Running Docker containers on a server with limited memory can lead to out-of-memory (OOM) issues, which can disrupt services and lead to downtime. This guide will show you how to increase swap space on an Ubuntu server to provide a buffer against OOM errors, using a real-world example from a server running multiple Docker containers.
Consider a typical scenario where an Ubuntu server is running several Docker containers:
fox@analytics:~$ sudo docker ps
CONTAINER ID IMAGE PORTS NAMES
7677a77e5606 plausible/analytics:v2.0 0.0.0.0:8000->8000/tcp plausible-plausible-1
0ea79b7d0c03 clickhouse/clickhouse-server:126.96.36.199-alpine 8123/tcp, 9000/tcp, 9009/tcp plausible-plausible_events_db-1
33f6e8a30da4 postgres:14-alpine 5432/tcp plausible-plausible_db-1
40400c725d7c bytemark/smtp 25/tcp plausible-mail-1
fox@analytics:~$ free -m
total used free shared buff/cache available
Mem: 981 693 62 70 225 66
Swap: 0 0 0
In this example, the server has less than 1 GB of RAM and no swap space configured. This configuration is prone to OOM errors, especially when the containers require more memory than what is available. In my case the service stayed online but the configuration management service salt-minion was unable to be reached to deploy new TLS certificates, the cert expired and prevented clients from sending metrics.
Creating and Enabling Swap Space
To prevent OOM errors, we'll create a swap file. This script automates the process, ensuring that your system has additional virtual memory to handle peak loads.
Save the script as setup_swap.sh and execute it on your server:
# Size of the swap file
# Create a swap file
fallocate -l $SWAP_SIZE $SWAP_FILE || dd if=/dev/zero of=$SWAP_FILE bs=1024 count=1048576
# Secure swap file permissions
chmod 600 $SWAP_FILE
# Set up a Linux swap area
# Enable the swap file
# Make the swap file permanent
echo "$SWAP_FILE none swap sw 0 0" | tee -a /etc/fstab
# Set the swappiness value
echo "vm.swappiness=$SWAPPINESS" | tee -a /etc/sysctl.conf
# Set the cache pressure value
echo "vm.vfs_cache_pressure=$CACHE_PRESSURE" | tee -a /etc/sysctl.conf
# Output the results
echo "Swap file created and enabled:"
echo "Swappiness set to $SWAPPINESS and cache pressure set to $CACHE_PRESSURE."
This script will create a 1 GB swap file, configure the system to use it, and adjust kernel parameters to optimize memory usage.
Understanding Swappiness and Cache Pressure
The swappiness parameter influences how often the system uses swap space. A value of 10 encourages the system to keep processes in RAM, resorting to swap only when necessary.
The vfs_cache_pressure setting determines how aggressively the kernel reclaims memory from the cache. A value of 50 provides a balance between reclaiming memory and maintaining cache for quick file access.
Monitoring Your System
After increasing the swap space, monitor your system's memory usage with:
This will help you understand if the swap space is sufficient or if further adjustments are needed.
By adding swap space and tuning kernel parameters, you've bolstered your server's ability to handle memory-intensive Docker containers. However, swap is not a replacement for physical RAM. If your server consistently uses a lot of swap, consider upgrading the RAM for better performance.
Stay proactive in managing your server's resources to ensure uninterrupted service for your Dockerized applications. Happy hosting!