Hadoop Distributed File System(HDFS) NameNode Maps a filename to list of Block IDs Maps each Block ID to DataNodes containing a replica of the block DataNode:Maps a Block ID to a physical location on disk Data Coherency Write-once-read-many access model Client can only append to existing files Distributed file systems good for millions of large files But have very high overheads and poor performance with billions of smaller tuples Database System Concepts-7th Edition 10.7 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 10.7 ©Silberschatz, Korth and Sudarshan th Edition Hadoop Distributed File System (HDFS) ▪ NameNode • Maps a filename to list of Block IDs • Maps each Block ID to DataNodes containing a replica of the block ▪ DataNode: Maps a Block ID to a physical location on disk ▪ Data Coherency • Write-once-read-many access model • Client can only append to existing files ▪ Distributed file systems good for millions of large files • But have very high overheads and poor performance with billions of smaller tuples