Replicated MySQL write performance query Whilst I've worked with MySQL for years, I've never had to created a replicated database, and after researching the topic I have a query about performance.

I'm thinking of an architecture based on 5 servers, one of them designated as the master and will handle ALL write transactions, whilst the other 4 will handle read requests.

My question is, if the master server is at 90% CPU usage handling writes, are the other servers in the cluster also under the same strain due to each write being sent to all other servers? or is something going on where the bulk of the processing is handles on the master so the slaves do less work processing the write transaction?
This depends on the query and the binary log format.

An UPDATE query that fetches lots of rows and changes very few will in binlog_format=ROW end up as UPDATE table SET x=y WHERE PK=z so will have a lower CPU/IO overhead. If it was using binlog_format=STATEMENT exactly the same query would end up on the slave.

Conversely a UPDATE query that updates a large number of rows, UPDATE table SET status=Fixed WHERE date &lt; NOW() will generate a large number on binary log entries (for each primary key) and might be slower in binlog_format=ROW than statement.

There's a good document on this in the mysql manual

Based on your galera tag, and the known limitations, you'll be using binlog_format=ROW and each table will have a primary key. Take note regarding transaction size so to avoid stalls due to large updates.