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From: Hannu Krosing <hannuk@google.com>
Date: Thu, 13 Nov 2025 21:26:46 +0100
Message-ID: 
 <CAMT0RQSNHFffbCmDNxQogVBD8H5gTDJNwhUR2btCVE+Lq1sGGw@mail.gmail.com>
Subject: Re: Patch: dumping tables data in multiple chunks in pg_dump
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Nathan Bossart <nathandbossart@gmail.com>
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Precedence: bulk

The reason for small chunk sizes is that they are determined by main
heap table, and that was just over 1GB

largetoastdb=3D> SELECT format('%I.%I', t.schemaname, t.relname) as table_n=
ame,
       pg_table_size(t.relid) AS table_size,
       sum(pg_relation_size(i.indexrelid)) AS total_index_size,
       pg_relation_size(t.relid) AS main_table_size,
       pg_relation_size(c.reltoastrelid) AS toast_table_size,
       pg_relation_size(oi.indexrelid) AS toast_index_size,
       t.n_live_tup AS row_count,
       count(*) AS index_count,
       array_to_json(array_agg(json_build_object(i.indexrelid::regclass,
pg_relation_size(i.indexrelid))), true) AS index_info
  FROM pg_stat_user_tables t
  JOIN pg_stat_user_indexes i ON i.relid =3D t.relid
  JOIN pg_class c ON c.oid =3D t.relid
  LEFT JOIN pg_stat_sys_indexes AS oi ON oi.relid =3D c.reltoastrelid
 GROUP BY 1, 2, 4, 5, 6, 7
 ORDER BY 2 DESC, 7 DESC
 LIMIT 25;
=E2=94=8C=E2=94=80[ RECORD 1 ]=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=AC=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=90
=E2=94=82 table_name       =E2=94=82 public.just_toasted                 =
=E2=94=82
=E2=94=82 table_size       =E2=94=82 56718835712                         =
=E2=94=82
=E2=94=82 total_index_size =E2=94=82 230064128                           =
=E2=94=82
=E2=94=82 main_table_size  =E2=94=82 1191559168                          =
=E2=94=82
=E2=94=82 toast_table_size =E2=94=82 54613336064                         =
=E2=94=82
=E2=94=82 toast_index_size =E2=94=82 898465792                           =
=E2=94=82
=E2=94=82 row_count        =E2=94=82 5625234                             =
=E2=94=82
=E2=94=82 index_count      =E2=94=82 1                                   =
=E2=94=82
=E2=94=82 index_info       =E2=94=82 [{"just_toasted_pkey" : 230064128}] =
=E2=94=82
=E2=94=94=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80=E2=94=80=E2=94=B4=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=98

