Experience with Hadoop?

Yes, you can use Hadoop on a local filesystem by using file URIs instead of hdfs URIs in various places. I think a lot of the examples that come with Hadoop do this.

This is probably fine if you just want to learn how Hadoop works and the basic map-reduce paradigm, but you will need multiple machines and a distributed filesystem to get the real benefits of the scalability inherent in the architecture.

Hadoop MapReduce can run ontop of any number of file systems or even more abstract data sources such as databases. In fact there are a couple of built-in classes for non-HDFS filesystem support, such as S3 and FTP. You could easily build your own input format as well by extending the basic InputFormat class.

Using HDFS brings certain advantages, however. The most potent advantage is that the MapReduce job scheduler will attempt to execute maps and reduces on the physical machines that are storing the records in need of processing. This brings a performance boost as data can be loaded straight from the local disk instead of transferred over the network, which depending on the connection may be orders of magnitude slower.

As Joe said, you can indeed use Hadoop without HDFS. However, throughput depends on the cluster's ability to do computation near where data is stored. Using HDFS has 2 main benefits IMHO 1) computation is spread more evenly across the cluster (reducing the amount of inter-node communication) and 2) the cluster as a whole is more resistant to failure due to data unavailability.

If your data is already partitioned or trivially partitionable, you may want to look into supplying your own partitioning function for your map-reduce task.