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TARGET LOAD STOPS AND MAKES NO FURTHER PROGRESS. WHY?

0

I am trying to get a DMS full load replication to occur.

The SOURCE_LOAD is succcessful. The TARGET_LOAD does this and then no further progress occurs. Note that the target load does successfuly connect to the target instance, create the table, create pirmary keys and create unique indexes indicating that DMS has ciomplete and correct network paths two and from both the source and target endpoints.

There is no evidence of REJECTS in the VPC logs for the relevant IP addresses.

Therefore network and RDS credentials appear to be A.O.K

However, the target load (of about 11 rows) never completes.

All that happens is that this messages is printed repeatedly

| 1762571198000 | 2025-11-08T03:06:38 [IO ]D: rep_net_server_select: Server poll timeout. failed (at_repnet.c:489)

Why is this happening?

| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Create primary key statement: ALTER TABLE "public"."affiliate" ADD PRIMARY KEY ( "id" )  (provider_syntax_manager.c:2227)                                                                                                                                        |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Adding columns to table_def public.affiliate  (odbc_util.c:1341)                                                                                                                                                                                                 |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Column id added to table_def  (odbc_util.c:1527)                                                                                                                                                                                                                 |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Column name added to table_def  (odbc_util.c:1527)                                                                                                                                                                                                               |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Column installer_company_id added to table_def  (odbc_util.c:1527)                                                                                                                                                                                               |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Column created_at added to table_def  (odbc_util.c:1527)                                                                                                                                                                                                         |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Added all columns to table_def successfully  (odbc_util.c:1538)                                                                                                                                                                                                  |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Table 'public.affiliate' has unique index 'affiliate_pkey'  (ar_odbc_conn.c:3153)                                                                                                                                                                                |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Got table indexes successfully  (odbc_util.c:1551)                                                                                                                                                                                                               |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Got table PK successfully  (odbc_util.c:1581)                                                                                                                                                                                                                    |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Adding PK to table definition  (odbc_util.c:1605)                                                                                                                                                                                                                |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  1 PK segments  (odbc_util.c:1635)                                                                                                                                                                                                                                |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Added PK to table_def successfully  (odbc_util.c:1662)                                                                                                                                                                                                           |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Adding unique index to table definition  (odbc_util.c:1671)                                                                                                                                                                                                      |
| 1762571130000 | 2025-11-08T03:05:30 [TARGET_LOAD     ]D:  Added Index to table_def successfully  (odbc_util.c:1701)                                                                                                                                                                                                        |
| 1762571131000 | 2025-11-08T03:05:31 [TARGET_LOAD     ]D:  Init CSV target for table id '1'  (csv_target.c:769)                                                                                                                                                                                                             |
| 1762571131000 | 2025-11-08T03:05:31 [TARGET_LOAD     ]D:  Going to create data file folder '/rdsdbdata/data/tasks/magellanic-umbriel-platfor-e92f/data_files/1  (csv_target.c:795)                                                                                                                                         |
| 1762571131000 | 2025-11-08T03:05:31 [TARGET_LOAD     ]D:  Target starts loading new table 'public'.'affiliate'  (endpointshell.c:3253)
Topics
Migration & Modernization
Tags
AWS Database Migration Service
Language
English

asked 10 months ago199 views

1 Answer
0

Your AWS DMS migration task appears to be stalled during the target load phase. There are several potential causes for this issue:

  1. Database Locking: There might be lock contention on the target database. If another application is accessing the target database and locking the table that DMS is trying to write to, it can cause the migration to stall. This creates delays in writing to all tables.

  2. Target Table Preparation Mode Settings: If the "Target table preparation mode" parameter is set to "Do nothing" or "Truncate" without having created primary and unique keys on the target tables, DMS will perform a full table scan for each update, which can significantly impact performance and cause the task to appear stuck.

  3. Resource Constraints: The target database might be experiencing high latency due to:

  • Backups running during the migration
  • Multiple availability zones (multi-AZ) being enabled
  • Database logging (such as audit or slow query logs) being enabled

  1. CDC Load Balancing Issues: If this is a CDC task, DMS uses a load balancing mechanism that might show 0% progress until enough changes accumulate or until you stop and resume the task.

  2. Triggers on Target Tables: If there are triggers on your target tables, they can interfere with the DMS replication process.

To troubleshoot this issue:

answered 10 months ago

  • Jon
    10 months ago

    There are no lock contentions on the database:

    There are the idle connections from the DMS instance:

    select pid, query_start, state, client_addr, query from pg_stat_activity where datname like 'umbriel_platform_clone'; pid | query_start | state | client_addr | query
    -------+-------------------------------+-------+---------------+----------------------------------------------------------- 14773 | 2025-11-08 03:05:30.076949+00 | idle | 10.21.245.232 | select pg_backend_pid() 14774 | 2025-11-08 03:05:30.054908+00 | idle | 10.21.245.232 | show max_identifier_length 14778 | 2025-11-08 03:05:30.927684+00 | idle | 10.21.245.232 | ALTER TABLE "public"."affiliate" ADD PRIMARY KEY ( "id" ) 14779 | 2025-11-08 03:05:31.000501+00 | idle | 10.21.245.232 | DEALLOCATE "_PLAN0x1516ac1ed490" (4 rows)

    The task preparation mode is DROP_AND_CREATE

    There is no "extensive logging" configured on the target.

    Numeron stop and restarts of the task have no effect. Stops do not usually succeed unless the instance itself is rebooted, indicating some kind of deadlock within the DMS instance itself.

    There data volumne is literally trivial 11 rows - it cannot be a resource issue with dms.t3.medium instance, surely?

    There are literally no additional cloudwatch log messages other than the repeated indicating of a server polling failure which presumably neans something to the C programmer responsible for this mess.

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