Re: Introduce XID age and inactive timeout based replication slot invalidation

On Tue, Feb 20, 2024 at 12:05 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
>> [...] and was able to produce something like:
> >
> > postgres=# select slot_name,slot_type,active,active_pid,wal_status,invalidation_reason from pg_replication_slots;
> >   slot_name  | slot_type | active | active_pid | wal_status | invalidation_reason
> > -------------+-----------+--------+------------+------------+---------------------
> >  rep1        | physical  | f      |            | reserved   |
> >  master_slot | physical  | t      |    1482441 | unreserved | wal_removed
> > (2 rows)
> >
> > does that make sense to have an "active/working" slot "ivalidated"?
>
> Thanks. Can you please provide the steps to generate this error? Are
> you setting max_slot_wal_keep_size on primary to generate
> "wal_removed"?

I'm able to reproduce [1] the state [2] where the slot got invalidated
first, then its wal_status became unreserved, but still the slot is
serving after the standby comes up online after it catches up with the
primary getting the WAL files from the archive. There's a good reason
for this state -
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/slotfuncs.c;h=d2fa5e669a32f19989b0d987d3c7329851a1272e;hb=ff9e1e764fcce9a34467d614611a34d4d2a91b50#l351.
This intermittent state can only happen for physical slots, not for
logical slots because logical subscribers can't get the missing
changes from the WAL stored in the archive.

And, the fact looks to be that an invalidated slot can never become
normal but still can serve a standby if the standby is able to catch
up by fetching required WAL (this is the WAL the slot couldn't keep
for the standby) from elsewhere (archive via restore_command).

As far as the 0001 patch is concerned, it reports the
invalidation_reason as long as slot_contents.data.invalidated !=
RS_INVAL_NONE. I think this is okay.

Thoughts?

[1]
./initdb -D db17
echo "max_wal_size = 128MB
max_slot_wal_keep_size = 64MB
archive_mode = on
archive_command='cp %p
/home/ubuntu/postgres/pg17/bin/archived_wal/%f'" | tee -a
db17/postgresql.conf

./pg_ctl -D db17 -l logfile17 start

./psql -d postgres -p 5432 -c "SELECT
pg_create_physical_replication_slot('sb_repl_slot', true, false);"

rm -rf sbdata logfilesbdata
./pg_basebackup -D sbdata

echo "port=5433
primary_conninfo='host=localhost port=5432 dbname=postgres user=ubuntu'
primary_slot_name='sb_repl_slot'
restore_command='cp /home/ubuntu/postgres/pg17/bin/archived_wal/%f
%p'" | tee -a sbdata/postgresql.conf

touch sbdata/standby.signal

./pg_ctl -D sbdata -l logfilesbdata start
./psql -d postgres -p 5433 -c "SELECT pg_is_in_recovery();"

./pg_ctl -D sbdata -l logfilesbdata stop

./psql -d postgres -p 5432 -c "SELECT pg_logical_emit_message(true,
'mymessage', repeat('aaaa', 10000000));"
./psql -d postgres -p 5432 -c "CHECKPOINT;"
./pg_ctl -D sbdata -l logfilesbdata start
./psql -d postgres -p 5432 -xc "SELECT * FROM pg_replication_slots;"

[2]
postgres=# SELECT * FROM pg_replication_slots;
-[ RECORD 1 ]-------+-------------
slot_name           | sb_repl_slot
plugin              |
slot_type           | physical
datoid              |
database            |
temporary           | f
active              | t
active_pid          | 710667
xmin                |
catalog_xmin        |
restart_lsn         | 0/115D21A0
confirmed_flush_lsn |
wal_status          | unreserved
safe_wal_size       | 77782624
two_phase           | f
conflict_reason     |
failover            | f
synced              | f
invalidation_reason | wal_removed
last_inactive_at    |
inactive_count      | 1

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com