scalable support for running many serial replication slaves on a cluster, including:
Even distribution of serial replication slaves among nodes in the cluster.
Even distribution of multiple serial replication slaves on the same node among CPU cores.
MariaDB Xpand 5.3
MariaDB Xpand 6.0
MariaDB Xpand 6.1
Serial replication slaves are balanced among nodes following two rules:
When a new serial replication slave is created, it starts on the node with the fewest currently enabled serial replication slaves.
After a group change, all serial replication slaves restart automatically and will be allocated to nodes according to rule 1 as if they were newly created.
Multiple serial replication slaves on the same node are balanced among CPU cores following three rules:
When a new serial replication slave is started, it starts on the CPU core that has the least number of serial replication slaves (either started or stopped).
Dropping a serial replication slave releases an allocated CPU core, allowing a new serial replication slave to take its place (an existing serial replication slave will not take its place).
After a group change, all serial replication slaves restart automatically and will allocate CPU cores according to rule 1 as if they were newly created.
These changes will only show performance improvements on clusters with many serial replication slaves running concurrently. For example, a 3-node cluster with 3 serial replication slaves will have 1 slave on each node. A 3-node cluster (each node with 4 CPU cores) with 9 serial replication slaves will have 3 slaves on each node, with one slave running on each of core 1, core 2, and core 3 (core 0 is not used by replication slaves). A 3-node cluster (each node with 3 CPU cores) with 9 serial replication slaves will have 3 slaves on each node, with two slaves running on core 1 and one slave on core 2.
A parallel replication slave does not have a single slave process that runs on a single node, the way a serial replication slave does.
For information on the architecture related to parallel replication with MariaDB Xpand, see "MariaDB Xpand Parallel Replication Architecture".