http://0rz.tw/714Eo ntpdate from cron -- DON'T DO THAT! To: [email protected] Subject: ntpdate from cron -- DON'T DO THAT! From: "N. Thomas" <[email protected]> Date: Sat, 21 Dec 2002 18:51:24 -0500 Message-id: <[email protected]> In-reply-to: <[email protected]> References: <[email protected]> * Sonny Kupka <[email protected]> [2002-12-21 10:46:12 -0600]: > I have ntpdate installed on woody.. and it's not automagically keeping my > system in sync.. Contrary to what you may have heard, ntpdate does not keep your system clock synced. Also ignore the foolish recommendations to run ntpdate from a cron job. ntpdate works like date(1), but it sets your clock's time to that of an ntp server (or servers) instead of having it specified by you. If you want to keep your clock in sync use ntpd -- that's what it was designed for. It uses many sophisticated algorithms and statistical methods to accomplish this. After some time, it can even figure out how "bad" your system clock is (i.e. its drift) and compensate for it, even if your network connection goes out. Unfortunately, some people, instead of taking the time to read the ntp documentation and writing a proper ntp.conf file, took the easy route and started running ntpdate from cron. This caused two problems, firstly it did not keep very good time: immediately after you called ntpdate, your clock would begin to drift again. And more importantly, every hour or so, the ntp servers were being affected by a "thunderclap" effect, the result of everybody putting: 0 * * * * /usr/local/bin/ntpdate or something similar into their crontab files. The ntp daemon does not do this as it randomizes the time it waits between queries. For this reason, Dr. Mills (ntp author) has deprecated ntpdate, and indeed, he will be removing it completely from a future release. -- 臧霸 吕布手下的猛将 对於台大有极深的怨恨 --

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