Hi!
"MARK" == MARK CALLAGHAN <mdcallag@gmail.com> writes:
MARK> It can be dangerous for us to run "set read_only" on a production server MARK> because it can block in close_cached_tables. More details about the pain MARK> this caused at a previous job are at: MARK> http://mysqlha.blogspot.com/2008/07/what-exactly-does-flush-tables-with.html MARK> Per the code in set_var.cc: MARK> /* MARK> Perform a 'FLUSH TABLES WITH READ LOCK'. MARK> This is a 3 step process: MARK> - [1] lock_global_read_lock() MARK> - [2] close_cached_tables() MARK> - [3] make_global_read_lock_block_commit() MARK> [1] prevents new connections from obtaining tables locked for write. MARK> [2] waits until all existing connections close their tables. MARK> [3] prevents transactions from being committed. MARK> */ MARK> Can there be a variant that doesn't do #2? My workload doesn't use MyISAM MARK> and I don't know if #2 is done because of MyISAM. Calling MARK> close_cached_tables seems like a heavy way to force LOCK TABLEs to be MARK> unlocked. Any long running queries will cause #2 to block. The reason for 2 is to ensure that that all table info is written to disk so that if you do a snapshot or copy of tables, you will get things in a consistent state. This is mostly for MyISAM and non transactional tables, but it will also speed up things for InnoDB tables and allow you to copy xtradb tables from one server to another (if you are using table spaces) without having to take down the server. It's possible to do a 'FLUSH TABLES FAST WITH READ LOCK' version that would only flush the header of MyISAM tables, which would probably help you, as long as you don't plan to copy any tables to any other server. Regards, Monty