Re: A few user-level questions on Streaming Replication and pg_upgrade

Gurjeet Singh wrote:
> Hi,
>
> Here are a few questions that were asked by a customer, who are trying
> to assess the pros and cons of using Postgres and its SR feature. I would
> like to get an opinion of someone more involved with the community than me.
>
> .) Will Postgres support Streaming Replication from 9.0.x to 9.1.x; i.e.
> across major releases.
>
> I am pretty sure the answer is "no, it won't", but just double-checking
> with the community.

[ CC to general removed --- emailing only hackers; cross-posting is
frowned upon. ]

Right.

> .) Is Streaming Replication supported across minor releases, in reverse
> direction; e.g. 9.0.3 to 9.0.1
>
> I think the answer is "it depends", since it would depend upon whether
> any SR related bug has been fixed in the 'greater' of the minor releases.
>
> I am assuming that smaller minor release to bigger minor release will
> always be supported (e.g. 9.0.1 to 9.0.3)

Yes. We could mention in the minor release notes if we break streaming
replication for a minor release --- or someone will tell us when we do.

> .) How reliable is `pg_upgrade -c` (dry run) currently; that is, how
> accurate is pg_upgrade at predicting any potential problem with the eventual
> in-place upgrade.
>
> I'd say it is as reliable as it gets since this is the official tool
> supported by the project, and it should not contain any known bugs. One has
> to use the latest and greatest 'minor' version of the tool for the major
> release they are upgrading to, though.

Well, we make no guarantees about the software at all, so it is hard to
make any guarantee about pg_upgrade either.

> I'd also like to mention a piece of information that may be surprising
> to some. Per Tom at a PGCon dinner, Postgres project does not promise
> continued guarantee of in-place upgrades across future major releases.
> Although the project will try hard to avoid having to make any changes that
> may affect in-place upgrade capability, but if a case can be made that a
> feature would give a significant improvement at the cost of compromising
> this capability, then the in-place upgrade capability may be forgone for
> that release.

Doesn't surprise me --- I know a time will come when we must change the
data format enough to break pg_upgrade's ability to perform major
upgrades. It is not 'if', but 'when'.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +