Galera Mariadb Multi-Master Replication

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I am trying to setup a cluster of 3 servers in 3 different locations; Dallas-US, London-UK, Mumbai-India. On each location I have setup a webserver and db server. On db server I have configured Galera Mariadb Multi-Master cluster to replicate db among all three servers. My each webservers are connected with local IP to their regional db server. I am expecting that my Dallas webserver will fetch db records from Dallas db server; London webserver from London db server and Mumbai webserver from Mumbai db server.

Everything is working well but I have found that mysql query takes much time above 100s while fetching record. I have tried Mariadb with single instance and its fetching data within 5s.

What am I doing wrong?

mariadb mysql
1 Answers

It is possible to configure Galera to be single-Master. It does not sound like you did that, but suggest double checking.

Given that all nodes are writable, here's a simplified view of what happens on each transaction.

  1. Do all the work to store/update the data on the Master you are connected to. (Presumably, it is the local machine.)
  2. At COMMIT time, make a single round-trip to each other nodes (probably ~200ms) to give them a chance to say "wait! that would cause a conflict".
  3. Usually step 2 will come back with "it's OK". At this point, the COMMIT returns success to the client.

(Note: If you are not using BEGIN...COMMIT, but instead auto_commit=ON, then there is an implicit COMMIT at the end of each DML statement.)

For a local read, the default action should return "immediately".

But, maybe you are concerned about the "critical read" problem. (cf wsrep_sync_wait) In this case, you want to make sure that a write has propagated to your server. This is likely to lead to a 200ms delay on the read because it waits for the "gcache" is caught up.

If you can assume that only read from the same server that they write to, consider setting wsrep_sync_wait=0. If anyone does a cross-datacenter write, then read, he could hit the "critical read" problem. (This is where he writes something, but may not see it on the next read.)

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