Centos 7

I guess this is the expected behavior ( https://mariadb.com/kb/en/set/#global-session ),
to change binlog_format dynamically: SET SESSION binlog_format='ROW';

As of gh-ost, I am no expert, but this looks like a workaround:
https://github.com/github/gh-ost/blob/master/doc/migrating-with-sbr.md - having a replica, enable log-slave-updates there + row format

Thanks for clarifying Alice much appreciated

We have used gh-ost in that manner and it is a viable workaround but is incredibly wasteful (both in terms of storage and additional I/O) as you have to enable binary logging on the slave also. It works very well otherwise and if you do switch to RBR on the slave, temporarily, to perform your gh-ost operation this works immediately as the connections for replication work are very short lived (so the change is pretty immediate)

I will get onto github about this as I think they should make this a warning in their documentation - it can (and in our case did) result in dataloss if you aren't aware of it

I'm happy for this to be closed out now

Thanks again

I'll close it as the described behavior is not a bug, but an intentionally implemented and documented behavior.

It is possible to consider some new feature that will change binlog format for all connections at once. But 1) it'll be a new feature. 2) it must not break existing applications, so should be backward compatible. 3) At the moment I do not see what the syntax should look like and what the implementation could be. Feel free to create a new MDEV with Type=Task for this new feature, if you'd want to start a discussion about how it could be done what the syntax could look like.