Reading from large text files

Question

Hello, I have a bit of a design problem that I need help with. Any suggestions are appreciated. I've written an application that will go through lines in a log file, parse the text, and insert appropriate data into a database. A new log file is generated from an auth server every 30 minutes, and is written to often. In the 1/2 hour period, one log file can get up to about 15,000 lines. I have no control over the size of these log files. I need to read the updates to the log file as quickly as possible, so I've used a FileSystemWatcher to fire the event when the file is changed. Each line in the log contains a timestamp (in chronological order), which I use to keep track of the last line I read. Of course, I still have to scan through all of the lines until I get to the ones that I have not yet read. I can't delete old lines of this file because when the auth server is done writing to it, I need to move it into another folder. If it's not obvious already, my problem is that towards the end of the 1/2 hour, this file is huge and reading from it every time there is an update hogs the CPU. Given the constraints, I'm not sure how else to handle this problem. Help!