SCP vs RSYNC - Which one to use in Production?

This tutorial covers the basics and differences between SCP and Rsync, when to use each, their performance, security considerations, practical examples, and tips for efficient file transfers, helping you choose the right tool for your needs.

SCP (Secure Copy Protocol) and Rsync are both widely used command-line tools for transferring files between computers in a network.

Both tools are essential for system administration, offering different advantages depending on the specific requirements of file transfer tasks, such as security, speed, and the volume of data being transferred.

 

Key differences between SYNC and RSYNC

1. Definition

SCP is a part of the SSH (Secure Shell) suite, providing a secure and authenticated method for copying files between a local and a remote host or between two remote hosts. It encrypts the files being transferred, ensuring that the data is secure in transit.

Rsync, on the other hand, is a utility that provides fast, incremental file transfer by transferring only the differences between the source and the destination. It can work over SSH to secure the transferred data and is often used for backing up and synchronizing files across systems due to its efficiency and flexibility.

 

2. Syntax

The basic syntax for SCP commands is as follows:

scp [OPTIONS] [[user@]source_host:]file1 [[user@]destination_host:]file2

• OPTIONS: Various options can be applied to modify the behavior of SCP, such as -P (specify the remote host ssh port), -p (preserve file modification and access times), and -r (recursively copy entire directories).
• user@: Optional. Specifies the username on the remote host. If not provided, SCP assumes the same username as the local system.
• source_host: The hostname or IP address of the source system. If omitted, SCP assumes the file is on the local system.
• destination_host: The hostname or IP address of the destination system.
• file1, file2: The source and destination file paths, respectively.

For Example:

scp /path/to/local/file.txt user@remote_host:/path/to/remote/directory

The basic syntax for Rsync is:

rsync [options] [[user@]source_host:]source_path [[user@]destination_host:]destination_path

• [options]: This includes all the Rsync-specific options like -a (archive mode), -v (verbose), -z (compress data during the transfer), etc.
• [[user@]source_host:]: Optionally specifies the username and host for the source location. If omitted, Rsync assumes a local path. Including the host implies remote access, typically secured over SSH.
• source_path: The path to the source file or directory. This can be a local path or a path on the source host if specified.
• [[user@]destination_host:]: Optionally specifies the username and host for the destination location. Similar to the source host, omitting this implies a local destination path.
• destination_path: The path where the files or directories will be copied to. This can be on the local system or on the specified destination host.

For Example:

rsync -avz /local/dir/ user@remote_host:/remote/dir/

 

3. Security

• SCP: It operates over SSH (Secure Shell) by default, providing a secure channel with encryption and authentication. The security level is tied to SSH, making it highly secure for transferring files.
• Rsync: By default, rsync uses the Rsync protocol but it also capable of running over SSH. rsync provides the same level of security as SCP when configured to do so. However, rsync can also run in daemon mode without SSH, which might be less secure if not properly configured with other security measures.

To use Rsync with SSH, you can simply add the -e (or --rsh) option to your Rsync command, specifying ssh as the remote shell:

rsync -avz -e ssh /path/to/source/ user@remote_host:/path/to/destination/

In this command:

• -a is for archive mode, which preserves permissions, timestamps, etc.
• -v enables verbose mode.
• -z enables compression during data transfer.
• -e ssh tells Rsync to use SSH for the data transfer.

If your remote host is listening on a non-standard SSH port, you can specify the port with the -e option like so:

rsync -avz -e 'ssh -p 2222' /path/to/source/ user@remote_host:/path/to/destination/

Here, -p 2222 specifies that SSH should connect to port 2222 on the remote host.

For more complex setups or enhanced security, you can pass additional SSH options using the -e flag. For example, to disable strict host key checking, you can use:

rsync -avz -e 'ssh -o StrictHostKeyChecking=no' /path/to/source/ user@remote_host:/path/to/destination/

 

4. Performance

• SCP: While secure and straightforward, SCP is not always the most efficient, especially for large datasets. It lacks the ability to compress data during transfer (though SSH compression can be enabled) and always copies files in their entirety, which can be slow and bandwidth-intensive for large files.
• Rsync performs a more complex operation by comparing the source and destination files before transferring. This comparison step can lead to better overall performance in terms of bandwidth usage but might be more CPU intensive during the comparison process.

