Re: Replication slot stats misgivings - Mailing list pgsql-hackers

On Tue, Apr 13, 2021 at 5:07 PM vignesh C <vignesh21@gmail.com> wrote:
>
> On Mon, Apr 12, 2021 at 7:03 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Mon, Apr 12, 2021 at 9:36 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > On Mon, Apr 12, 2021 at 5:29 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > >
> > > > On Mon, Apr 12, 2021 at 8:08 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > >
> > > > > On Mon, Apr 12, 2021 at 4:34 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > > > >
> > > > > > On Mon, Apr 12, 2021 at 6:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > > > >
> > > > > > > On Mon, Apr 12, 2021 at 10:27 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > > > > > >
> > > > > > > > On Sat, Apr 10, 2021 at 9:53 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > It seems Vignesh has changed patches based on the latest set of
> > > > > > > > > comments so you might want to rebase.
> > > > > > > >
> > > > > > > > I've merged my patch into the v6 patch set Vignesh submitted.
> > > > > > > >
> > > > > > > > I've attached the updated version of the patches. I didn't change
> > > > > > > > anything in the patch that changes char[NAMEDATALEN] to NameData (0001
> > > > > > > > patch) and patches that add tests.
> > > > > > > >
> > > > > > >
> > > > > > > I think we can push 0001. What do you think?
> > > > > >
> > > > > > +1
> > > > > >
> > > > > > >
> > > > > > > > In 0003 patch I reordered the
> > > > > > > > output parameters of pg_stat_replication_slots; showing total number
> > > > > > > > of transactions and total bytes followed by statistics for spilled and
> > > > > > > > streamed transactions seems appropriate to me.
> > > > > > > >
> > > > > > >
> > > > > > > I am not sure about this because I think we might want to add some
> > > > > > > info of stream/spill bytes in total_bytes description (something like
> > > > > > > stream/spill bytes are not in addition to total_bytes).
> > > >
> > > > BTW doesn't it confuse users that stream/spill bytes are not in
> > > > addition to total_bytes? User will need to do "total_bytes +
> > > > spill/stream_bytes" to know the actual total amount of data sent to
> > > > the decoding output plugin, is that right?
> > > >
> > >
> > > No, total_bytes includes the spill/stream bytes. So, the user doesn't
> > > need to do any calculation to compute totel_bytes sent to output
> > > plugin.
> >
> > The following test for the latest v8 patch seems to show different.
> > total_bytes is 1808 whereas spill_bytes is 13200000. Am I missing
> > something?
> >
> > postgres(1:85969)=# select pg_create_logical_replication_slot('s',
> > 'test_decoding');
> >  pg_create_logical_replication_slot
> > ------------------------------------
> >  (s,0/1884468)
> > (1 row)
> >
> > postgres(1:85969)=# create table a (i int);
> > CREATE TABLE
> > postgres(1:85969)=# insert into a select generate_series(1, 100000);
> > INSERT 0 100000
> > postgres(1:85969)=# set logical_decoding_work_mem to 64;
> > SET
> > postgres(1:85969)=# select * from pg_stat_replication_slots ;
> >  slot_name | total_txns | total_bytes | spill_txns | spill_count |
> > spill_bytes | stream_txns | stream_count | stream_bytes | stats_reset
> >
-----------+------------+-------------+------------+-------------+-------------+-------------+--------------+--------------+-------------
> >  s         |          0 |           0 |          0 |           0 |
> >       0 |           0 |            0 |            0 |
> > (1 row)
> >
> > postgres(1:85969)=# select count(*) from
> > pg_logical_slot_peek_changes('s', NULL, NULL);
> >  count
> > --------
> >  100004
> > (1 row)
> >
> > postgres(1:85969)=# select * from pg_stat_replication_slots ;
> >  slot_name | total_txns | total_bytes | spill_txns | spill_count |
> > spill_bytes | stream_txns | stream_count | stream_bytes | stats_reset
> >
-----------+------------+-------------+------------+-------------+-------------+-------------+--------------+--------------+-------------
> >  s         |          2 |        1808 |          1 |         202 |
> > 13200000 |           0 |            0 |            0 |
> > (1 row)
> >
>
> Thanks for identifying this issue, while spilling the transactions
> reorder buffer changes gets released, we will not be able to get the
> total size for spilled transactions from reorderbuffer size. I have
> fixed it by including spilledbytes to totalbytes in case of spilled
> transactions. Attached patch has the fix for this.
> Thoughts?

I've not looked at the patches yet but as Amit mentioned before[1],
it's better to move 0002 patch to after 0004. That is, 0001 patch
changes data type to NameData, 0002 patch adds total_txn and
total_bytes, and 0003 patch adds regression tests. 0004 patch will be
the patch using HTAB (was 0002 patch) and get reviewed after pushing
0001, 0002, and 0003 patches. 0005 patch adds more regression tests
for the problem 0004 patch addresses.

Regards,

[1] https://www.postgresql.org/message-id/CAA4eK1Kd4ag6Vc6jO%2BntYmTMiR70x3t_%2BYQRMDP%3D9T5a2uzUHg%40mail.gmail.com

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/