The option in the chrony configuration file that changes the initial interval of polls to a NTP server in order to speed up the initial synchronization is iburst. The iburst option allows chronyd to send four requests to the server at intervals of 2 seconds or less, instead of the interval specified by the minpoll option, which is usually 64 seconds. This way, chronyd can make the first update of the clock shortly after start1. The iburst option is recommended for all servers, especially if the network connectivity is not reliable1.
The other options are not valid or do not have the same effect as iburst. The quickstart option does not exist in the chrony configuration file. The fast option is used to specify a fast initial correction of the system clock, but it does not change the polling interval1. The fsync option is used to enable or disable synchronization of the system clock to the real-time clock (RTC) every 11 minutes1. The flood option is used to enable a mode of operation where chronyd sends a burst of requests to the server at a high rate, which can be useful for testing or initial synchronization of a very inaccurate clock1.
Reference:
LPI Linux Essentials: 1.4. Using sudo
LPI Linux Administrator: 102.5. Use Debian package management
LPI Linux Engineer: 201.1. Measure and Troubleshoot Resource Usage
LPI Linux Professional Certification Program
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