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troubleshooting section about incorrect timestamps

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Scott Lamb 2021-07-13 11:18:17 -07:00
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guide/troubleshooting.md
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@ -16,6 +16,7 @@ need more help.
* [`Error: pts not monotonically increasing; got 26615520 then 26539470`](#error-pts-not-monotonically-increasing-got-26615520-then-26539470)
* [Out of disk space](#out-of-disk-space)
* [Database or filesystem corruption errors](#database-or-filesystem-corruption-errors)
* [Incorrect timestamps](#incorrect-timestamps)
* [Configuration interface problems](#configuration-interface-problems)
* [`moonfire-nvr config` displays garbage](#moonfire-nvr-config-displays-garbage)
* [Browser user interface problems](#browser-user-interface-problems)
@ -320,6 +321,47 @@ After the system as a whole is verified healthy, run `moonfire-nvr check` while
Moonfire NVR is stopped to verify integrity of the SQLite database and sample
file directories.
#### Incorrect timestamps
Moonfire NVR uses the system clock when a run of recordings starts to determine
the run's initial timestamp. If the system clock is stepped after the run
starts, Moonfire NVR will keep using timestamps based on the old (usually
incorrect) setting.
This is most noticeable on the Raspberry Pi or other cheap SBCs which don't
come with a battery-backed real-time clock (RTC). Instead, they save the
current time periodically and restore it on bootup. Their clocks often are a
few hours behind on startup following a power outage. You may notice in
`journalctl` logs messages similar to the following when the clock is fixed:
```
Aug 14 21:05:51 moonfire moonfire-nvr[710]: Aug 14 21:05:51.538 INFO reserved 590d892d-b2e8-4e6c-9e1b-c4418d0abd69
Aug 14 22:37:39 moonfire systemd[1]: Time has been changed
Aug 14 22:38:48 moonfire moonfire-nvr[710]: Aug 14 22:38:48.965 INFO Committing extra transaction because there's no cached uuid
```
Note the 1.5-hour gap between messages; this is roughly how much the clock was
adjusted.
The exact message may differ based on your Linux distribution and message;
here's another variation:
```
Jul 13 10:05:52 pi4 systemd-timesyncd[340]: Synchronized to time server for the first time [2600:3c00::e:d0bb]:123 (2.debian.pool.ntp.org).
```
Here's what you can do:
* *recover*: restart Moonfire NVR to pick up the new timestamp.
* *prevent*: add a RTC module or fresh battery so your clock is correct
at boot time. There's a
[guide](https://github.com/scottlamb/moonfire-nvr/wiki/System-setup#realtime-clock-on-raspberry-pi)
on the wiki.
Currently Moonfire NVR doesn't have any logic to detect this happening or
mechanism to fix old timestamps after the fact. Ideas and help welcome; see
[issue #9](https://github.com/scottlamb/moonfire-nvr/issues/9).
### Configuration interface problems
#### `moonfire-nvr config` displays garbage