[ClusterLabs] Question about MySQL agent behaviour, in particular <resource_name>_mysql_master_IP

[Ken Gaillot]

On 05/08/2017 02:02 AM, Les Green wrote:
> Hi All,
> 
> I've recently had an issue when I've needed to add
> <resource_name>_mysql_master_IP to hosts in a MySQL HA cluster as I
> needed to move a running cluster onto a VPN.
> 
> It seems <resource_name>_REPL_INFO is only set when a resource is
> promoted, so as a test, I:
> * Put the system in maintenance mode
> * Manually removed the <resource_name>_REPL_INFO from the CIB
> * Added the appropriate <resource_name>_mysql_master_IP attributes for
> each node.
> * Manually configured MySQL to be using IP based replication
> * Then took the cluster out of maintenance mode.
> 
> The replication continued ok, but as predicted the
> <resource_name>_REPL_INFO was not populated. When putting the current
> master into standby, the slave was promoted correctly and the
> <resource_name>_REPL_INFO populated correctly.
> 
> HOWEVER, while doing this, I noticed in the code, if the master hostname
> changes, the replication for the slave is restarted at the master
> position, not the slave position. See:
> https://github.com/ClusterLabs/resource-agents/blob/master/heartbeat/mysql#L535
> 
> Why on earth would we ever do this? This seems rife for a disaster of
> lost writes. I feel we should only ever be using the current slave
> position as the start position for a CHANGE MASTER command.
> 
> Can somebody set me straight on this matter? I feel maybe I'm missing
> something significant.
> 
> Cheers, Les

I've never used the mysql agent, but maybe master_log_pos in this case
actually stores the slave position in the master log? If not, you might
want to open an issue at
https://github.com/ClusterLabs/resource-agents/issues

Here's the commit that added the relevant code:

https://github.com/ClusterLabs/resource-agents/commit/0c07e8655031f161bb8b54683d9fec6ec9a1f29b

Interestingly, it says "only 2-node setup is supported". I don't think
that's still true, but I don't know what was involved in the change.