Re: pg_restore scan

I've been given a database dump file daily and I've been asked to restore
it.
I tried everything I could to speed up the process, including using -j 40.

I discovered that at the later stage of the restore process, the
following behaviour repeated a few times :
40 x pg_restore process doing 100% CPU
40 x postgres process doing COPY but using 0% CPU
..... and zero disk write activity

I don't see this behaviour when restoring the database that was dumped with
-Fd.
Also with an un-piped backup file, I can restore a specific table without
having to wait for hours.

--

On Fri, 19 Sept 2025 at 01:54, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 9/18/25 05:58, R Wahyudi wrote:
> > Hi All,
> >
> > Thanks for the quick and accurate response! I never been so happy
> > seeing IOwait on my system!
>
> Because?
>
> What did you find?
>
> >
> > I might be blind as I can't find information about 'offset' in pg_dump
> > documentation.
> > Where can I find more info about this?
>
> It is not in the user documentation.
>
> From the thread Ron referred to, there is an explanation here:
>
> https://www.postgresql.org/message-id/366773.1756749256%40sss.pgh.pa.us
>
> I believe the actual code, for the -Fc format, is in pg_backup_custom.c
> here:
>
>
> https://github.com/postgres/postgres/blob/master/src/bin/pg_dump/pg_backup_custom.c#L723
>
> Per comment at line 755:
>
> "
> If possible, re-write the TOC in order to update the data offset
> information. This is not essential, as pg_restore can cope in most
> cases without it; but it can make pg_restore significantly faster
> in some situations (especially parallel restore). We can skip this
> step if we're not dumping any data; there are no offsets to update
> in that case.
> "
>
> >
> > Regards,
> > Rianto
> >
> > On Wed, 17 Sept 2025 at 13:48, Ron Johnson <ronljohnsonjr(at)gmail(dot)com
> > <mailto:ronljohnsonjr(at)gmail(dot)com>> wrote:
> >
> >
> > PG 17 has integrated zstd compression, while --format=directory lets
> > you do multi-threaded dumps. That's much faster than a single-
> > threaded pg_dump into a multi-threaded compression program.
> >
> > (If for _Reasons_ you require a single-file backup, then tar the
> > directory of compressed files using the --remove-files option.)
> >
> > On Tue, Sep 16, 2025 at 10:50 PM R Wahyudi <rwahyudi(at)gmail(dot)com
> > <mailto:rwahyudi(at)gmail(dot)com>> wrote:
> >
> > Sorry for not including the full command - yes , its piping to a
> > compression command :
> > | lbzip2 -n <threadsforbzipgoeshere>--best > <filenamegoeshere>
> >
> >
> > I think we found the issue! I'll do further testing and see how
> > it goes !
> >
> >
> >
> >
> >
> > On Wed, 17 Sept 2025 at 11:02, Ron Johnson
> > <ronljohnsonjr(at)gmail(dot)com <mailto:ronljohnsonjr(at)gmail(dot)com>>
> wrote:
> >
> > So, piping or redirecting to a file? If so, then that's the
> > problem.
> >
> > pg_dump directly to a file puts file offsets in the TOC.
> >
> > This how I do custom dumps:
> > cd $BackupDir
> > pg_dump -Fc --compress=zstd:long -v -d${db} -f ${db}.dump
> > 2> ${db}.log
> >
> > On Tue, Sep 16, 2025 at 8:54 PM R Wahyudi
> > <rwahyudi(at)gmail(dot)com <mailto:rwahyudi(at)gmail(dot)com>> wrote:
> >
> > pg_dump was done using the following command :
> > pg_dump -Fc -Z 0 -h <host> -U <user> -w -d <database>
> >
> > On Wed, 17 Sept 2025 at 08:36, Adrian Klaver
> > <adrian(dot)klaver(at)aklaver(dot)com
> > <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:
> >
> > On 9/16/25 15:25, R Wahyudi wrote:
> > >
> > > I'm trying to troubleshoot the slowness issue
> > with pg_restore and
> > > stumbled across a recent post about pg_restore
> > scanning the whole file :
> > >
> > > > "scanning happens in a very inefficient way,
> > with many seek calls and
> > > small block reads. Try strace to see them. This
> > initial phase can take
> > > hours in a huge dump file, before even starting
> > any actual restoration."
> > > see : https://www.postgresql.org/message-id/
> > E48B611D-7D61-4575-A820- <https://
> >
> www.postgresql.org/message-id/E48B611D-7D61-4575-A820->
> > > B2C3EC2E0551%40gmx.net <http://40gmx.net>
> > <https://www.postgresql.org/message-id/ <https://
> > www.postgresql.org/message-id/>
> > > E48B611D-7D61-4575-A820-B2C3EC2E0551%40gmx.net
> > <http://40gmx.net>>
> >
> > This was for pg_dump output that was streamed to a
> > Borg archive and as
> > result had no object offsets in the TOC.
> >
> > How are you doing your pg_dump?
> >
> >
> >
> > --
> > Adrian Klaver
> > adrian(dot)klaver(at)aklaver(dot)com
> > <mailto:adrian(dot)klaver(at)aklaver(dot)com>
> >
> >
> >
> > --
> > Death to <Redacted>, and butter sauce.
> > Don't boil me, I'm still alive.
> > <Redacted> lobster!
> >
> >
> >
> > --
> > Death to <Redacted>, and butter sauce.
> > Don't boil me, I'm still alive.
> > <Redacted> lobster!
> >
>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>