7.2 Distributed file systems

Distributed file systems are a crucial component of modern computing, enabling shared access to files across networks. They provide transparency, scalability, and fault tolerance, allowing users to interact with remote files as if they were local.

These systems face challenges like maintaining data consistency, dealing with network limitations, and ensuring security. Popular implementations like NFS and HDFS showcase different approaches to addressing these challenges, balancing performance and reliability in distributed environments.

Distributed File Systems: Concepts and Design

Key Principles and Features

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• Distributed file systems (DFS) allow multiple clients to access shared files and resources over a network providing a unified view of data across multiple servers
• Transparency hides the complexities of distribution from users encompassing:
  • Location transparency masks physical storage locations
  • Access transparency provides uniform operations regardless of client location
  • Naming transparency maintains consistent file naming across the system
• Scalability allows addition of new storage nodes and clients without significant performance degradation or system reconfiguration
• Fault tolerance mechanisms ensure data availability and system reliability during hardware failures or network partitions
• Consistency models define how changes to data propagate and become visible across multiple clients balancing between strong consistency and high performance

Caching and Security Strategies

• Caching strategies reduce network traffic and improve access latency by storing frequently accessed data closer to clients
  • Client-side caching caches data on individual client machines
  • Server-side caching caches frequently accessed data on file servers
• Security considerations protect data integrity and confidentiality across distributed environments including:
  • Authentication verifies user identities (Kerberos)
  • Authorization controls access to files and directories
  • Encryption secures data in transit and at rest (SSL/TLS)

Distributed File Systems: Advantages vs Challenges

Advantages of Distributed File Systems

• Improved scalability allows seamless expansion of storage capacity and performance by adding new nodes to the system
• Enhanced availability and fault tolerance provide continuous access to data even during hardware failures or network issues
  • Replication across multiple nodes ensures data redundancy
  • Automatic failover mechanisms maintain system operation
• Increased performance through parallel access and load balancing across multiple servers
  • Concurrent read/write operations on different nodes
  • Distribution of workload among available resources

Challenges in Distributed File Systems

• Maintaining data consistency across distributed nodes leads to complex synchronization mechanisms and potential conflicts
  • Concurrent updates may result in inconsistent states
  • Resolving conflicts requires sophisticated algorithms (vector clocks)
• Network latency and bandwidth limitations impact performance and responsiveness especially for geographically dispersed systems
  • High latency in wide-area networks affects real-time operations
  • Limited bandwidth constrains data transfer rates
• Implementing effective security measures proves challenging due to the distributed nature of data and secure communication across untrusted networks
  • Ensuring end-to-end encryption without compromising performance
  • Managing access control across multiple administrative domains
• Management complexity increases requiring sophisticated tools and protocols for monitoring backup and recovery across multiple nodes
  • Coordinating maintenance activities across distributed components
  • Implementing efficient backup strategies for large-scale systems

Architecture of Distributed File Systems: NFS and HDFS

Network File System (NFS) Architecture

• NFS consists of clients servers and a protocol for communication allowing transparent access to remote files as if they were local
• Uses Remote Procedure Calls (RPCs) for client-server communication supporting stateless operation for improved fault tolerance
• Client-side caching improves performance but requires cache coherence mechanisms
  • Write-through caching ensures immediate updates to the server
  • Callback-based invalidation notifies clients of changes
• NFS versions evolve to address performance and security concerns
  • NFSv4 introduces stateful operation and integrated security

Hadoop Distributed File System (HDFS) Architecture

• Designed for storing and processing large datasets across clusters of commodity hardware
• HDFS architecture includes:
  • NameNode for metadata management storing file system namespace and block locations
  • Multiple DataNodes for storing actual data blocks typically in 64MB or 128MB sizes
• Employs a write-once read-many access model optimized for large sequential reads and writes
• Implements data replication across multiple DataNodes to ensure fault tolerance and high availability
  • Default replication factor of 3 with configurable settings
  • Rack-aware replica placement for improved reliability
• HDFS client interacts with NameNode for metadata operations and directly with DataNodes for data transfer
  • Clients can read data from the nearest replica
  • Write operations involve a pipeline of DataNodes for replication

Consistency and Replication in Distributed File Systems

Consistency Models and Strategies

• Consistency models range from strong consistency (linearizability) to weaker models like eventual consistency each with trade-offs between performance and data coherence
• Read and write quorums ensure operations are performed on a sufficient number of replicas to maintain consistency and availability
  • Read quorum (R) + Write quorum (W) > Total replicas (N) for strong consistency
• Lease-based consistency protocols provide time-bounded guarantees on data freshness and help manage cache coherence across distributed clients
  • Clients acquire leases for exclusive or shared access to data
  • Leases expire after a predetermined time reducing the need for constant communication
• Eventual consistency models prioritize availability and partition tolerance over immediate consistency requiring careful application design to handle potential inconsistencies
  • Used in large-scale distributed systems (Amazon Dynamo)
  • Conflicts resolved through techniques like vector clocks or last-writer-wins

Replication and Conflict Resolution

• Replication strategies balance data availability fault tolerance and performance with techniques such as:
  • Primary-backup replication designates a primary copy for writes
  • Quorum-based replication requires agreement among a subset of replicas
• Conflict resolution mechanisms handle concurrent updates from multiple clients employing techniques like:
  • Versioning maintains multiple versions of data (Git)
  • Last-writer-wins policies prioritize the most recent update
• Optimistic replication strategies allow improved performance by permitting updates to propagate asynchronously at the cost of potential conflicts
  • Suitable for scenarios with infrequent conflicts (collaborative editing)
  • Requires efficient conflict detection and resolution mechanisms