When Does flush() Not Guarantee Disk Writing in Python?

Understanding Python's File Objects: The Mystery of flush()

Python's documentation for File Objects states that the flush() method does not necessarily write data to disk. This may seem counterintuitive, considering the assumption that flushing forces data to be written to the disk. However, the reality is more complex due to the presence of multiple buffering layers.

Buffering and Disk Writing

File writing typically involves two levels of buffering: internal buffers and operating system buffers. Internal buffers are created by Python and are designed to enhance performance by avoiding frequent system calls for every write. Data is written to the internal buffer, and when it fills up, it is transferred to the operating system buffer using system calls.

The operating system maintains its own buffers. This means that data flushed to the internal buffer may not immediately be written to the disk. Instead, it may reside in the operating system buffers, increasing the risk of data loss in the event of a system failure.

The Role of flush() and os.fsync()

To ensure that data is reliably written to disk, the flush() and os.fsync() methods are available. The flush() method forces data from the internal buffer to the operating system buffer, ensuring that other processes accessing the file can read it. However, it does not guarantee that the data has reached the disk.

To write data directly to the disk, bypassing the operating system buffers, the os.fsync() method must be called. This method ensures that all operating system buffers are synchronized with the storage media, effectively transferring the data to disk.

When to Use flush() and os.fsync()

In most cases, using either flush() or os.fsync() is unnecessary. However, they become crucial in scenarios where it is critical for data to be reliably stored on the disk. For example, when dealing with sensitive or critical files, or in situations where unexpected system shutdowns or crashes may occur, both methods should be employed to ensure data integrity.

Additional Note

It is important to consider the widespread use of cached disks in modern systems. These disks introduce additional layers of caching and buffering, which may further complicate the behavior of flush() and os.fsync(). Consult operating system documentation for specific details on how cached disks affect file writing operations.

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