Slave负责存储文件的具体内容，以块为单位，每块64MB
研究论文：
# Sanjay Ghemawat, Howard Gobioff, and Shun-Tak Leung. "The Google file system." ACM SIGOPS operating systems review. Vol. 37. No. 5. ACM, 2003.
# Jeffrey Dean and Sanjay Ghemawat. "MapReduce: simplified data processing on large clusters." Communications of the ACM 51.1 (2008): 107-113.