Re: use CREATE DATABASE STRATEGY = FILE_COPY in pg_upgrade

On Fri, Jun 07, 2024 at 11:10:25AM +0200, Matthias van de Meent wrote:
> My primary concern isn't the IO, but the O(shared_buffers) that we
> have to go through during a checkpoint. As I mentioned upthread, it is
> reasonably possible the new cluster is already setup with a good
> fraction of the old system's shared_buffers configured. Every
> checkpoint has to scan all those buffers, which IMV can get (much)
> more expensive than the IO overhead caused by the WAL_LOG strategy. It
> may be a baseless fear as I haven't done the performance benchmarks
> for this, but I wouldn't be surprised if shared_buffers=8GB would
> measurably impact the upgrade performance in the current patch (vs the
> default 128MB).

I did a handful of benchmarks on an r5.24xlarge that seem to prove your
point.  The following are the durations of the pg_restore step of
pg_upgrade:

* 10k empty databases, 128MB shared_buffers
  WAL_LOG:   1m 01s
  FILE_COPY: 0m 22s

* 10k empty databases, 100GB shared_buffers
  WAL_LOG:   2m 03s
  FILE_COPY: 5m 08s

* 2.5k databases with 10k tables each, 128MB shared_buffers
  WAL_LOG:   17m 20s
  FILE_COPY: 16m 44s

* 2.5k databases with 10k tables each, 100GB shared_buffers
  WAL_LOG:   16m 39s
  FILE_COPY: 15m 21s

I was surprised with the last result, but there's enough other stuff
happening during such a test that I hesitate to conclude much.

> I'll note that the documentation for upgrading with pg_upgrade has the
> step for updating postgresql.conf / postgresql.auto.conf only after
> pg_upgrade has run already, but that may not be how it's actually
> used: after all, we don't have full control in this process, the user
> is the one who provides the new cluster with initdb.

Good point.  I think it's clear that FILE_COPY is not necessarily a win in
all cases for pg_upgrade.

>>> If such a change were implemented (i.e. no checkpoints for FILE_COPY
>>> in binary upgrade, with a single manual checkpoint after restoring
>>> template1 in create_new_objects) I think most of my concerns with this
>>> patch would be alleviated.
>>
>> Yeah, I think that's a valid point. The second checkpoint is to ensure
>> that the XLOG_DBASE_CREATE_FILE_COPY never gets replayed. However, for
>> binary upgrades, we don't need that guarantee because a checkpoint
>> will be performed during shutdown at the end of the upgrade anyway.
> 
> Indeed.

It looks like pg_dump always uses template0, so AFAICT we don't even need
the suggested manual checkpoint after restoring template1.

I do like the idea of skipping a bunch of unnecessary operations in binary
upgrade mode, since it'll help me in my goal of speeding up pg_upgrade.
But I'm a bit hesitant to get too fancy here and to introduce a bunch of
new "if (IsBinaryUpgrade)" checks if the gains in the field won't be all
that exciting.  However, we've already sprinkled such checks quite
liberally, so maybe I'm being too cautious...

-- 
nathan