I think there is confusion on how we are defining colocated clients. This is because of the way I first understood and answered your question. Your edge nodes are different from what Hadoop calls colocated client which is probably what you have read somewhere.
When a Map Reduce job runs, it spawns number of mappers. each mapper is usually reading data on the node it is running on (the local data). Each of these mappers is a client or colocated client assuming its reading local data. However, these mappers were not taking advantage of the fact that data they are reading is local to where these mappers are running (A mapper in some cases might read data from a remote machine which is going to be inefficient if it happens). The mappers were using the same TCP protocol in reading local data as they would use to read remote data. It was determined that performance can be improved about 20-30% only by making these mappers read data blocks directly off of disk if they can be made aware of the local data. Hence this change was made to improve that performance. If you would like more details, please see the following JRA.
https://issues.apache.org/jira/browse/HDFS-347
If you scroll down in this jira then you will see a design document. That design document will clear any confusion you may have.
