dfs -getMerge, which calls FileUtil.CopyMerge, contains this javadoc:
Get all the files in the directories that match the source file pattern
* and merge and sort them to only one file on local fs
* srcf is kept.
However, it only concatenates the set of input files, rather than merging them in sorted order.
Ideally, the copyMerge should be equivalent to a map-reduce job with IdentityMapper and IdentityReducer with numReducers = 1. However, not having to run this as a map-reduce job has some advantages, since it increases cluster utilization during reduce phase.