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English
In file system programming, fsync() and fdatasync() are two system calls used to ensure that file data is persisted to disk. While they serve a similar purpose of writing file buffer data to disk, they differ in the specific content they execute and their performance. In this article, we will discuss in detail the differences between these two system calls and how to choose which one to use in different scenarios.
fsync()
The fsync() function is used to synchronize both the file data and metadata (such as modification time, permissions, etc.) associated with a specified file descriptor to the disk. This operation ensures that both the file's data and its related metadata have the latest copies on the disk. After calling fsync(), the operating system waits until all data is written to the disk and the metadata has been updated.
fdatasync()
The fdatasync() function is more streamlined, as it ensures that only the actual file data (not metadata) is written to disk. In other words, fdatasync() guarantees the integrity of the file's contents but does not force the synchronization of its metadata, such as modification times and permissions, which may not be updated.
fsync() ensures that both the file content and its metadata (such as modification times, permissions, etc.) are synchronized to the disk.
fdatasync() only focuses on the synchronization of the file content, and does not concern the update of the file's metadata.
Since fdatasync() does not need to synchronize metadata, its execution is relatively faster. In contrast, fsync() has to wait for the metadata to be written, so it is typically slower than fdatasync(). For applications that care only about the data content and do not need metadata updates, fdatasync() is more efficient.
fsync() may incur unnecessary system call overhead in some cases because it forces the update of all file information.
fdatasync() only performs data synchronization, so its cost is lower, especially when dealing with large amounts of data, leading to better performance.
If the application only cares about the persistence of file content and does not need to update file metadata, fdatasync() is a better choice. For example, in scenarios like writing log files, the main concern is whether the log content has been written to the disk, not whether the metadata is modified.
If the application requires both the file's content and metadata (e.g., modification time, permissions) to be synchronized to the disk, then fsync() should be used. For instance, in database systems or file management systems, it may be necessary to ensure consistency between file contents and metadata like timestamps.
If performance is a critical factor and file metadata updates are not important, fdatasync() is the more suitable option. For example, during batch data processing or when writing large files, using fdatasync() can improve efficiency and reduce unnecessary overhead.
fsync() and fdatasync() both ensure that file data is written to disk, guaranteeing file persistence, but they differ in the scope of synchronization.
fsync() synchronizes both data and metadata, whereas fdatasync() only synchronizes data, and metadata may not be updated.
In performance-critical scenarios where metadata synchronization is not necessary, using fdatasync() can provide better performance.
In scenarios where both data and metadata need to be consistent, fsync() is the better choice.
When choosing which function to use, you should base your decision on the specific application scenario and the required level of data integrity.
