SST

SST (State Snapshot Transfer) is the mechanism by which a Galera node joining the cluster (or one that has been offline too long) receives a complete copy of the entire dataset from a donor node.

How it works #

When a node joins the cluster and the gap of missing transactions exceeds the gcache size, the cluster initiates an SST. The donor node creates a full snapshot of the database and transfers it to the receiving node. Available methods include: mariabackup (doesn’t block the donor), rsync (fast but blocks the donor for reads), and mysqldump (slow and blocking).

What it’s for #

SST is essential for two scenarios: adding a new node to the cluster (first join) and recovering a node that has been offline so long that missing transactions are no longer available in the donor’s gcache.

When to use it #

SST is triggered automatically by Galera when needed. The SST method choice (wsrep_sst_method) is made during configuration. In production, mariabackup is the recommended choice because it doesn’t block the donor node, avoiding cluster degradation during the transfer.