Hadoop Distributed File System (HDFS) Overview
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What is the primary role of the scanner in the Hadoop Distributed File System (HDFS)?

  • To increase the speed of data transfer between nodes.
  • To maintain a backup of data blocks regularly.
  • To compress data for more efficient storage.
  • To identify and correct bad blocks before they are accessed. (correct)

    • Which command would you use to list the contents of a directory named 'lab1' in HDFS?

  • hadoop fs -display lab1
  • hadoop fs -ls lab1 (correct)
  • hadoop fs -get lab1
  • hadoop fs -view lab1

    • What mechanism does HDFS employ to manage disk bandwidth on the DataNode while scanning for errors?

  • Throttling mechanism. (correct)
  • Load balancing.
  • Redundancy controls.
  • Data sharding.

    • Which command is used to upload a file named 'data.txt' into the 'lab1' directory in HDFS?

    <p>hadoop fs -put data.txt lab1</p> Signup and view all the answers

    What kind of errors is the HDFS scanner primarily concerned with detecting?

    <p>Checksum errors in data blocks.</p> Signup and view all the answers

    What is a key architectural goal of HDFS?

    <p>Detecting faults and enabling automatic recovery</p> Signup and view all the answers

    Which of the following describes the typical file size in HDFS?

    <p>Gigabytes to terabytes</p> Signup and view all the answers

    HDFS is designed primarily for which type of data access?

    <p>High throughput for batch processing</p> Signup and view all the answers

    What type of access model does HDFS use for files?

    <p>Write-once-read-many (WORM) access</p> Signup and view all the answers

    How does HDFS address potential hardware failures?

    <p>By implementing quick fault detection and recovery</p> Signup and view all the answers

    Which of the following assumptions is NOT part of HDFS design?

    <p>Support for small datasets</p> Signup and view all the answers

    What is a relaxed requirement of HDFS compared to POSIX file systems?

    <p>Coherency model for data modification</p> Signup and view all the answers

    Which characteristic most accurately reflects HDFS architecture?

    <p>Designed for handling massive data storage</p> Signup and view all the answers

    What is the primary benefit of keeping computation close to where the data is located?

    <p>It minimizes network congestion and increases throughput.</p> Signup and view all the answers

    What role does the NameNode play in the HDFS architecture?

    <p>It manages the HDFS namespace and metadata.</p> Signup and view all the answers

    How does HDFS handle large files?

    <p>It splits them into large blocks typically of 128MB.</p> Signup and view all the answers

    What is the replication factor in HDFS?

    <p>The number of replicas maintained for a file.</p> Signup and view all the answers

    What happens when a file is opened for writing in HDFS?

    <p>A lease is granted to the writing client for exclusive access.</p> Signup and view all the answers

    What communication do DataNodes have with the NameNode?

    <p>They send heartbeats periodically.</p> Signup and view all the answers

    Which of the following describes the default block placement strategy in HDFS?

    <p>One replica is stored on a different rack.</p> Signup and view all the answers

    What is the purpose of the hflush() function in HDFS?

    <p>To guarantee data visibility to new readers.</p> Signup and view all the answers

    How is the distance between two nodes in HDFS defined?

    <p>By the sum of their distances to their closest common ancestor.</p> Signup and view all the answers

    What occurs if a DataNode fails in HDFS?

    <p>File replication handles the loss automatically.</p> Signup and view all the answers

    What is the main trade-off in HDFS block placement?

    <p>Minimizing write cost versus maximizing availability and read bandwidth.</p> Signup and view all the answers

    What function does the HDFS client perform?

    <p>Exports the HDFS file system interface.</p> Signup and view all the answers

    Why does the last block's content in HDFS might not be visible until the file is closed?

    <p>HDFS trades some visibility semantics for performance.</p> Signup and view all the answers

    What is the primary function of the CheckpointNode in HDFS?

    <p>To merge existing checkpoints and journal into a new checkpoint</p> Signup and view all the answers

    How does HDFS handle a corrupted block when reading a file?

    <p>The client notifies the NameNode and fetches another replica</p> Signup and view all the answers

    What does HDFS federation allow in large clusters?

    <p>It partitions the data storage by separating namespaces</p> Signup and view all the answers

    What is one major advantage of implementing HDFS High Availability (HA)?

