Replace Memory Issue VB.net - vb.net

Question

i was kindly helped before with this code, however I have hit a stumbling block, and im not sure the correct way to go. I have the code below which does a find and replace on over 120k of find and replaces. The problem is the text file is HUGE easily over 5 gig of log files so i get a memory issue which is not surprise. So do i load the data in blocks if that is even possible?, if so how.
Private Sub CmdBtnTestReplace_Click(sender As System.Object, e As System.EventArgs) Handles CmdBtnTestReplace.Click
Dim fName As String = "c:\backup\logs\master.txt"
Dim wrtFile As String = "c:\backup\logs\masterUserFormatted.txt"
Dim strRead As New System.IO.StreamReader(fName)
Dim strWrite As New System.IO.StreamWriter(wrtFile)
Dim s As String
s = strRead.ReadToEnd()
'runs through over 120k of find and replaces
For Each row As DataGridViewRow In DataGridView1.Rows
If Not row.IsNewRow Then
Dim Find1 As String = row.Cells(0).Value.ToString
Dim Replace1 As String = row.Cells(1).Value.ToString
Cursor.Current = Cursors.WaitCursor
'replace using string from 1st column and replaces with string from 2nd column.
s = s.Replace(Find1, Replace1)
End If
Next
strWrite.Write(s)
strRead.Close()
strWrite.Close()
Cursor.Current = Cursors.Default
MessageBox.Show("Finished Replacing")
End Sub

Answer 1

If the input file is a simple multi-line text file, where no individual line is too big to load into memory at once, and the search string is never going to span multiple lines, then reading only one line at a time will be the simplest solution. For instance:
Dim fName As String = "c:\backup\logs\master.txt"
Dim wrtFile As String = "c:\backup\logs\masterUserFormatted.txt"
Dim strRead As New System.IO.StreamReader(fName)
Dim strWrite As New System.IO.StreamWriter(wrtFile)
Cursor.Current = Cursors.WaitCursor
While True
Dim line As String = strRead.ReadLine()
If line IsNot Nothing Then
For Each row As DataGridViewRow In DataGridView1.Rows
If Not row.IsNewRow Then
Dim Find1 As String = row.Cells(0).Value.ToString
Dim Replace1 As String = row.Cells(1).Value.ToString
line = line.Replace(Find1, Replace1)
End If
Next
strWrite.WriteLine(line)
Else
Exit While
End If
End While
strRead.Close()
strWrite.Close()
Cursor.Current = Cursors.Default
MessageBox.Show("Finished Replacing")
It's worth mentioning that the StreamReader and StreamReader implement IDisposable. As such, it would be preferable to enclose them in a Using block rather than explicitly calling Close yourself.