Here are a few options for an English title in a question format, capturing the essence of the article: **Short & Direct:** * **How to Preserve Sparse Files When Using io.Copy() in Go?** * **Can

Sparse Files Inflated by io.Copy()

Background

io.Copy() transfers raw bytes between an io.Reader and an io.Writer. However, it lacks the ability to handle the location of holes within files, which is considered side-channel information that syscalls like read(2) conceal from users.

Addressing the Issue

To manage sparse files, you must employ the syscall package and manually manipulate parameters. The SEEK_HOLE and SEEK_DATA values from lseek(2) are essential for this process.

Implementation

• Obtain SEEK_HOLE and SEEK_DATA values for your supported platforms.
• Acquire the file descriptor from the opened file using os.File's Fd() method.
• Use syscall.Seek() to detect data regions and retrieve data from them.

Preserving Sparseness

Transferring files as sparse is more complex. On Linux, you can attempt to punch holes using fallocate(2) with FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE. If that fails, write zeroed blocks to the destination to encourage the OS to convert them to a hole.

Precautions

• Some filesystems, such as FAT, do not support holes.
• Consider using syscall.Rename() or os.Rename() to move files between directories within the same filesystem without copying data.

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