In my development Zabbix 2.2.1 environment, I've noticed some strange behavior from the history syncers. I always have one history syncer that is stuck being busy. Doing a 'ps -eaf | grep zabbix_server | grep "history syncer"' shows that it is always busy syncing history:

zabbix 3383 3302 94 09:08 ? 00:41:46 /usr/local/zabbix/sbin/zabbix_server: history syncer #2 [synced 29 items in 0.005907 sec, syncing history]

I've attached the ltrace and strace for that specific PID. I have a log file as well with debug logging enabled, but I prefer not to paste it here since it has hostnames and such in it. Please let me know somewhere non-public that I can send that if you want it. It does show many different things that the particular process was doing.

It does appear that the stuck process can cycle around. Previously it was history syncer #2 that had issues. Just now I can see that history syncer #4 has the problem.