[roncesvalles]

You need replication for HA. Otherwise the only HA you have is the RAID setup on the one machine. The reason people go horizontal is rarely scale. And if you're doing that you might as well use the passive replica as a read replica.

[brightball]

Read replication is easy.

Horizontal scaling for DB typically means distributing the write load across instances either via sharding or master-master. I could be wrong of course, but that’s how I read it.

MySQL is very good at master-master out of the box.

[roncesvalles]

To be pedantic, master-master implies the same row can be committed to at the same time at different master nodes (without an immediate causal connection between the two). Very few databases allow this.