To block all write access and to mark all tables as ‘properly closed’ on disk, we execute the FLUSH TABLES WITH READ LOCK. Note that the tables can still be used for read operations. Open tables will be closed and all tables will be locked for all databases with a global lock.
FLUSH TABLES and RELOAD privilege are required to execute this operation.
To release the lock, you may use UNLOCK TABLES, which implicitly commits any active transaction only if any tables currently have been locked with the LOCK TABLES. If the UNLOCK TABLES is executed after FLUSH TABLES WITH READ LOCK, then the commit will not occur because FLUSH TABLES WITH READ LOCK does not acquire table locks.
Cheers!
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