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doc: add FAQ section on minimizing service downtime

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Luke Valenta 2023-11-01 16:40:35 -04:00 committed by Miroslav Lichvar
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doc/faq.adoc
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@ -497,6 +497,58 @@ pidfile /var/run/chronyd-server1.pid
driftfile /var/lib/chrony/drift-server1
----
=== How can `chronyd` be configured to minimise downtime during restarts?
The `dumpdir` directive in _chrony.conf_ provides `chronyd` a location to save
a measurement history of the sources it uses when the service exits. The `-r`
option then enables `chronyd` to load state from the dump files, reducing the
synchronisation time after a restart.
Similarly, the `ntsdumpdir` directive provides a location for `chronyd` to save
NTS cookies received from the server to avoid making a NTS-KE request when
`chronyd` is started. When operating as an NTS server, `chronyd` also saves
cookies keys to this directory to allow clients to continue to use the old keys
after a server restart for a more seamless experience.
On Linux systems,
https://www.freedesktop.org/software/systemd/man/latest/sd_listen_fds.html[systemd
socket activation] provides a mechanism to reuse server sockets across
`chronyd` restarts, so that client requests will be buffered until the service
is again able to handle the requests. This allows for zero-downtime service
restarts, simplified dependency logic at boot, and on-demand service spawning
(for instance, for separated server `chronyd` instances run with the `-x`
flag).
With socket activation, the service manager (systemd) creates sockets and
passes file descriptors to them to the process via the `LISTEN_FDS` environment
variable. Before opening new sockets, `chronyd` first checks for and attempts
to reuse matching sockets passed from the service manager. For instance, if an
IPv4 datagram socket bound on `bindaddress` and `port` is available, it will be
used by the NTP server to accept incoming IPv4 requests.
An example systemd socket unit is below, where `chronyd` is configured with
`bindaddress 0.0.0.0`, `bindaddress ::`, `port 123`, and `ntsport 4460`.
----
[Unit]
Description=chronyd server sockets
[Socket]
Service=chronyd.service
# IPv4 NTP server
ListenDatagram=0.0.0.0:123
# IPv6 NTP server
ListenDatagram=[::]:123
# IPv4 NTS-KE server
ListenStream=0.0.0.0:4460
# IPv6 NTS-KE server
ListenStream=[::]:4460
BindIPv6Only=ipv6-only
[Install]
WantedBy=sockets.target
----
=== Should be a leap smear enabled on NTP server?
With the `smoothtime` and `leapsecmode` directives it is possible to enable a