It can be dangerous for us to run "set read_only" on a production server because it can block in close_cached_tables. More details about the pain this caused at a previous job are at:
http://mysqlha.blogspot.com/2008/07/what-exactly-does-flush-tables-with.html
Per the code in set_var.cc:
/*
Perform a 'FLUSH TABLES WITH READ LOCK'.
This is a 3 step process:
- [1] lock_global_read_lock()
- [2] close_cached_tables()
- [3] make_global_read_lock_block_commit()
[1] prevents new connections from obtaining tables locked for write.
[2] waits until all existing connections close their tables.
[3] prevents transactions from being committed.
*/
Can there be a variant that doesn't do #2? My workload doesn't use MyISAM and I don't know if #2 is done because of MyISAM. Calling close_cached_tables seems like a heavy way to force LOCK TABLEs to be unlocked. Any long running queries will cause #2 to block.