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Galera Cache sizing

To synchronize the data between the Galera Cluster and a new or re-entering Galera node Galera Cluster uses 2 different mechanisms:

  • For full synchronization of data: Snapshot State Transfer (SST).
  • For delta synchronization of data: Incremental State Transfer (IST).

The Incremental State Transfer (IST) is relevant when a node is already known to the Galera Cluster and just left the cluster short time ago. This typically happens in a maintenance window during a rolling cluster restart.

The Galera Cache is a round-robin file based cache that keeps all the write-sets (= transactions + meta data) for a certain amount of time. This time, which should be bigger than your planned maintenance window, depends on the size of the Galera Cache (default 128 Mbyte) and the traffic which will happen during your maintenance window.

If your traffic is bigger than the Galera Cache can keep Galera Cluster will fall-back from IST to SST which is a very expensive operation for big databases.

The size of the Galera Cache can be calculated of the delta of the sum of the following 2 Galera status informations before and after the maintenance window:

Galera Cache size = delta(wsrep_replicated_bytes + wsrep_received_bytes)

Ideally you determine these values before your change happens in a time window where you have roughly the same traffic as during your maintenance window.

If you do not have a Galera Cluster in place yet or if you do not have those values available you can also use the numbers of the traffic written to the binary log or the number of the traffic written to InnoDB transaction log (Innodb_os_log_written).

As a rough estimate we have evaluated the following formulas for you:

Binary Log Traffic x 1.3 = Wsrep traffic (+/- 10%)

or

InnoDB Log File traffic x 0.6 = Wsrep traffic (+/- 10%)

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Comments

Yesterday we did a first real life test of the formulas and the predictions of all 3 methods where quite congruent... So I personally trust them!
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