## Why Do Sparse Files Become Large When Copied with io.Copy()?

Sparse Files Remain Large When Copied Using io.Copy()

When copying sparse files using io.Copy(), they unexpectedly become large at the destination. What can be done to prevent this?

Background

io.Copy() transfers raw bytes, unaware of sparse file properties. Sparse files are stored efficiently, with holes in the data. io.Copy() cannot communicate this hole information, resulting in a loss of sparseness during the copy process.

Solution

To address this issue, you need to bypass io.Copy() and work directly with the syscall package. Here's how:

• Detect Holes: Use the SEEK_HOLE and SEEK_DATA special values in lseek(2) to locate holes and data regions within the sparse file.
• Customize Seek Values: Platform-specific SEEK_HOLE and SEEK_DATA values are necessary. Determine these values for the supported platforms.
• Modify the Read Pattern: Identify data-containing regions and read data from them.
• Consider File Punching: On Linux, you can attempt to punch a hole at the end of the destination file using fallocate(2). If unsupported, write zeroed blocks to simulate a hole.

Additional Considerations

• Filesystem Support: Not all filesystems support holes, such as FAT32. Check if the destination filesystem supports holes.
• Source and Destination Differences: Verify if the source and destination files reside on the same filesystem. If so, consider using syscall.Rename() or os.Rename() to move the file without copying.

For more insights, refer to the Go issue #13548 on writing sparse files in tar archives.

The above is the detailed content of ## Why Do Sparse Files Become Large When Copied with io.Copy()?. For more information, please follow other related articles on the PHP Chinese website!