Understanding Why Rsync Runs Through All Files but Fails to Copy Them

Rsync is a powerful and widely-used file copying tool for UNIX-like systems. However, it is not always as straightforward as it might seem, especially when it comes to file back-ups. In this article, we will explore the reasons why rsync might look at all the files but fail to copy them. We will also discuss the implications of improper configuration and potential solutions to these issues.

The Basics of Rsync

Rsync uses the -a (archive) option to enable the preservation of file attributes and permissions. Without this option, rsync will examine the files but will not initiate the transfer. Therefore, the basic command might look like:

[bash]rsync [source] [destination]

This behavior can be misunderstood, leading to frustration when no files are copied. It's crucial to understand that rsync checks the files to ensure that only those that need to be updated are copied.

Common Reasons for Rsync Not Copying Files

There are several reasons why rsync might run through all the files but fail to copy them:

File Permissions: Rsync needs the correct permissions to read and write files. Ensure that you have the necessary permissions to access and copy the files. Local vs. Remote Files: When copying files over a network, time synchronization between the source and destination systems is essential. Any time difference can lead to rsync not detecting changes and thus not copying the files. File Already Up-to-Date: If the files already exist on the destination and are not older than the source, rsync will not copy them. This behavior is designed to avoid unnecessary data transfer.

Time Synchronization and Its Impact

Proper time synchronization between different systems is crucial for accurate file comparisons. If the clocks are not in sync, rsync might incorrectly conclude that files have not changed, leading to no copies being made. To mitigate this, you can use tools like nTP (Network Time Protocol) to synchronize the clocks of the systems involved.

[bash]sudo service ntpd start

Handling Large Numbers of Files

When dealing with large numbers of files, such as over 40 million files, rsync can become quite resource-intensive. Rsync must first check all the files to determine which ones need to be copied, which can take a considerable amount of time.

Note: According to some estimates, checking 40 million files could take 8 to 15 days. Once the check is complete, rsync will then copy any new or updated files.

Resuming after Network Dropouts

Network dropouts or interruptions can cause rsync to abort its transfer. Unfortunately, rsync does not provide a built-in mechanism for resuming the transfer from where it left off. This means that when the network reconnects, rsync will have to start the entire process over again.

Note: Storing a checkpoint or using a more robust backup solution with resume capabilities may mitigate these issues.

Conclusion

Understanding why rsync runs through all files but fails to copy them is crucial for effective file management and backup procedures. Paying attention to file permissions, time synchronization, and the specifics of large file transfers can significantly enhance the reliability and efficiency of your backup processes. By implementing best practices and using the right tools, you can ensure that your backups run smoothly and effectively.