Finally find the solution to transfer session data from database to files
You can run following script to transfer session data from database to files
<?php
ini_set('memory_limit', '500M');
ini_set('max_execution_time', 0);
error_reporting(E_ALL | E_STRICT);
ini_set('display_errors', 1);
require_once 'app/Mage.php';
umask(0);
Mage::app();
$resource = Mage::getSingleton('core/resource');
$readConnection = $resource->getConnection('core_read');
$table = $resource->getTableName('core/session');
$query = 'SELECT * FROM ' . $table;
$session = $readConnection->fetchAll($query);
$filePath = 'var/session/';
foreach ($session as $sess) {
$file = $filePath.'sess_'.$sess['session_id'];
$fp = fopen($file, 'w');
fwrite($fp, $sess['session_data']);
fclose($fp);
chmod($file, 0777);
}
?>