Re: pg_restore scan

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