Re: Clarification in pg10's pgupgrade.html step 10 (upgrading standby servers)

På tirsdag 12. september 2017 kl. 19:19:22, skrev Bruce Momjian <
bruce(at)momjian(dot)us <mailto:bruce(at)momjian(dot)us>>:
On Thu, Aug  3, 2017 at 11:37:32AM +0200, Michael Paquier wrote:
> On Mon, Jul 31, 2017 at 6:13 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Fri, Jul 28, 2017 at 10:35 AM, Andreas Joseph Krogh
> > <andreas(at)visena(dot)com> wrote:
> >> I'm reading https://www.postgresql.org/docs/10/static/pgupgrade.html to
try
> >> to understand how to upgrade standby-servers using pg_upgrade with pg10.
> >>
> >> The text in step 10 sais:
> >> "You will not be running pg_upgrade on the standby servers, but rather
> >> rsync", which to me sounds like rsync, in step 10-f, should be issued on
the
> >> standy servers. Is this the case? If so I don't understand how the
standby's
> >> data is upgraded and what "remote_dir" is. If rsync is supposed to be
issued
> >> on the primary then I think it should be explicitly mentioned, and step
10-f
> >> should provide a clarer example with more detailed values for the
> >> directory-structures involved.
> >>
> >> I really think section 10 needs improvement as I'm certainly not
comfortable
> >> upgrading standbys following the existing procedure.
> >
> > Yeah, I don't understand it either, and I have never been convinced
> > that there's any safe way to do it other than recloning the standbys
> > from the upgraded master.
>
> Here are my 2c on the matter. 10-f means that the upgraded node may
> have generated WAL with wal_level = minimal, which, at least it seems
> to me, that we have a risk of having inconsistent data pages if only a
> rsync is used on the old standbys. Like Robert, the flow we used in
> the products I work on is to re-create standbys from scratch after the
> upgrade using a fresh backup, with a VM cloning. An upgrade here is an
> in-place process not only linked to Postgres, so standby VMs are made
> of many services, some are being linked to Postgres. So this choice is
> mainly decided by those dependencies, still it feels safer anyway.

I have applied the attached doc patch back to 9.5 to clarify
pg_upgrade's rsync instructions and explain how it works.

Improvements?
 
 
Thanks, that certainly improves things.
But; I still find the rsync-command in f) confusing;
1. Why --size-only? From rsync manual: "skip files that match in size", is
this safe??
2. Why is old_pgdata in the rsync-command, why is it needed to sync it?
 
There are many ways to do/configure things it seems, resulting in many ifs and
buts which makes section 10 rather confusing. I really think a complete
example, with absolute paths, would be clarifying.
 
I'm afraid many will still re-create standbys from scratch without a really
good and complete example to follow.

--
Andreas Joseph Krogh