WAN Latency¶
Length: 221 words; Published: June 24, 2015; Updated: October 22, 2019; Category: Performance; Type: Best Practices
When using Galera Cluster over a WAN, remember that WAN links can have exceptionally high latency. You can check this by taking Round-Trip Time (RTT) measurements between cluster nodes. If there is a latency, you can correct for this by adjusting all of the temporal parameters.
Recommendations
To take RTT measurements, use ping on each cluster node to ping the others. For example, if you were to log into the node at 192.168.1.1, you might execute something like the following from the command-line:
$ ping -c 3 192.168.1.2
PING 192.168.1.2 (192.168.1.2) 58(84) bytes of data.
64 bytes from 192.168.1.2: icmp_seq=1 ttl=64 time=0.736 ms
64 bytes from 192.168.1.2: icmp_seq=2 ttl=64 time=0.878 ms
64 bytes from 192.168.1.2: icmp_seq=3 ttl=64 time=12.7 ms
--- 192.168.1.2 ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 0.736/4.788/12.752/5.631 ms
Repeat this on each node in the cluster and note the highest value among them.
Parameters that relate to periods and timeouts, such as evs.join_retrans_period, must all use values that exceed the highest RTT measurement in the cluster.
wsrep_provider_options="evs.join_retrans_period=PT0.5S"
This allows the cluster to compensate for the latency issues of the WAN links between the cluster nodes.
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