1. What is HDFS?
   • HDFS stands for Hadoop Distributed File System. It is a distributed file system designed to store and manage large volumes of data across multiple nodes in a Hadoop cluster.
2. What is the primary motivation behind using HDFS?
   • HDFS is designed for fault tolerance and scalability, making it suitable for storing and processing massive datasets on a cluster of commodity hardware.
3. Explain the architecture of HDFS.
   • HDFS follows a master/slave architecture. The NameNode manages metadata, and DataNodes store the actual data. Clients communicate with the NameNode for metadata and DataNodes for data retrieval and storage.
4. What is the role of the NameNode in HDFS?
   • The NameNode manages metadata, such as file names, permissions, and the block locations. It does not store the actual data but directs clients to the DataNodes for data retrieval.
5. What is a DataNode in HDFS?
   • A DataNode is a slave node that stores data in HDFS. It manages the actual data blocks and responds to read and write requests from clients.
6. How does HDFS ensure fault tolerance?
   • HDFS achieves fault tolerance through data replication. Each data block is replicated to multiple DataNodes across the cluster. If a node fails, another copy of the data is readily available.
7. What is a block in HDFS?
   • A block is the minimum unit of data storage in HDFS, typically set to 128 MB or 256 MB. Files are divided into blocks, and each block is replicated across multiple DataNodes.
8. How does HDFS handle large files?
   • HDFS divides large files into blocks, and each block is distributed across the cluster. This parallelism allows for efficient processing of large files in a distributed environment.
9. Explain the process of reading data from HDFS.
   • Clients communicate with the NameNode to get the metadata. The NameNode provides the locations of the data blocks, and the client reads directly from the DataNodes in parallel.
10. How is data written to HDFS?
    • Clients communicate with the NameNode to create a new file. The NameNode allocates data blocks and provides their locations. Clients then write data directly to the designated DataNodes.
11. What is the default replication factor in HDFS?
    • The default replication factor in HDFS is 3, meaning each data block is replicated to three different DataNodes.
12. Explain the importance of block replication in HDFS.
    • Block replication ensures fault tolerance and data availability. If a DataNode or block becomes unavailable, there are still other replicas that can be used.
13. How does HDFS handle data locality?
    • HDFS aims to maximize data locality by scheduling tasks on nodes where the data resides. This minimizes data transfer across the network and improves performance.
14. What is the Secondary NameNode in HDFS, and what is its role?
    • The Secondary NameNode is a helper node that periodically merges the edit log with the fsimage to prevent the edit log from becoming too large. It does not act as a failover NameNode.
15. Explain the process of recovering from a NameNode failure.
    • In the event of a NameNode failure, the cluster can be configured with a standby NameNode that can take over. Additionally, regular backups and the Secondary NameNode can aid in recovery.
16. What is the purpose of the fsimage file in HDFS?
    • The fsimage file in HDFS is a snapshot of the file system namespace. It represents the metadata of the file system at a particular point in time.
17. How does HDFS handle write operations?
    • HDFS supports append operations and allows multiple writers to append data to the same file concurrently. The system ensures consistency and integrity during these operations.
18. Explain the role of the rack awareness feature in HDFS.
    • Rack awareness is a feature that improves data locality by considering the physical location of nodes in racks. It optimizes block placement to minimize data transfer across racks.
19. What is the significance of the balancer in HDFS?
    • The balancer is a tool in HDFS that redistributes data blocks across DataNodes to ensure uniform disk space utilization. It helps maintain balance and prevents hotspots.
20. How does HDFS handle security?
    • HDFS provides security through Kerberos authentication and Access Control Lists (ACLs). It ensures that only authorized users have access to the data.