Re: doc: Mention clock synchronization recommendation for hot_standby_feedback

On Thu, Dec 5, 2024, at 6:43 AM, Jakub Wartak wrote:
> One of our customers ran into a very odd case, where hot standby feedback backend_xmin propagation stopped working due to major (hours/days) clock time shifts on hypervisor-managed VMs. This happens (and is fully reproducible) e.g. in scenarios where standby connects and its own VM is having time from the future (relative to primary) and then that time goes back to "normal". In such situation "sends hot_standby_feedback xmin" timestamp messages are stopped being transferred, e.g.:

Is it worth a WARNING message if there is a "huge" time difference
between the servers? We already have the reply time in the message so
it is a matter of defining the "huge" interval plus a roundtrip. We also
need to avoid spamming the log.

Your patch looks good to me. Should it be converted into a
<note>...</note>? (See synchronous_standby_names [1] for an example.)

[1] https://www.postgresql.org/docs/current/runtime-config-replication.html


--
Euler Taveira
EDB   https://www.enterprisedb.com/