Re: backup manifests - Mailing list pgsql-hackers

On Thu, Apr 2, 2020 at 2:55 PM Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Apr 2, 2020 at 2:23 PM Andres Freund <andres@anarazel.de> wrote:
> > > That might make the window fairly wide on normal systems, but I'm not
> > > sure about Raspberry Pi BF members or things running
> > > CLOBBER_CACHE_ALWAYS/RECURSIVELY. I guess I could try it.
> >
> > You can set checkpoint_timeout to be a day. If that's not enough, well,
> > then I think we have other problems.
>
> I'm not sure that's the only issue here, but I'll try it.

I ran into a few problems here. In trying to set this up manually, I
always began with the following steps:

====
# (1) create cluster
initdb

# (2) add to configuration file
log_checkpoints=on
checkpoint_timeout=1d
checkpoint_completion_target=0.99

# (3) fire it up
postgres
createdb
====

If at this point I do "pg_basebackup -D pgslave -R -c spread", it
completes within a few seconds anyway, because there's basically
nothing dirty, and no matter how slowly you write out no data, it's
still pretty quick. If I run "pgbench -i" first, and then
"pg_basebackup -D pgslave -R -c spread", it hangs, apparently
essentially forever, because now the checkpoint has something to do,
and it does it super-slowly, and "psql -c checkpoint" makes it finish
immediately. However, this experiment isn't testing quite the right
thing, because what I actually need is a slow backup off of a
cascading standby, so that I have time to promote the parent standby
before the backup completes. I tried continuing like this:

====
# (4) set up standby
pg_basebackup -D pgslave -R
postgres -D pgslave -c port=5433

# (5) set up cascading standby
pg_basebackup -D pgslave2 -d port=5433 -R
postgres -c port=5434 -D pgslave2

# (6) dirty some pages on the master
pgbench -i

# (7) start a backup of the cascading standby
pg_basebackup -D pgslave3 -d port=5434 -R -c spread
====

However, the pg_basebackup in the last step completes after only a few
seconds. If it were hanging, then I could continue with "pg_ctl
promote -D pgslave" and that might give me what I need, but that's not
what happens.

I suspect I'm not doing quite what you had in mind here... thoughts?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company