I am checking of a particular field type in the schema using: 

drupal_get_schema('MY_FIELD');


I see in the database the field type is int, but when I check after:

$schema[$table]['fields']['MY_FIELD']['type'] 


is giving me always 'datetime'.

I assume is a cache issue: I am using: 

cache_clear_all('schema', 'cache');


but still no luck.


Question: How to wipe RAM on shutdown (prevent Cold Boot Attacks)?
Answer: Take a look at this thread

http://ubuntuforums.org/showthread.php?t=1500936

If you still want to do it then:


  Look for secure-delete in Synaptic 
  
  The Secure-Delete package comes with four commands:
  
  
    srm - Secure remove; used for deleting files or directories currently on your hard disk;
    
    smem - Secure memory wiper; used to wipe traces of data from your computer's memory (RAM);
    
    sfill - Secure free space wiper; used to wipe all traces of data from the free space on your disk;
    
    sswap - Secure swap wiper; used to wipe all traces of data from your swap partition.
  


Source: http://ubuntuforums.org/showthread.php?t=1008111

And to make it run when you want:


  To execute a script upon rebooting Ubuntu
  
  
    Put your script in /etc/rc0.d
    Make it executable (sudo chmod +x myscript)
    Note: The scripts in this directory are executed in alphabetical order
    
    The name of your script must begin with K99 to run at the right time. 
    To execute a script at shutdown
    
    Put your script in /etc/rc6.d
    Make it executable (sudo chmod +x myscript)
    Note: The scripts in this directory are executed in alphabetical order
    
    The name of your script must begin with K99 to run at the right time. 
  


Source: http://en.kioskea.net/faq/3348-ubuntu-executing-a-script-at-startup-and-shutdown

If you do not know how to make a script:


  Make a text file in an easy to find directory:
  
  Code:
  
  
    #!/bin/bash
    
    COMMAND1
    
    COMMAND2
    
    COMMAND3`
  
  
  Use the shebang line that I have given. Replace COMMAND1 and etc with the command you would like the script to run.
  
  Save the text file and then you have to make it executable. For this I will assume that the the file is called "test.sh". To make it executable:
  
  
    chmod 775 test.sh
  
  
  Then you can run the script:
  
  
    ./test.sh
  


Source: http://ubuntuforums.org/showthread.php?t=928839

Hope this helps,

Leinardo