Backing Up a Master or Slave by Making It Read Only
It is possible to back up either master or slave servers in a replication setup by acquiring a global read
lock and manipulating the read_only  system variable to change the read-only state of the
server to be backed up:
1. Make the server read-only, so that it processes only retrievals and blocks updates.
2. Perform the backup.
The following instructions describe how to do this for a master server and for a slave server. For both
scenarios discussed here, suppose that you have the following replication setup:
• A master server M1
• A slave server S1 that has M1 as its master
• A client C1 connected to M1
• A client C2 connected to S1
In either scenario, the statements to acquire the global read lock and manipulate the read_only 
variable are performed on the server to be backed up and do not propagate to any slaves of that
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