Performance improvement on vb.net code - vb.net

Question

I need to write 50 million records with 72 columns into text file, the file size is growing as 9.7gb .
I need to check each and every column length need to format as according to the length as defined in XML file.
Reading records from oracle one by one and checking the format and writing into text file.
To write 5 crores records it is taking more than 24 hours. how to increase the performance in the below code.
Dim valString As String = Nothing
Dim valName As String = Nothing
Dim valLength As String = Nothing
Dim valDataType As String = Nothing
Dim validationsArray As ArrayList = GetValidations(Directory.GetCurrentDirectory() + "\ReportFormat.xml")
Console.WriteLine("passed xml")
Dim k As Integer = 1
Try
Console.WriteLine(System.DateTime.Now())
Dim selectSql As String = "select * from table where
" record_date >= To_Date('01-01-2014','DD-MM-YYYY') and record_date <= To_Date('31-12-2014','DD-MM-YYYY')"
Dim dataTable As New DataTable
Dim oracleAccess As New OracleConnection(System.Configuration.ConfigurationManager.AppSettings("OracleConnection"))
Dim cmd As New OracleCommand()
cmd.Connection = oracleAccess
cmd.CommandType = CommandType.Text
cmd.CommandText = selectSql
oracleAccess.Open()
Dim Tablecolumns As New DataTable()
Using oracleAccess
Using writer = New StreamWriter(Directory.GetCurrentDirectory() + "\FileName.txt")
Using odr As OracleDataReader = cmd.ExecuteReader()
Dim sbHeaderData As New StringBuilder
For i As Integer = 0 To odr.FieldCount - 1
sbHeaderData.Append(odr.GetName(i))
sbHeaderData.Append("|")
Next
writer.WriteLine(sbHeaderData)
While odr.Read()
Dim sbColumnData As New StringBuilder
Dim values(odr.FieldCount - 1) As Object
Dim fieldCount As Integer = odr.GetValues(values)
For i As Integer = 0 To fieldCount - 1
Dim vals As Array = validationsArray(i).ToString.ToUpper.Split("|")
valName = vals(0).trim
valDataType = vals(1).trim
valLength = vals(2).trim
Select Case valDataType
Case "VARCHAR2"
If values(i).ToString().Length = valLength Then
sbColumnData.Append(values(i).ToString())
'sbColumnData.Append("|")
ElseIf values(i).ToString().Length > valLength Then
sbColumnData.Append(values(i).ToString().Substring(0, valLength))
'sbColumnData.Append("|")
Else
sbColumnData.Append(values(i).ToString().PadRight(valLength))
'sbColumnData.Append("|")
End If
Case "NUMERIC"
valLength = valLength.Substring(0, valLength.IndexOf(","))
If values(i).ToString().Length = valLength Then
sbColumnData.Append(values(i).ToString())
'sbColumnData.Append("|")
Else
sbColumnData.Append(values(i).ToString().PadLeft(valLength, "0"c))
'sbColumnData.Append("|")
End If
'sbColumnData.Append((values(i).ToString()))
End Select
Next
writer.WriteLine(sbColumnData)
k = k + 1
Console.WriteLine(k)
End While
End Using
writer.WriteLine(System.DateTime.Now())
End Using
End Using
Console.WriteLine(System.DateTime.Now())
'Dim Adpt As New OracleDataAdapter(selectSql, oracleAccess)
'Adpt.Fill(dataTable)
Return Tablecolumns
Catch ex As Exception
Console.WriteLine(System.DateTime.Now())
Console.WriteLine("Error: " & ex.Message)
Console.ReadLine()
Return Nothing
End Try