What is the need to do the database master-slave backup, how to resolve

What happens when you need to do a database master-slave backup
I only know can be used to do hot backup.
What is the difference between MySQL read-write separation and MySQL master-slave backup?

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Although our project uses the master-slave synchronization, but did not hear about the backup ... DBA thing ...
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Of course, if more than one slave server, and the read operation is greater than the write operation, you can imagine the ability to increase the load.
If more than one server can perform a custom allocation algorithm, or MySQL will handle the default

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