Re: Update low-level backup documentation to match actual behavior

Hi Michael,

Thanks for reviewing!

On 8/29/17 9:44 PM, Michael Paquier wrote:
> On Tue, Aug 29, 2017 at 10:59 PM, David Steele <david(at)pgmasters(dot)net> wrote:
>>
>> Attached is the 9.6 patch. It required a bit more work in func.sgml
>> than I was expecting so have a close look at that. The rest was mostly
>> removing irrelevant hunks.
>
> + switch to the next WAL segment. On a standby, it is not possible to
> + automatically switch WAL segments, so you may wish to run
> + <function>pg_switch_wal</function> on the primary to perform a manual
> + switch. The reason for the switch is to arrange for
> [...]
> + WAL segments have been archived. If write activity on the primary
> is low, it
> + may be useful to run <function>pg_switch_wal</> on the primary in order to
> + trigger an immediate segment switch of the last required WAL
> It seems to me that both portions are wrong. There is no archiving
> wait on standbys for 9.6, and
I think its clearly stated here that pg_stop_backup() does not wait for
WAL to archive on a standby. Even, it is very important for the backup
routine to make sure that all the WAL *is* archived.

> pg_stop_backup triggers by itself the
> segment switch, so saying that enforcing pg_switch_wal on the primary
> is moot.

pg_stop_backup() does not perform a WAL switch on the standby which is
what this sentence is referring to. I have separated this section out
into a new paragraph to (hopefully) make it clearer.

> pg_switch_xlog has been renamed to pg_switch_wal in PG10, so
> the former name applies.

Whoops!

New patch is attached.

Thanks,
--
-David
david(at)pgmasters(dot)net