When Canary#writeSniffing is enabled, Canary#checkWriteTableDistribution will make sure the regions of write table distributed on all region servers as:
The master will also work as a regionserver, so that ClusterStatus#getServers will contain the master. On the other hand, write table of Canary will not be assigned to master, making numberOfCoveredServers always smaller than numberOfServers and admin.balancer always be invoked in each sniffing period. This may cause frequent region moves. A simple fix is excluding master from numberOfServers.