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 000114600AE 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://docs.airbyte.com/integrations/sources/postgres/postgres-troubleshooting#under-cdc-incremental-mode-there-are-still-full-refresh-syncs> - https://fivetran.com/docs/databases/postgresql/troubleshooting/last-tracked-lsn-error Thank you very much for your time and assistance. Thanks, Kaushik Iska
Re: Intermittent Issue with WAL Segment Removal in Logical Replication
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 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 000100060059 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. Thanks, Kaushik On Tue, Dec 26, 2023 at 10:36 AM Kaushik Iska wrote: > 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 000114600AE 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://docs.airbyte.com/integrations/sources/postgres/postgres-troubleshooting#under-cdc-incremental-mode-there-are-still-full-refresh-syncs> >- > > > > https://fivetran.com/docs/databases/postgresql/troubleshooting/last-tracked-lsn-error > > > > Thank you very much for your time and assistance. > > Thanks, > > Kaushik Iska > >
Re: Intermittent Issue with WAL Segment Removal in Logical Replication
I am unfortunately not really familiar with Google Cloud SQL internals as well. But we have seen this happen on Amazon RDS as well. Could it be possible that we are requesting a future WAL segment, say WAL upto X is written and we are asking for X + 1? It could be that the error message is misleading. I do not have the information from pg_replication_slots as I have terminated the test. I am fairly certain that I can reproduce this again. I will gather both the restart_lsn and contents of pg_wal for the failed segment. Is there any other information that would help debug this further? On Fri, Dec 29, 2023 at 4:16 PM Tomas Vondra wrote: > 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 000100060059 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 >
