Common Misuses During Full Data Import

This document introduces the common error scenarios in using Loader or TiDB Data Migration (DM) during the full data import process, and also provides their reasons and solutions.

Error: Try adjusting the `max_allowed_packet` variable

The following error is reported during full data import:

packet for query is too large. Try adjusting the 'max_allowed_packet' variable

Reasons

Both MySQL client and MySQL/TiDB Server have max_allowed_packet quotas. If any of the max_allowed_packet quotas is violated, the client receives a corresponding error message. Currently, the latest version of Syncer, Loader, DM, and TiDB Server all have a default max_allowed_packet quota of 64M.
- Please use the latest version, or the latest stable version of the tool. See the download page.
The full data import processing module in Loader or DM does not support splitting dump sqls files.

Solutions

For the above reasons, it is recommended to use the -s, --statement-size option which Mydumper offers to control the size of Insert Statement: Attempted size of INSERT statement in bytes, default 1000000.

According to the default configuration of --statement-size, the size of Insert Statement that Mydumper generates is as close as 1M. This default configuration ensures that this error will not occur in most cases.

Sometimes the following WARN log appears during the dump process. The WARN log itself does not affect the dump process but indicates that the dumped table might be a wide table.
```
Row bigger than statement_size for xxx
```
If a single row of a wide table exceeds 64M, you need to modify and enable the following two configurations:
- Execute set @@global.max_allowed_packet=134217728 (134217728 = 128M) in TiDB Server.
- Add the configuration like max-allowed-packet=128M to the Loader configuration file or DB configuration in DM task configuration file, and then restart the progress or task.