These applications write their data only once but they read it one or more times and require these reads to be satisfied at streaming speeds. HDFS supports write-once-read-many semantics on files.
这些应用都是写数据一次，读却是一次到多次，并且读的速度要满足流式读。
2
Semaphores do not say which entities, nor how (read vs write), only how many.