What Is the Difference Between fdatasync and fsync? When to Use Each One?

When working with file operations, we often come across the functions fdatasync and fsync. Both are related to file data synchronization. However, while their functionality seems similar, there are important differences between the two. Understanding these differences helps us select the most appropriate function to achieve data persistence in programming.

1. Basic Overview

The fsync function

The fsync function flushes all data of the file referenced by a file descriptor—including file metadata—from the kernel’s cache to disk. This ensures that all file content is safely written to disk, preventing data loss in case of a crash or power failure. fsync synchronizes not only file content but also metadata such as modification times and permissions.

The fdatasync function

The fdatasync function is similar to fsync, but it only synchronizes the file’s data, not its metadata. In other words, it ensures that the actual file contents are written to disk but excludes metadata like modification times or permissions. This typically makes fdatasync more efficient, as it reduces the amount of data that needs to be synchronized.

2. Key Differences

• What gets synchronized:

  • fsync synchronizes both data and metadata, ensuring all file information is written to disk.
  • fdatasync synchronizes only the data, excluding metadata.

• Performance impact:
  Because fsync synchronizes more information, its performance is usually lower than fdatasync. If metadata synchronization is not required, fdatasync offers better efficiency.

• Use cases:

  • fsync is best suited for scenarios requiring both file data and metadata (such as timestamps and permissions) to be persistent. It is commonly used in applications requiring strict data integrity, such as databases and file system operations.
  • fdatasync is suitable for cases where only file content matters and metadata synchronization can be ignored. Examples include certain file transfer applications or logging programs where only the data itself matters, not modification times or permissions.

3. Use Case Analysis

When to use fsync

fsync is typically used when complete consistency of a file is required, such as:

• Database operations: Databases often call fsync after writing data to ensure that both data files and related metadata (like transaction logs and timestamps) are safely persisted. This prevents data loss in case of crashes or power failures.
• File system operations: When a file system must guarantee file integrity—especially when involving metadata such as permissions and timestamps—it relies on fsync.

When to use fdatasync

fdatasync is usually applied in performance-sensitive situations where only the file’s content data needs to be synchronized. For example:

• Log file writing: When writing logs, applications typically care only about the content, not metadata. Using fdatasync improves write performance.
• Video and audio streams: In handling media streams, only the file content needs to be written to disk. Metadata such as timestamps is less important, so fdatasync improves efficiency.

4. Performance Comparison

From a performance perspective, fdatasync is generally more efficient than fsync because it synchronizes less data. For large files in particular, fsync may introduce longer delays, while fdatasync completes synchronization faster.

That said, performance differences also depend on factors such as disk type, operating system implementation, and the specific scenario. In high-performance storage systems, the advantage of fdatasync may not be significant.

5. Conclusion

• fsync synchronizes both file data and metadata, ensuring no data loss during crashes or power failures. It is ideal for scenarios with strict consistency requirements, such as databases and file systems.
• fdatasync synchronizes only the file’s data, excluding metadata. It is suitable for use cases focused solely on content, such as logging or media processing.

By understanding the differences between the two and choosing the appropriate function for the task, we can manage file synchronization more effectively in programming.

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