M09-File Systems

CS 439 Principles of Computing Systems

File System Design Elements

Key components of file system design include:

• API: A set of methods for interacting with the file system that abstracts the underlying complexities, enabling a uniform interface for file operations.

• Naming Schemes: Mechanisms used to identify files and directories, which can range from simple flat structures to complex hierarchical trees that improve organization and retrieval.

• User Interfaces: Interfaces designed for users to interact with the file system, enabling intuitive navigation, searches, and file manipulation.

• Permissions and Security: Systems regulating access to files and directories, ensuring that unauthorized users cannot access sensitive information. This typically includes mechanisms like access control lists (ACLs) and mandatory access controls (MAC).

• Accounting and Reliability: Features that monitor resource usage (e.g., disk space for users) and ensure the integrity and availability of data over time.

• Backup Strategies: Methods employed to create copies of data to protect against loss or corruption, including full, incremental, and differential backups.

• Performance Considerations: Factors that affect how efficiently the file system interacts with hardware, such as read/write speeds and the impact of file fragmentation.

• Data Layout and Storage Units: How data is physically organized on storage devices, including block size and how files are segmented.

• Compression Techniques: Methods for reducing the size of files to save space, which can influence reading and writing speeds, as well as resource usage.

• Versioning Features: Capabilities that allow users to maintain multiple versions of a file, including history tracking for retrieval purposes.

• Logging for Reliability: Implementing logging mechanisms to record operations, which aids in recovery processes during failures.

File System Examples

MacOS APFS

• API Methods: Includes essential methods such as read, write, seek, and create that enable interaction with the file system.

• Naming: Features a hierarchical directory structure that provides a clear organization of files and directories.

• User Interface: Offers intuitive navigation features that simplify user interaction, including search functionalities and visual representations of files.

• Permissions: Implements the rwxrwxrwx model which grants varying levels of access to different users.

• Accounting: Supports quotas that help manage disk space usage per user or group to prevent any one user from monopolizing resources.

• Reliability: Utilizes a log-based structure that ensures data durability and integrity even in the event of system crashes.

• Backup: Provides both full and incremental backup options, enhancing data protection strategies.

• Security: Features file and volume-level encryption to secure data from unauthorized access.

• Resilience and Performance: Optimized for SSDs, offering features like cloning (creating copies without additional disk space use) and shared data access that enhance performance.

• Versioning and Implementation: Integrates Time Machine for seamless backups and uses Zlib for efficient file-level compression.

ZFS

• API Methods: Similar core methods as APFS, including read, write, seek, and create for consistent functionality.

• Naming: Utilizes a hierarchical directory format for better data organization.

• User Interface: Includes comprehensive management tools that facilitate the administration of file systems.

• Permissions: Adopts the rwxrwxrwx access control model for fine-grained security management.

• Accounting: Implementation of quotas for effective resource management across users and groups.

• Reliability: Employs Mirroring and RAID-Z paradigms to enhance data protection and uptime.

• Backup: Supports both full and incremental backup strategies to ensure data recoverability.

• Security: Incorporates block-level encryption mechanisms to safeguard information.

• Performance and Versioning: Utilizes snapshots for efficient version management without significant performance compromise.

• Implementation: Employs block-level compression that optimizes storage use without loss of data integrity.

The Flat File System

Overview

Implements a straightforward file system on disk without advanced naming controls. This type of file system allows upper software layers to define special files and manage attributes such as location, size, modified dates, etc.

Examples of Allocation

1. Contiguous Allocation

   • Data Structures: Files are allocated on contiguous sectors of the disk.

   • Pros: Fast read/write operations due to sequential access patterns.

   • Cons: Can lead to disk space fragmentation, complicating file growth and necessitating slow disk compaction processes.

2. Linked Allocation

   • Structure: Sectors are linked into a list, where each sector points to the next.

   • Pros: Files can grow or shrink with ease, eliminating external fragmentation.

   • Cons: Random access requires traversal of the linked list; sectors must reserve additional space for links, which can reduce storage efficiency.

3. File Allocation Table (FAT)

   • Method: Utilizes linked allocation, but maintains the links in a separate table for management ease.

   • Benefits: Simplifies overall management and helps prevent external fragmentation issues.

4. Indexed Allocation

   • Structure: Utilizes indices with both direct and indirect indexing techniques.

   • Performance: Improves access efficiency and reduces fragmentation by minimizing the need to traverse linked lists.

Permissions and Features

Each file corresponds to an inode, containing essential attributes including:

• User ID (identifying file owner)

• Timestamps (accessed, modified, created) for tracking

• Protection bits that define the access permissions

• Reference count indicating how many instances of the file exist

• File type and size

Free Space Management

• Bit Vectors: Represent sectors as a series of bits, where each bit indicates availability or allocation status.

• Linked Lists: Maintain lists of free sectors, allowing efficient tracking of unallocated space.

• Challenges include effectively managing metadata for free spaces while minimizing overhead.

Naming Considerations

Files are identified by unique file IDs assigned by the file system. A naming layer is responsible for translating these IDs into a human-readable format, facilitating user interaction.

Advanced File Concepts

• Sparse Files: Files may contain empty "holes" that optimize memory usage but can complicate retrieval in sequential or linked storage systems.

• File Block: Varies by file system implementation (e.g., Linux, APFS, ZFS) with trade-offs between storage efficiency and performance considerations.

• File Extent: Groups of consecutive blocks to expedite access operations, utilized in systems like EXT4 and APFS.

SSD Optimization Techniques

• Low Latency: SSDs eliminate concerns related to mechanical disk seeks or rotational delays, enabling faster access times.

• TRIM Command: Assists in managing write amplification and garbage collection processes to maintain performance over time.

Reliability Issues in File Systems

The Reliability Problem

Operations often involve several independent disk writes, leading to consistency problems if interrupted (such as during power loss).

Logging

Implementing logs can ensure consistency and recovery of file system operations following interruptions. Logging provides performance benefits by reducing write latencies while increasing write counts, though challenges arise on SSDs due to their architecture.

Failure Recovery

File systems can recover by reading logs to ensure consistency and executing necessary updates when discrepancies are detected.

Implementation of Logging

Logs must be managed to prevent indefinite growth and ensure efficient data throughput.

Shadow Paging

Offers atomic modifications with minimal increases in bandwidth consumption but performs less efficiently compared to logging due to non-sequential writes involved in its processes.

Log-Based File Systems

Designed to focus on reliability by preserving the overall structure of the file system rather than just the contents of individual files.

Log-Structured File Systems

These systems prioritize performance and are particularly suitable for SSDs, focusing on efficiency and minimizing fragmentation issues.

Conclusion

As file systems continue to evolve, the reliability challenges differ between HDD