In fact, I tried to write the easiest explanation of my problem in order to be understood... My project is developed with Symfony and Doctrine (BERK, i know ...).
The project has more than 2 years and Doctrine makes some bad decisions and lock all the table for a while. We are developing the project without Doctrine but it will not be available within 1 year...
To be more precise : We have a database with more than 400 Gb and ONE table with more than 100 Gb of data. This is huge for doctrine. When the cron runs, it writes a lot on the disks in temporary file (although we have 128 GB of Ram...). Of course, each table is well indexes...
That is why I'm thinking about replication : My server A (master) is for my users... and my server B is a server reserved for calculations (and this server B which writes on the base)