Re: Replication slot stats misgivings

On Mon, Apr 12, 2021 at 7:03 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Apr 12, 2021 at 9:36 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Mon, Apr 12, 2021 at 5:29 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Apr 12, 2021 at 8:08 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > > On Mon, Apr 12, 2021 at 4:34 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > >
> > > > > On Mon, Apr 12, 2021 at 6:19 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > On Mon, Apr 12, 2021 at 10:27 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > > > >
> > > > > > > On Sat, Apr 10, 2021 at 9:53 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)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?

Regards,
Vignesh