We are using two MYSQL servers in ring replication where master is also a slave of the other master. Both have slaves connected to each other. We want to run pt-online-schema-change to add couple of columns but we are not sure if it may break the replication due to ring replication in place. Any advice would be great.

  • I got lost on "slaves connected to each other".
    – Rick James
    Mar 16, 2018 at 13:14

1 Answer 1


If you use a dsn table as a recursion method, then pt-online-schema-change can do the job for you. This is documented here:


If you use the hosts method, though, pt-osc will detect that SHOW SLAVE STATUS returns more than 1 row, but it considers the topology is using replication channels. You would get this error:

This server returned more than one row for SHOW SLAVE STATUS but "channel"
was not specified on the command line at bin/pt-online-schema-change line 4494.

For setting up topologies to test different circumstances, dbdeployer is a useful tool https://github.com/datacharmer/dbdeployer#replication-topologies.

Disclosure: I work for Percona

  • 1
    As an aside, overall the use of ring replication isn't thought to be ideal. You may have a specific need (or might have inherited it), I realize. Here is a discussion on that percona.com/blog/2014/10/07/…
    – greenweeds
    Mar 14, 2018 at 19:41
  • Hey, Thanks for your feedback. I agree with you that this tool will not work, perhaps we can call it a bug as there is no channel replication setup, instead its a ring. Though I just joined them 2 days ago and was curious as if this can occur. I am in process of taking ring replication out of the scenario but will take time to convince the client. Thanks for pointing out though! Mar 15, 2018 at 8:04
  • No problem I have added this to our bug reports if you want to follow it jira.percona.com/browse/PT-1505
    – greenweeds
    Mar 15, 2018 at 12:09
  • Hi again, one of our team observed an alternative way of using pt-osc to do what you need, I have updated the answer. Hope this helps.
    – greenweeds
    Mar 15, 2018 at 15:05

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