Re: Patch: dumping tables data in multiple chunks in pg_dump

Added to https://commitfest.postgresql.org/patch/6219/

On Thu, Nov 13, 2025 at 9:26 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
>
> The reason for small chunk sizes is that they are determined by main
> heap table, and that was just over 1GB
>
> largetoastdb=> SELECT format('%I.%I', t.schemaname, t.relname) as table_name,
> 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 = t.relid
> JOIN pg_class c ON c.oid = t.relid
> LEFT JOIN pg_stat_sys_indexes AS oi ON oi.relid = c.reltoastrelid
> GROUP BY 1, 2, 4, 5, 6, 7
> ORDER BY 2 DESC, 7 DESC
> LIMIT 25;
> ┌─[ RECORD 1 ]─────┬─────────────────────────────────────┐
> │ table_name │ public.just_toasted │
> │ table_size │ 56718835712 │
> │ total_index_size │ 230064128 │
> │ main_table_size │ 1191559168 │
> │ toast_table_size │ 54613336064 │
> │ toast_index_size │ 898465792 │
> │ row_count │ 5625234 │
> │ index_count │ 1 │
> │ index_info │ [{"just_toasted_pkey" : 230064128}] │
> └──────────────────┴─────────────────────────────────────┘
>
> On Thu, Nov 13, 2025 at 9:24 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> >
> > 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(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres -f
> > /tmp/ltoastdb-1-plain.dump largetoastdb
> > real 3m59.465s
> > user 3m43.304s
> > sys 0m15.844s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=9095 -j 4 -f /tmp/ltoastdb-4.dump
> > largetoastdb
> > real 1m18.320s
> > user 3m49.236s
> > sys 0m19.422s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=9095 -j 8 -f /tmp/ltoastdb-8.dump
> > largetoastdb
> > real 0m42.028s
> > user 3m55.299s
> > sys 0m24.657s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=9095 -j 16 -f /tmp/ltoastdb-16.dump
> > largetoastdb
> > real 0m42.575s
> > user 4m11.011s
> > sys 0m26.110s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=1000 -j 16 -f /tmp/ltoastdb-16-1kpages.dump
> > largetoastdb
> > real 0m29.641s
> > user 6m16.321s
> > sys 0m49.345s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=1000 -j 8 -f /tmp/ltoastdb-8-1kpages.dump
> > largetoastdb
> > real 0m35.685s
> > user 3m58.528s
> > sys 0m26.729s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=1000 -j 4 -f /tmp/ltoastdb-4-1kpages.dump
> > largetoastdb
> > real 1m3.737s
> > user 3m50.251s
> > sys 0m18.507s
> >
> > hannuk(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > --format=directory -h 10.58.80.2 -U postgres
> > --huge-table-chunk-pages=1000 -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 PM Hannu Krosing <hannuk(at)google(dot)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 = 4172 GB/h)
> > >
> > > $ time ./pg_dump --format=directory -h 10.58.80.2 -U postgres
> > > --huge-table-chunk-pages=131072 -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(at)pgn2:~/work/postgres/src/bin/pg_dump$ time ./pg_dump
> > > --format=directory -h 10.58.80.2 -U postgres
> > > --huge-table-chunk-pages=131072 -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 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> > > >
> > > > I just ran a test by generating a 408GB table and then dumping it both ways
> > > >
> > > > $ time pg_dump --format=directory -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=directory -h 10.58.80.2 -U postgres
> > > > --huge-table-chunk-pages=131072 -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 PM Ashutosh Bapat
> > > > <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
> > > > >
> > > > > Hi Hannu,
> > > > >
> > > > > On Tue, Nov 11, 2025 at 9:00 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> > > > > >
> > > > > > Attached is a patch that adds the ability to dump table data in multiple 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 format
> > > > > > 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