Re: pg_upgrade with -j shows no perf improvement

On Mon, Jun 17, 2013 at 12:18 PM, Lonni J Friedman <netllama(at)gmail(dot)com>wrote:

> On Sat, Jun 15, 2013 at 8:03 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > On Fri, Jun 14, 2013 at 02:29:24PM -0700, Lonni J Friedman wrote:
> >> Greetings,
> >> I'm in the early stages of preparing to upgrade a production 9.2
> >> cluster to 9.3, by testing the beta of 9.3. All of my testing is
> >> happening on RHEL6-x86_64 on a dedicated server with 128GB RAM and 2x
> >> Intel Xeon E5-2670 CPUs, with all of $PGDATA residing on an 8 disk
> >> RAID10 array.
> >>
> >> Currently, a full pg_basebackup of my data is approaching 800GB in
> >> size (uncompressed), so this isn't a tiny, trivial database.
> >>
> >> I was curious about how much of a performance gain I'd get from
> >> upgrading with the new -j option to pg_upgrade, so first I performed
> >> the upgrade without it to get a baseline. The command I ran for the
> >> upgrade is as follows:
> >> time pg_upgrade -v -d /var/lib/pgsql/9.2/data -D
> >> /var/lib/pgsql/9.3/data -b /usr/pgsql-9.2/bin -B /usr/pgsql-9.3/bin
> >>
> >> time reported the following afterward the upgrade had completed
> successfully:
> >> real 24m59.255s
> >> user 0m17.069s
> >> sys 15m25.153s
> >>
> >>
> >> I then repeated the upgrade (after blowing away $PGDATA, and running
> >> initdb again for 9.3), and re-ran pg_upgrade with the same command as
> >> above, only with '-j4' appended to the end. Surprisingly, the
> >> completion time was less than 30 seconds faster. I repeated a third
> >> time with '-j8', and that was about the same completion time as with
> >> '-j4'. I guess I could repeat with 'j2', but I'd be surprised if it
> >> was dramatically faster when -j4 was only marginally so. It seems
> >> like the parallelism of the -j option doesn't seem to be helping much
> >> at all, in my case.
> >>
> >> Is this expected, or is it possible that there's a bug somewhere? Let
> >> me know if I can provide any logs from the upgrade.
> >
> > The documentation states:
> >
> > The <option>--jobs</> option allows multiple CPU cores to be used
> > for copying/linking of files and to dump and reload database schemas
> > in parallel; a good place to start is the maximum of the number of
> > CPU cores and tablespaces. This option can dramatically reduce the
> > time to upgrade a multi-database server running on a multiprocessor
> > machine.
> >
> > My guess is that you didn't have many tablespaces or databases, or the
> > copy time overwhelmed the performance improvement of the parallelism.
> > I am not surprised you didn't see a big win. Can you test --link
> > mode?
>
> I only have 1 tablespace, although I have 9 databases. However, one
> of the databases is about 95% of the total on-disk space, so that's
> probably the explanation of why -j isn't helping me?
>
>
That is probably why you DID had some improvement, although so little.

> I don't have sufficient disk space to efficiently test --link mode,
> unless there's some way to quickly roll back to the pre-upgrade
> version of the database after a --link mode upgrade has completed
> successfully that I'm not seeing?
>
>
I didn't got it. AFAIK, in link mode it would take **less** space than
normal mode, not the opposite. Am I wrong?

Unless you want to keep a backup of the old cluster on the same machine,
but even on that case you could take this backup compressed, although the
overall time would be worst.

Regards,
--
Matheus de Oliveira
Analista de Banco de Dados
Dextra Sistemas - MPS.Br nível F!
www.dextra.com.br/postgres