    <p>It ensures continuous service with minimal interruption during NameNode failures</p> Signup and view all the answers

    What role does ZooKeeper play in HDFS High Availability?

    <p>It oversees the failover process between NameNodes</p> Signup and view all the answers

    What is a critical requirement for HDFS High Availability?

    <p>NameNodes must share highly available storage for journals</p> Signup and view all the answers

    What is the purpose of the balancer in HDFS?

    <p>It redistributes blocks to balance load among DataNodes</p> Signup and view all the answers

    What happens to the replica of a block when a DataNode fails?

    <p>The NameNode marks the replica as unavailable or corrupt</p> Signup and view all the answers

    What does the block scanner do in HDFS?

    <p>It verifies the integrity of blocks on a DataNode</p> Signup and view all the answers

    What is the role of the failover controller in HDFS HA?

    <p>It initiates failover when the Active NameNode fails</p> Signup and view all the answers

    Which statement about block locations in HDFS is true regarding BackupNode?

    <p>It stores a read-only version of the NameNode's namespace state</p> Signup and view all the answers

    What causes an ungraceful failover in HDFS?

    <p>Slow network or network partition issues</p> Signup and view all the answers

    How can block caching improve the performance of HDFS?

    <p>It allows applications to schedule tasks on the DataNode with cached blocks</p> Signup and view all the answers

    Study Notes

    Hadoop Distributed File System (HDFS) Overview

    • HDFS is a distributed file system for commodity hardware
    • Designed for large datasets and batch processing
    • Similar to POSIX but with relaxed requirements
    • Scalable to 100+ PB storage and thousands of servers
    • Supports close to a billion files and blocks

    HDFS Assumptions and Goals

    • Commodity Hardware: Hardware failures are expected; fault detection and recovery are essential.
    • Streaming Data Access: Optimized for batch processing, not interactive use. Some POSIX semantics are relaxed for higher throughput.
    • Large Datasets: Typical file sizes are gigabytes to terabytes. High aggregate data bandwidth and scaling to many nodes are priorities.
    • Simple Coherency Model: Write-once-read-many (WORM) access. Files are not modifiable except for appends and truncates, simplifying coherency issues.
    • Moving Computation is Cheaper than Moving Data: Prefer executing computation closer to the data to minimize network congestion and enhance throughput.

    HDFS Architecture

    • Master-Worker Architecture: One NameNode and many DataNodes.
    • NameNode: Manages the file system metadata (file system tree and file/directory metadata). Stores persisted information. Loads entire namespace into memory at startup and can be reconstructed from DataNodes.
    • DataNode: Stores actual data blocks. Blocks are replicated (typically 3 times). DataNodes send periodic heartbeats and block reports to the NameNode.
    • HDFS Client: File system interface accessed by applications. Hides the distributed nature of the system.

    HDFS Resilience

    • NameNode: Persists checkpoints and journal logs to disk for recovery. A BackupNode has a read-only, synchronized copy of the namespace state.
    • DataNode: Data integrity checked with checksums. Data loss triggers replica retrieval and creation on other DataNodes. NameNode tracks unavailable/corrupted replicas.

    HDFS Optimizations

    • Block Caching: Frequently accessed blocks can be cached in DataNode memory. Configurable on a per-file basis.
    • HDFS Federation: Allows scaling the cluster with multiple NameNodes. Each NameNode manages a portion of the namespace, and block pools are not partitioned across NameNodes.
    • HDFS High Availability (HA): Active-standby NameNode configuration. Standby takes over quickly on active failure (under ~1 minute).
    • Balancer: Redistributes blocks to balance DataNode workloads to enhance locality and minimize strain on overutilized nodes.
    • Block Scanner: Periodic verification of data blocks on DataNodes to catch and fix corruption.

    HDFS Usage

    • Hadoop FS Shell: Commands for direct interaction with HDFS (e.g., hadoop fs -mkdir, hadoop fs -ls, hadoop fs -put, hadoop fs -cat).

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    Description

    Explore the fundamentals of the Hadoop Distributed File System (HDFS), designed for large datasets and batch processing. Understand its architecture, key features, and operational assumptions that prioritize fault tolerance and data accessibility. This quiz will test your knowledge on how HDFS optimizes data management across distributed systems.

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