dgraph-io/dgraph · error

while writing data file

Error message

while writing data file

What it means

In generateSchemaAndData, the error returned by dumpTables (either its row-dump pass or its constraints pass) is re-wrapped with "while writing data file", indicating the data output phase failed. Both inner passes already add "while dumping table <name>" context, producing a full wrap chain to the root cause.

Source

Thrown at dgraph/cmd/migrate/run.go:172

	schemaWriter, schemaCancelFunc, err := getFileWriter(schemaOutput)
	if err != nil {
		return err
	}
	defer schemaCancelFunc()
	dataWriter, dataCancelFunc, err := getFileWriter(dataOutput)
	if err != nil {
		return err
	}
	defer dataCancelFunc()

	dumpMeta.dataWriter = dataWriter
	dumpMeta.schemaWriter = schemaWriter

	if err := dumpMeta.dumpSchema(); err != nil {
		return errors.Wrapf(err, "while writing schema file")
	}
	if err := dumpMeta.dumpTables(); err != nil {
		return errors.Wrapf(err, "while writing data file")
	}
	return nil
}

View on GitHub (pinned to 759e242be6)

Solutions

1. Print with %+v and read the chain: data file phase -> while dumping table X -> root cause.
2. Fix the specific table's data or DB connectivity issue named in the chain.
3. Check disk space/writability for the --data output path.
4. Rerun the migration after fixing; remove/overwrite the partial output files.

Example fix

// before
err := migrate.Run()  // "while writing data file: while dumping table orders: nil value found"
// after
-- fix source: SELECT ... COALESCE(nullable_col, default) or clean NULLs, then rerun migrate

if err := canWrite(filepath.Dir(dataPath)); err != nil {
	return fmt.Errorf("cannot write data to %s: %w", dataPath, err)
}
// plus pre-dump data checks for NULLs and orphans (see errors 111/112)

func isDataWriteErr(err error) bool {
	return strings.Contains(err.Error(), "while writing data file")
}

if err := migrate.Run(); err != nil {
	if isDataWriteErr(err) {
		log.Printf("data phase failed, cause: %+v", errors.Cause(err))
	}
}

When it happens

Trigger: dumpTables returns any error: SQL read failure on a table, NULL/invalid value conversion via getValue, orphaned FK during constraint resolution, or dataWriter Flush failure.

Common situations: Mid-migration DB connection loss; bad row data (NULLs) in one table; unwritable/full disk for the data output file; orphaned foreign keys.

Related errors

• while writing schema
• while dumping table %s
• while writing schema file
• not allowed to overwrite %s
• nil value found

AI-assisted analysis of dgraph-io/dgraph@759e242be6 (2026-09-01). Data as JSON: /api/errors/536e9b9cbc47198c. Report an issue: GitHub.