Re: Further Hot Standby documentation required

On Mon, 2010-05-03 at 12:17 +0300, Heikki Linnakangas wrote:

> >> * wal_level doesn't describe what the impacts are on a standby if the
> >> level is changed on the primary, nor is there a caution or a warning of
> >> any kind. For example, if a standby is setup with hot_standby = on and
> >> the primary is set wal_level = archive, does the standby start working
> >> if the primary changes wal_level = hot_standby? What happens if the
> >> primary is set wal_level = hot_standby and is then changed to archive?
>
> Hmm, feels like it should rather be documented in the hot_standby
> setting, it affects the standby not the master after all.

Danger of action at a distance. The change is on the master, but the
effect is on the standby. The person changing the master must be warned
of the danger that they will bring down the standby, so it must go with
the parameter, not only with the HS docs.

Doesn't this new behaviour prevent startup-from-a-shutdown-checkpoint?
Whether it does or not, we need it to be documented.

> >> * wal_level doesn't explicitly describe that the levels are in sequence
> >> and that hot_standby is a superset of archive.
>
> It does say:
> "The default value is <literal>minimal</>, which writes only the
> information needed to recover from a crash or immediate shutdown.
> <literal>archive</> adds logging required for WAL archiving, and
> <literal>hot_standby</> further adds information required to run
> read-only queries on a standby server."
>
> Want to propose a better wording?

"so at least archive level"

changed to

"so a setting of either archive or hot_standby"

That way there is no concept of an ordering required to understand the
sentence because it is explicit.

--
Simon Riggs www.2ndQuadrant.com