Thanks Fajar for quick response! Now I know there exists mutual influence among virtual servers. Oddly, instead of seeing the error message " There are no DB handles to use ", there are bulk of "Discarding duplicate request from NMS - ID xx due to unfinished request xxxx", then after about short period of time, it showed msg "WARNING: Unresponsive child for request xxxx, in component authorized module", can I still say that is due to problematic DB?

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That is, even virtual server ?A? only has few operation to check, while ?B? has much more things need to check and unfortunately maybe it could have low performance DB backend, does B might occupy resource in whole Authorize section so that A also become more and more busier?

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Yes

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In fact, I got many times of Error msg ?child is hung for request in component authorize module? in radius.log. I would not think the loading in our server was really much high enough for causing late response to simulated request from our NMS per **minute**. Our wifi auth request per day is among 2000~3000 in average.

only 2k per day? That's tiny. Something if seriously wrong with your db if it can't handle that much requests.

IIRC one way to isolate problematic db is to use small num_sql_socks in sql.conf (default is 5). If the db is slow, it should bomb out quickly with "There are no DB handles to use" errors, but there should be only a small count of FR threads interacting with that db so that processes not using it (e.g. other virtual server) should still work just fine.

-- Fajar