Let's run a simple test, I havea file of around 1GB. We will try to transfer this file from client to server (10.39.251.204) using both scp and rsync and observe the CPU impact. I am going to use pidstat to capture the CPU usage of specific PID over the period of time taken to transfer the file:

Here is my script to capture the CPU usage:

#!/bin/bash

#scp -r /opt/shrey/ZTS_1.19.66/INSTALL_MEDIA/IMAGES/ncm-rocky-1.15.49.tar.gz 10.39.251.204:/opt/deepak/ &
rsync -avz /opt/shrey/ZTS_1.19.66/INSTALL_MEDIA/IMAGES/ncm-rocky-1.15.49.tar.gz 10.39.251.204:/opt/deepak/ &
SCP_PID=$!

MAX_CPU=0
while kill -0 $SCP_PID 2> /dev/null; do
    CPU_USAGE=$(pidstat -p $SCP_PID 1 1 | grep -v Linux | grep -v '#' | awk '{print $8}')
    MAX_CPU=$(echo "$CPU_USAGE $MAX_CPU" | awk '{if ($1 > $2) print $1; else print $2}')
    sleep 1
done

echo "Peak CPU Usage: $MAX_CPU%"

In this script I have added both rsync and scp command, which I will execute one after the other:

For scp I got the PEAK CPU Usage as 9% while for rsync I received peak CPU usage of 101% so as explained, rsync may help you save some bandwidth but it will require more CPU for the processing.

For SCP:

For RSYNC:

 

5. Scalability

• SCP: Scaling with SCP can be challenging due to its lack of incremental copying and synchronization features. For large-scale operations, managing bandwidth and storage efficiently becomes difficult.
• Rsync: Rsync is better suited for scaling, thanks to its incremental update feature and the ability to efficiently synchronize directories and files across multiple locations. It's more adaptable to various network conditions and storage configurations.

 

6. Handling Interrupted Transfers

• SCP does not have a built-in mechanism to resume interrupted transfers. If a transfer is stopped, it needs to be restarted from the beginning.
• Rsync has the ability to resume interrupted transfers, making it more reliable for transferring large files or over unreliable connections.

Let's check with an example:

We will attempt to perform scp using same file and destination multiple times. As you can check in the output, each transaction took around the same time i.e. 1.5s, this is because scp will attempt to copy the entire file with every attempt.

Now let's re-attempt the same transaction using rsync. Here as you can observe the difference, the first transaction took around 3.8s while the subsequent requests take around 0.3s as the destination already contains the file and there is nothing to transfer.

 

7. Transfer Efficiency and Speed

• SCP is generally faster for single files or when transferring data for the first time because it copies the entire content without checking the differences.
• Rsync excels in efficiency, especially for subsequent transfers, by only copying the differences between the source and the destination, which can significantly reduce the transfer time and bandwidth usage.

We attempt to transfer file using scp which took around 1.5s

now with the same file rsync is taking around 3.8s

So this proves that SCP is faster for single file transaction.

 

Decision Factors

• Use SCP if:
  • You're transferring files for the first time or infrequently - SCP is ideal for one-off transfers where efficiency gains from incremental updates are not applicable.
  • Simplicity is key - If you prefer straightforward command syntax without needing to manage file synchronization complexities, SCP offers an easier approach.
  • Security is a top priority - Although both SCP and Rsync can use SSH for secure transfers, SCP's integration with SSH makes it a robust choice for secure file transfers without additional configuration.
  • You don't need to sync directories - For direct file transfers without the need to check for updates or changes in the files or directories, SCP is perfectly suitable.
  • Bandwidth and transfer size are not concerns - If your network can handle the load and you're not worried about optimizing for bandwidth, SCP's method of transferring the whole file without differential data checks works well.
• Use Rsync if:
  • You're regularly updating or syncing files - Rsync is designed to update files by transferring only the changes, making it highly efficient for regular backups or synchronization tasks.
  • Bandwidth efficiency is crucial - In environments where network bandwidth is limited or costly, Rsync's ability to transfer only modified parts of files can significantly reduce network load.
  • You need to resume interrupted transfers - Rsync can pick up where it left off in the event of a connection interruption, saving time and bandwidth by not starting transfers from scratch.
  • File permissions and detailed attributes matter - Rsync offers granular control over how file permissions, ownership, and timestamps are handled, ensuring that file metadata is preserved or modified according to your needs.
  • You're dealing with large volumes of data or numerous files - For syncing large directories or handling vast amounts of data, Rsync's incremental transfer capability and options for recursive directory handling make it a superior choice.
  • You need to manage file exclusions or patterns - Rsync supports complex include/exclude patterns for transfers, giving you precise control over which files or directories are synchronized.