Currently, every HFile.Reader has a single DFSInputStream, which it uses to service all gets and scans. For gets, we use the positional read API (aka "pread") and for scans we use a synchronized block to seek, then read. The advantage of pread is that it doesn't hold any locks, so multiple gets can proceed at the same time. The advantage of seek+read for scans is that the datanode starts to send the entire rest of the HDFS block, rather than just the single hfile block necessary. So, in a single thread, pread is faster for gets, and seek+read is faster for scans since you get a strong pipelining effect.
However, in a multi-threaded case where there are multiple scans (including scans which are actually part of compactions), the seek+read strategy falls apart, since only one scanner may be reading at a time. Additionally, a large amount of wasted IO is generated on the datanode side, and we get none of the earlier-mentioned advantages.
In one test, I switched scans to always use pread, and saw a 5x improvement in throughput of the YCSB scan-only workload, since it previously was completely blocked by contention on the DFSIS lock.