High Speed Data Logging

If you want to be able to retrieve the data easily, then flat (text) files are generally not the way to go. I would suggest finding an off the shelf historian product that meets your budgetary needs (as they vary widely in price). The big players like Rockwell, GE, etc all have their own. Historian products often come with pre-packaged reports too, so that may cover your needs for getting the data out easily. However, if you want the most flexibility for data retrieval, its best to have it stored in a relational database like SQL Server. Some historians do store in relational databases, while others use proprietary file systems that you may not be able to access in an ad-hoc way. So look into the type of data storage when you check out some Historians.

If you can't find a Historian that fits your budget, you can consider writing something custom too. Depending on the data source that you need to interface with, it may not an overly complicated task, but then you have the onus of supporting custom code.

I agree with Jeff, you won't want the headaches of handling text or csv files for storing and retrieving this type of data. A configurable off-the-shelf (COTS) historian package would be the best way to go. The decision of relational versus non-relational database hinges on the rate of data collection, retrieval method and the length of time the data needs to remain online (dictates database size, etc.), as well as the requirement for any contextual data (shift, part #, serial #, etc.). Both provide robust data collection methods for archiving and redundancy. Most products also include easy to use Microsoft Excel menus for data retrieval without having to know database query language. If your data logging PC is using a common HMI program such as RSView, InTouch or iFIX you can log data directly to your historian from the PLC or from the HMI tag database already configured.