On Thu, Nov 13, 2025 at 9:24=E2=80=AFPM Hannu Krosing <hannuk@google.com> w=
rote:
>
> Ran another test with a 53GB database where most of the data is in TOAST
>
> CREATE TABLE just_toasted(
>   id serial primary key,
>   toasted1 char(2200) STORAGE EXTERNAL,
>   toasted2 char(2200) STORAGE EXTERNAL,
>   toasted3 char(2200) STORAGE EXTERNAL,
>   toasted4 char(2200) STORAGE EXTERNAL
> );
>
> and the toast fields were added in somewhat randomised order.
>
> Here the results are as follows
>
> Parallelism   |   chunk size (pages)   |    time (sec)
>  1        |    -         |     240
>  2        |  1000    |     129
>  4        |  1000    |      64
>  8        |  1000    |      36
> 16       |  1000    |      30
>
>  4        |  9095    |      78
>  8        |  9095    |      42
> 16       |  9095    |      42
>
> The reason larger chunk sizes performed worse was that they often had
> one or two stragglers left behind which
>
> Detailed run results below:
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres -f
> /tmp/ltoastdb-1-plain.dump largetoastdb
> real    3m59.465s
> user    3m43.304s
> sys     0m15.844s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D9095 -j 4 -f /tmp/ltoastdb-4.dump
> largetoastdb
> real    1m18.320s
> user    3m49.236s
> sys     0m19.422s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D9095 -j 8 -f /tmp/ltoastdb-8.dump
> largetoastdb
> real    0m42.028s
> user    3m55.299s
> sys     0m24.657s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D9095 -j 16 -f /tmp/ltoastdb-16.dump
> largetoastdb
> real    0m42.575s
> user    4m11.011s
> sys     0m26.110s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D1000 -j 16 -f /tmp/ltoastdb-16-1kpages.dump
> largetoastdb
> real    0m29.641s
> user    6m16.321s
> sys     0m49.345s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D1000 -j 8 -f /tmp/ltoastdb-8-1kpages.dump
> largetoastdb
> real    0m35.685s
> user    3m58.528s
> sys     0m26.729s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D1000 -j 4 -f /tmp/ltoastdb-4-1kpages.dump
> largetoastdb
> real    1m3.737s
> user    3m50.251s
> sys     0m18.507s
>
> hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> --format=3Ddirectory -h 10.58.80.2 -U postgres
> --huge-table-chunk-pages=3D1000 -j 2 -f /tmp/ltoastdb-2-1kpages.dump
> largetoastdb
> real    2m8.708s
> user    3m57.018s
> sys     0m18.499s
>
> On Thu, Nov 13, 2025 at 7:39=E2=80=AFPM Hannu Krosing <hannuk@google.com>=
 wrote:
> >
> > Going up to 16 workers did not improve performance , but this is
> > expected, as the disk behind the database can only do 4TB/hour of
> > reads, which is now the bottleneck. (408/352/*3600 =3D 4172 GB/h)
> >
> > $ time ./pg_dump --format=3Ddirectory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=3D131072 -j 16 -f /tmp/parallel16.dump largedb
> > real    5m44.900s
> > user    53m50.491s
> > sys     5m47.602s
> >
> > And 4 workers showed near-linear speedup from single worker
> >
> > hannuk@pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=3Ddirectory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=3D131072 -j 4 -f /tmp/parallel4.dump largedb
> > real    10m32.074s
> > user    38m54.436s
> > sys     2m58.216s
> >
> > The database runs on a 64vCPU VM with 128GB RAM, so most of the table
> > will be read in from the disk
> >
> >
> >
> >
> >
> >
> > On Thu, Nov 13, 2025 at 7:02=E2=80=AFPM Hannu Krosing <hannuk@google.co=
m> wrote:
> > >
> > > I just ran a test by generating a 408GB table and then dumping it bot=
h ways
> > >
> > > $ time pg_dump --format=3Ddirectory -h 10.58.80.2 -U postgres -f
> > > /tmp/plain.dump largedb
> > >
> > > real    39m54.968s
> > > user    37m21.557s
> > > sys     2m32.422s
> > >
> > > $ time ./pg_dump --format=3Ddirectory -h 10.58.80.2 -U postgres
> > > --huge-table-chunk-pages=3D131072 -j 8 -f /tmp/parallel8.dump largedb
> > >
> > > real    5m52.965s
> > > user    40m27.284s
> > > sys     3m53.339s
> > >
> > > So parallel dump with 8 workers using 1GB (128k pages) chunks runs
> > > almost 7 times faster than the sequential dump.
> > >
> > > this was a table that had no TOAST part. I will run some more tests
> > > with TOASTed tables next and expect similar or better improvements.
> > >
> > >
> > >
> > > On Wed, Nov 12, 2025 at 1:59=E2=80=AFPM Ashutosh Bapat
> > > <ashutosh.bapat.oss@gmail.com> wrote:
> > > >
> > > > Hi Hannu,
> > > >
> > > > On Tue, Nov 11, 2025 at 9:00=E2=80=AFPM Hannu Krosing <hannuk@googl=
e.com> wrote:
> > > > >
> > > > > Attached is a patch that adds the ability to dump table data in m=
ultiple chunks.
> > > > >
> > > > > Looking for feedback at this point:
> > > > >  1) what have I missed
> > > > >  2) should I implement something to avoid single-page chunks
> > > > >
> > > > > The flag --huge-table-chunk-pages which tells the directory forma=
t
> > > > > dump to dump tables where the main fork has more pages than this =
in
> > > > > multiple chunks of given number of pages,
> > > > >
> > > > > The main use case is speeding up parallel dumps in case of one or=
 a
> > > > > small number of HUGE tables so parts of these can be dumped in
> > > > > parallel.
> > > >
> > > > Have you measured speed up? Can you please share the numbers?
> > > >
> > > > --
> > > > Best Wishes,
> > > > Ashutosh Bapat