From what I can tell from just looking at the code , the locking isn't at the database layer per say, it's at the MySQL connection per zone In the zonedb.cpp. there is only one connection per zone, at least from what I see.
Fsync is a delay or latency issue on the DB when dealing with transactions. Every single query in save is a transaction. (All 13+of them.) You can have a system that can do 500 transactions per sec but can do 100,000 inserts per sec if you bulk up statements.
A small latency can have a massive impact when locks are involved.
If there is latency at the DB it queues up on the zone depending on how many are in each zone. Less people in the zone the less this impacts them
Lowering the latency by limiting the fsync on the transaction call can ease the pressure on the lock on the connection which prevents stalling of the character saved. Or that is at least the idea.
**Note** this lock prevents all queries in the zone , not jsut during saves.
Also note the removal of the table scanning of the pet tables (adding indexes) helps to lower latency of the call as well
It can be easy to confuse work with latency/locks. You can have a slow system doing no work.
Honestly I would like to do more work on this and know it's a stop gap but figured doing 13x less transaction s per save was a win when someone in this thread noted commenting how some of the saves improved their latency. (Mine improved 2-3x)
I know it can be many issues and I may be barking up the wrong tree , but this is simply another option that does have a very clear improvement in performance around the zone locks.
Side note, A single mysql connection for a process is generally a less than idea situation. It is too much of a blockage area for network IO. Locks should be kept for nano/microseconds, not milliseconds. Possibly make seperate connections for read/writes depending on how the threading is setup on the zone process.. (note I have not really looked at the threading model of the zone yet, so this may be moot and may be my misunderstanding)
Note on a phone so sorry for formatting/bad Grammer. Very small window
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