Horton

RE: speed up a logical replica setup

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: 'Euler Taveira' <euler@eulerto.com>, Alexander Lakhin <exclusion@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Date: 2024-07-17T07:53:22Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_createsubscriber: Remove obsolete comment

  2. pg_createsubscriber: Fix an unpredictable recovery wait time.

  3. Fix unstable test in 040_pg_createsubscriber.

  4. Fix the testcase introduced in commit 81d20fbf7a.

  5. Further weaken new pg_createsubscriber test on Windows.

  6. Temporarily(?) weaken new pg_createsubscriber test on Windows.

  7. Make pg_createsubscriber warn if publisher has two-phase commit enabled.

  8. Make pg_createsubscriber more wary about quoting connection parameters.

  9. pg_createsubscriber: Remove failover replication slots on subscriber

  10. pg_createsubscriber: Remove replication slot check on primary

  11. pg_createsubscriber: Only --recovery-timeout controls the end of recovery process

  12. pg_createsubscriber: creates a new logical replica from a standby server

  13. Add some const decorations

  14. Add option force_initdb to PostgreSQL::Test::Cluster:init()

  15. Remove MSVC scripts

Dear Alexander, Euler, Amit,

I also analyzed this failure, let me share it. Here, I think events in below
ordering were occurred.

1. Backend created a publication on $db2,
2. BGWriter generated RUNNING_XACT record, then
3. Backend created a replication slot on $db2.

In this case, the recovery_target_lsn is ahead of the RUNNING_XACT record generated
at step 3. Also, since both bgwriter and slot creation mark the record as
*UNIMPORTANT* one, the writer won't start again even after the
LOG_SNAPSHOT_INTERVAL_MS. The rule is written in BackgroundWriterMain():

```
			/*
			 * Only log if enough time has passed and interesting records have
			 * been inserted since the last snapshot.  Have to compare with <=
			 * instead of < because GetLastImportantRecPtr() points at the
			 * start of a record, whereas last_snapshot_lsn points just past
			 * the end of the record.
			 */
			if (now >= timeout &&
				last_snapshot_lsn <= GetLastImportantRecPtr())
			{
				last_snapshot_lsn = LogStandbySnapshot();
				last_snapshot_ts = now;
			}
```

Therefore, pg_createsubscriber waited until a new record was replicated, but no
activities were recorded, causing a timeout. Since this is a timing issue, Alexander
could reproduce the failure with shorter time duration and parallel running.

IIUC, the root cause is that pg_create_logical_replication_slot() returns a LSN
which is not generated yet. So, I think both mine [1] and Euler's approach [2]
can solve the issue. My proposal was to add an extra WAL record after the final
slot creation, and Euler's one was to use a restart_lsn as the recovery_target_lsn.
In case of primary server, restart_lsn is set to the latest WAL insert position and
then RUNNING_XACT record is generated later.

How do you think?

[1]: https://www.postgresql.org/message-id/OSBPR01MB25521B15BF950D2523BBE143F5D32@OSBPR01MB2552.jpnprd01.prod.outlook.com
[2]: https://www.postgresql.org/message-id/b1f0f8c7-8f01-4950-af77-339df3dc4684%40app.fastmail.com

Best regards,
Hayato Kuroda
FUJITSU LIMITED