scp stalled while copying large files
Categories:
Troubleshooting SCP Stalls with Large File Transfers
Learn why SCP (Secure Copy Protocol) transfers of large files might stall and discover effective strategies to diagnose and resolve these common issues, ensuring reliable data movement.
SCP (Secure Copy Protocol) is a widely used command-line utility for securely copying files and directories between local and remote hosts. While generally robust, users often encounter frustrating stalls when transferring large files. These stalls can be caused by a variety of factors, ranging from network congestion and firewall rules to SSH configuration limits and disk I/O bottlenecks. This article will guide you through understanding the common culprits and provide practical solutions to ensure your large file transfers complete successfully.
Understanding the SCP Process and Potential Bottlenecks
Before diving into solutions, it's helpful to understand how SCP works. SCP leverages SSH for data transfer and authentication. This means that any issues affecting your SSH connection can also impact SCP. When transferring large files, the continuous stream of data can expose underlying network instabilities or resource limitations that might not be apparent during smaller transfers. The process involves several stages, each a potential point of failure or slowdown.
flowchart TD
A[Initiate SCP Command] --> B{SSH Handshake & Authentication}
B --> C[Establish Secure Channel]
C --> D[File Transfer Begins]
D --> E{Data Blocks Sent & Acknowledged}
E --"No Acknowledgment/Timeout"--> F[Stall/Hang]
E --"Acknowledgment Received"--> D
D --> G[File Transfer Complete]Simplified SCP File Transfer Workflow
Common Causes of SCP Stalls
Several factors can contribute to SCP stalling, especially with large files. Identifying the root cause is crucial for applying the correct fix. Here are the most frequent culprits:
1. Network Issues
Network instability, high latency, packet loss, or insufficient bandwidth are primary causes. Firewalls or network proxies can also interfere with long-running connections.
2. SSH Configuration Limits
SSH has various timeouts and keep-alive settings that, if not configured appropriately, can cause connections to drop or stall during prolonged inactivity or slow transfers. The ClientAliveInterval and ClientAliveCountMax on the server, and ServerAliveInterval and ServerAliveCountMax on the client, are particularly relevant.
3. Disk I/O Bottlenecks
If either the source or destination disk cannot read or write data fast enough, it can cause the SCP process to wait, leading to a perceived stall. This is more common with older HDDs or heavily utilized storage systems.
4. System Resources
Lack of available memory or CPU on either the client or server can slow down the SSH encryption/decryption process, leading to transfer delays.
5. Large File Handling
SCP, by default, might not be optimized for extremely large files. Some SSH implementations or network devices might have issues with very long-lived, high-throughput connections.
Diagnosing and Resolving SCP Stalls
Here's a systematic approach to diagnose and resolve SCP stalling issues.
1. Step 1: Check Network Connectivity and Performance
Use ping to check basic connectivity and latency. Use traceroute (or tracert on Windows) to identify network hops and potential bottlenecks. For bandwidth testing, tools like iperf3 can help determine the actual network throughput between the two hosts, independent of SCP.
2. Step 2: Enable Verbose SCP Output
The -v flag with scp provides verbose debugging output, which can offer clues about where the transfer is hanging. Look for messages indicating timeouts, connection resets, or specific SSH errors.
3. Step 3: Adjust SSH Keep-Alive Settings
Configure SSH to send keep-alive packets to prevent the connection from timing out due to inactivity. This can be done on the client side or server side. For client-side, use the -o option with scp.
4. Step 4: Monitor System Resources
On both the source and destination machines, use tools like top, htop, iostat, or sar to monitor CPU, memory, and disk I/O usage during the transfer. High I/O wait times or CPU utilization can indicate a bottleneck.
5. Step 5: Consider Alternative Transfer Methods
If SCP continues to be problematic, consider alternatives like rsync (which can resume interrupted transfers and is often more efficient for large files), sftp (interactive and more robust for some scenarios), or even tar combined with netcat for very high-speed, unencrypted transfers over a trusted network.
# Verbose SCP transfer
scp -v large_file.tar.gz user@remote_host:/path/to/destination
# SCP with client-side keep-alive options
scp -o ServerAliveInterval=30 -o ServerAliveCountMax=5 large_file.tar.gz user@remote_host:/path/to/destination
# Example rsync command for large files (resumable)
rsync -avzP large_file.tar.gz user@remote_host:/path/to/destination
Examples of SCP with verbose output and keep-alive, and an rsync alternative.
rsync, the -P flag combines --partial (keep partially transferred files) and --progress (show progress during transfer), which is extremely useful for large files and unstable connections.Advanced SSH Configuration for Stability
For persistent issues, you might need to adjust SSH server-side configurations. This typically involves editing the /etc/ssh/sshd_config file on the remote server.
# /etc/ssh/sshd_config (on the remote server)
# Specifies the number of seconds that the sshd(8) daemon will wait before
# sending a null packet to the client to keep the connection alive.
ClientAliveInterval 60
# Specifies the number of client alive messages (see above) which may be
# sent by the server without receiving any messages back from the client.
# If this threshold is reached, sshd will disconnect the client.
ClientAliveCountMax 3
# Restart SSH service after making changes
sudo systemctl restart sshd # For systemd-based systems
# sudo service ssh restart # For SysVinit-based systems
SSH server-side keep-alive configuration.
These settings tell the SSH server to send a 'keep-alive' message to the client every 60 seconds. If the client doesn't respond after 3 such messages (i.e., 180 seconds of no response), the server will disconnect. Adjust these values based on your network's stability, but be careful not to set them too low, as it can lead to premature disconnections.