We currently have a push log and a pull log, but do not separately log intracluster synchronization events. We've encountered several specific cases where having this kind of log would be helpful:
- In PHI943, an install was accidentally aborting locks early. Having timing information in the sync log would let us identify this more quickly.
- In PHI889, an install hit an issue with MaxStartups configuration in sshd. A log would let us identify when this is an issue.
- In PHI889, I floated a "push the linux kernel + fetch timeout" theory. A sync log would let us see sync/fetch timeouts to confirm this being a problem in practice.
- A sync log will help us assess, develop, test, and monitor intracluster routing sync changes (likely those in T13211) in the future.
Some of these events are present in the pull log already, but only if they make it as far as running a git upload-pack subprocess (not the case with MaxStartups problems) -- and they can't record end-to-end timing.
No UI yet, I'll add that in a future change.