Обсуждение: Intermittent Issue with WAL Segment Removal in Logical Replication

Dear PostgreSQL Community,

I am seeking guidance regarding a recurring issue we've encountered with WAL segment removal during logical replication using pgoutput plugin. We sporadically encounter an error indicating that a requested WAL segment has already been removed. This issue arises intermittently when executing START_REPLICATION. An example error message is as follows:

requested WAL segment 000000010000146000000AE has already been removed

Please note that this error is not specific to the segment mentioned above; it serves as an example of the type of error we are experiencing.

Additional Context:

• max_slot_wal_keep_size is -1, logical_decoding_work_mem is 4 GB.

• The error seems to appear randomly and is not consistent.

• After a couple of retries, the replication process eventually succeeds.

• For one of the users it seems to be happening every 16 hours or so.

Our approach involves starting with START_REPLICATION 0, replicating data in batches, and then restarting at the last LSN of the previous batch. We are trying to understand the root cause behind the intermittent removal of WAL segments during logical replication. Specifically, we are looking for insights into:

• The potential reasons for the WAL segments being reported as removed.

• Why this error occurs intermittently and why replication succeeds after several retries.

• Any advice on troubleshooting and resolving this issue, or insights into whether it might be related to our specific replication setup or a characteristic of pgoutput, would be highly valuable.

Related Posts

• https://issues.redhat.com/browse/DBZ-590 

• Troubleshooting Postgres Sources | Airbyte Documentation

• https://fivetran.com/docs/databases/postgresql/troubleshooting/last-tracked-lsn-error 

Thank you very much for your time and assistance.

Thanks,

Kaushik Iska

On 12/27/23 16:31, Kaushik Iska wrote:
> Hi all,
>
> I'm including additional details, as I am able to reproduce this issue a
> little more reliably.
>
> Postgres Version: POSTGRES_14_9.R20230830.01_07
> Vendor: Google Cloud SQL
> Logical Replication Protocol version 1
>

I don't know much about Google Cloud SQL internals. Is it relatively
close to Postgres (as e.g. RDS) or are the internals very different /
modified for cloud environments?

> Here are the logs of attempt succeeding right after it fails:
>
> 2023-12-27 01:12:40.581 UTC [59790]: [6-1] db=postgres,user=postgres
> STATEMENT:  START_REPLICATION SLOT peerflow_slot_wal_testing_2 LOGICAL
> 6/5AE67D79 (proto_version '1', publication_names
> 'peerflow_pub_wal_testing_2') <- FAILS
> 2023-12-27 01:12:41.087 UTC [59790]: [7-1] db=postgres,user=postgres
> ERROR:  requested WAL segment 000000010000000600000059 has already been
> removed
> 2023-12-27 01:12:44.581 UTC [59794]: [3-1] db=postgres,user=postgres
> STATEMENT:  START_REPLICATION SLOT peerflow_slot_wal_testing_2 LOGICAL
> 6/5AE67D79 (proto_version '1', publication_names
> 'peerflow_pub_wal_testing_2')  <- SUCCEEDS
> 2023-12-27 01:12:44.582 UTC [59794]: [4-1] db=postgres,user=postgres
> LOG:  logical decoding found consistent point at 6/5A31F050
>
> Happy to include any additional details of my setup.
>

I personally don't see how could this fail and then succeed, unless
Google does something smart with the WAL segments under the hood. Surely
we try to open the same WAL segment (given the LSN is the same), so how
could it not exist and then exist?

As Ron already suggested, it might be useful to see information for the
replication slot peerflow_slot_wal_testing_2 (especially the restart_lsn
value). Also, maybe show the contents of pg_wal (especially for the
segment referenced in the error message).

Can you reproduce this outside Google cloud environment?


regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company