Why does io.Copy() create large sparse files, and how can you efficiently copy them while preserving their sparseness?

io.Copy() Creates Large Sparse Files: A Comprehensive Guide

Background on File Sparseness

io.Copy() operates at the byte level, transferring raw data between an input and output stream. It lacks the ability to handle file sparseness, which is an optimization technique to store data efficiently by creating holes (empty areas) in files.

Challenges with io.Copy()

Therefore, when copying sparse files using io.Copy(), the destination files become large as there's no mechanism to preserve the hole structure. io.Copy() treats sparse files as if they were filled with data, even though they contain empty areas.

Workaround Using Syscalls

To overcome this limitation, one must bypass io.Copy() and implement file copying manually using the syscall package. Specifically, the SEEK_HOLE and SEEK_DATA values should be used in conjunction with lseek(2) to locate holes and data within the source files.

Platform-Specific Considerations

The SEEK_HOLE and SEEK_DATA values vary across platforms, so it's essential to determine their specific values for the target systems. These values can be obtained from header files or system documentation. For instance, Linux systems typically define these values in /usr/include/unistd.h.

Creating Platform-Specific Files

To ensure platform compatibility, it's recommended to create platform-specific files containing the SEEK_HOLE and SEEK_DATA values. This allows developers to easily switch between different platforms without modifying the core code.

Procedure for Reading Sparse Files

When reading sparse files, the key is to identify data-containing regions and read data from those areas. This involves seeking to the next data region using SEEK_HOLE and then reading data until reaching the next hole using SEEK_DATA.

Transferring Sparse Files

Transferring sparse files as sparse requires an additional step. Depending on the target filesystem, fallocate(2) can be used to create holes in the destination file. If fallocate(2) is not supported, it's possible to fill the hole with zeroed blocks and hope that the operating system converts them to actual holes.

Filesystem Considerations

It's important to note that some filesystems do not support holes. If the target filesystem falls into this category, it's not possible to create sparse files using this technique.

Additional Tips

• Consider using os.Rename() to move files within the same filesystem, avoiding the need for copying.
• Refer to Go issue #13548 for further insights into creating sparse tar files.

The above is the detailed content of Why does io.Copy() create large sparse files, and how can you efficiently copy them while preserving their sparseness?. For more information, please follow other related articles on the PHP Chinese website!