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we are currently testing if moving from mariaDB master/slave replication to a 3-node-galera-cluster is an option for us.

General replication in the cluster works fine like expected. Load test shows that write-performance does not meet our requirements though. To migrate data from the master/slave-system to the cluster one cluster node is set up as slave to that m/s-system (backup from master is imported, slave replication on the cluster node starts from the point where the backup is taken). The cluster node never catches up to the master (seconds-behind-master is increasing). So we looked for tuning-options for our cluster. Setting repl.commit_order=2 improved performance, replication from the master to the cluster node went fine. But we then experienced that the other cluster nodes weren't replicating any more (though their state was synced and the wsrep%-variables did not show anything unusual, no throttling), replication within the cluster seemed to have stalled entirely. When resetting repl.commit_order replication worked again. Our cluster was not replicating neither when we had just the cluster operational (without replicating from the master node) with repl.commit_order=2.

Here excerpts from the server.cnf:

[galera]
# Mandatory settings
wsrep_provider         = /usr/lib64/galera/libgalera_smm.so
wsrep_cluster_address  = "gcomm://10.0.0.10,10.0.0.8,10.0.0.11"
wsrep_cluster_name       = foobar
wsrep_node_name          = foobar-db1
wsrep_node_address       = 10.0.0.10
binlog_format            = row
default_storage_engine   = InnoDB
innodb_autoinc_lock_mode = 2

wsrep_slave_threads      = 16
wsrep_provider_options   = 'gcs.fc_limit=80;gcs.fc_factor=0.8;repl.commit_order=2'

# SST settings
wsrep_sst_method         = xtrabackup-v2
wsrep_sst_receive_address= 10.0.0.10
wsrep_sst_auth           = galera:somepw

wsrep_retry_autocommit   = 10

# required since 10.1
wsrep_on=ON

Any ideas?

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(Is your question mostly about the "migration"? Or about write performance on the resulting Cluster?)

Look at your transactions. Optimal is some sort of "medium" size. Too short leads to long latency. Too long leads to extra risk of deadlocks/failure at COMMIT time.

Look at the network. You are dependent on how far apart the nodes are. If there are next to each other, you do not get the HA benefits. If they are too far apart, you have latency issues.

Look at the write queries. If they are likely to conflict, but you are sending them to different nodes, then you may be adding to the problem, relative to somehow focusing some sets of queries onto a single node.

Do you have "critical reads"? How are you handling them?

More Galera tips .

  • Thanks for your input, Rick! Migration is working fine (in theory, due to the write performance issues the cluster is either neither catching up to the replication master or the nodes are not replicating at all (one node receiving the replication data is doing just fine, the others are "stalled") - depending on the "repl.commit_order"-configuration. – Heiko Großmann Apr 19 '17 at 16:01

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