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We found this severely annoying and now use My SQL's event scheduler to update a timer table on each master every second, so we can actually see actual delay from the global master (in a non-ring topology) or delay from any peer in a ring.I belive the issue is not related to long running queries.That is related to a known bug, fixed in My SQL 5.5 :id=38551In short, in all My SQL versions before 5.5 , the query cache mutex is acquired even if query_cache_size=0 and query_cache_type=OFF : always! It often happens to see and hear of replication SQL thread almost always in state “invalidating query cache entries (table)” .While sometime this works, others don’t; the query cache is “disabled” and you still see the SQL thread in state “invalidating query cache entries (table)”.Replication delay for server2 starts growing, replication delay for server3 stays zero : server2 is done with query one, starts processing query two. Server3 will be done with query 3, replication delay jumps back to zero, and then back up to 10 as it processes the next query.So, the jumpy behaviour is caused by not using a global timestamp for replication delay, but simply the delay behind the last "hop" in the replication chain.
The classic tip is “set global query_cache_size=0” .
XX Waiting for query cache lock INSERT INTO `db B`.`table B` (...) VALUES (...) ON DUPLICATE KEY UPDATE ... Simultaneous SELECT and INSERT in one database seems to cause a dead lock with query cache update by INSERT ON DUPLICATE KEY UPDATE in a different database.
If I turn off either the Replication or the Query Cache then the lock up does not happen.
In basic terms the processing steps involved are compression of time series data in per second resolution into per minute, hour and day resolutions. It is the Query #3 that eventually produces the error "", presumably because it is the first one to timeout.
When My SQL locks up, I execute SHOW PROCESSLIST command and I see the following queries: N User Time Status SQL query 1 system user XX update INSERT INTO `db A`.`table A` (...) VALUES (...) 2 ???? It looks like some sort of dead lock, but I cannot understand why.