Galera Cluster

Quorum calculation — Nodes available

PmaControl computes the actually usable nodes in the Galera quorum:

Nodes available (X) =
  mysql_available = 1
  AND wsrep_cluster_status = Primary
  AND (
    wsrep_local_state_comment = Synced
    OR (Donor/Desync/Unsync AND wsrep_desync = OFF)
  )

This rule excludes nodes:

• Non-Primary — Node outside the quorum
• Disconnected — Node disconnected from the cluster
• Inconsistent — Inconsistent data
• Donor with wsrep_desync=ON — Intentionally desynchronized

The denominator Y (total) is always the total number of detected nodes, even if some are offline.

wsrep segments

Galera segments (gmcast.segment) are displayed as subgraphs in Dot3:

• SEGMENT_OK — All nodes in the segment are Synced
• SEGMENT_KO — No node in the segment is available
• SEGMENT_PARTIAL — Only some nodes are available

IST / SST flow

PmaControl detects state transfers between nodes:

• IST (Incremental State Transfer) — Incremental transfer via gcache
• SST (State Snapshot Transfer) — Full transfer (mariabackup, rsync, mysqldump)

Dot3 displays a hint link donor→joiner when an SST/IST is in progress.

garb arbitrator

PmaControl detects the Galera arbitrator (garbd) via wsrep_incoming_addresses. The arbitrator appears in Dot3 as a virtual node — it participates in quorum voting without storing data.

wsrep-notify

The wsrep-notify.sh script (in documentation/wsrep-notify/) can be configured on each Galera node to log state changes in /var/log/galera.log.

# Dans my.cnf
[mysqld]
wsrep_notify_cmd = /usr/local/bin/wsrep-notify.sh