What does the swapon command do?

The swapon command enables swap space on Linux systems, activating swap partitions or swap files to provide additional virtual memory.

How do I enable all swap spaces?

Use 'swapon -a' to enable all swap spaces listed in /etc/fstab, or 'swapon /dev/sdX' to enable a specific swap partition.

How do I check active swap spaces?

Use 'swapon --show' or 'swapon -s' to display all currently active swap spaces with their sizes and usage information.

swapon Command

The swapon command enables swap space on Linux systems. It activates swap partitions or swap files, making them available for the kernel to use as virtual memory when physical RAM is insufficient.

Syntax

swapon [OPTIONS] [DEVICE...] swapon -a swapon -s

Description

The swapon command is used to enable swap space on Linux systems. It can activate individual swap devices or all swap spaces defined in /etc/fstab. The command also provides functionality to display information about active swap spaces.

Key features:

Enable specific swap partitions or files
Enable all swap spaces from /etc/fstab
Display active swap space information
Set swap priorities for optimization
Support for various swap device types
Integration with system boot process

Note: Swap devices must be properly formatted with mkswap before they can be enabled with swapon.

Common Options

Option	Description
`-a, --all`	Enable all swap spaces listed in /etc/fstab
`-d, --discard[=policy]`	Enable discard/TRIM support for SSD swap
`-e, --ifexists`	Silently skip devices that do not exist
`-f, --fixpgsz`	Reinitialize swap space if page size has changed
`-o, --options opts`	Specify mount options
`-p, --priority priority`	Set swap priority (0-32767)
`-s, --summary`	Display summary of swap usage
`--show[=column]`	Display swap information in table format
`-v, --verbose`	Display verbose output
`-h, --help`	Display help message
`-V, --version`	Display version information

Examples

Display active swap spaces

                        # Show swap summary
                        swapon -s
                        
                        # Show detailed swap information
                        swapon --show
                        
                        # Alternative method
                        cat /proc/swaps
                    

Check currently active swap spaces

Enable specific swap partition

sudo swapon /dev/sdb1

Enable a specific swap partition

Enable specific swap file

sudo swapon /swapfile

Enable a swap file

Enable all swap spaces

sudo swapon -a

Enable all swap spaces listed in /etc/fstab

Enable swap with priority

sudo swapon -p 10 /dev/sdb1

Enable swap with specific priority (higher numbers = higher priority)

Enable swap with verbose output

sudo swapon -v /dev/sdb1

Enable swap with detailed output

Enable swap with discard support

sudo swapon -d /dev/sdb1

Enable swap with TRIM/discard support for SSDs

Enable multiple swap devices

sudo swapon /dev/sdb1 /dev/sdc1

Enable multiple swap devices at once

Understanding swapon Output

Summary Output (-s option)

                        $ swapon -s
                        Filename                Type        Size    Used    Priority
                        /dev/sdb1              partition   2097148 0       -2
                        /swapfile              file        1048572 0       -3
                    

Summary showing swap devices, types, sizes, and usage

Detailed Output (--show option)

                        $ swapon --show
                        NAME      TYPE      SIZE USED PRIO
                        /dev/sdb1 partition   2G   0B   -2
                        /swapfile file      1024M   0B   -3
                    

Detailed table format with human-readable sizes

Custom Column Display

                        # Show specific columns
                        $ swapon --show=NAME,SIZE,USED
                        NAME      SIZE USED
                        /dev/sdb1   2G   0B
                        /swapfile 1024M   0B
                        
                        # Show all available columns
                        $ swapon --show=NAME,TYPE,SIZE,USED,PRIO,UUID
                    

Customize output to show specific information

Creating and Managing Swap

Creating Swap Partition

Complete swap partition setup

                        # 1. Create partition (using fdisk)
                        sudo fdisk /dev/sdb
                        # Create new partition, set type to 82 (Linux swap)
                        
                        # 2. Format as swap
                        sudo mkswap /dev/sdb1
                        
                        # 3. Enable swap
                        sudo swapon /dev/sdb1
                        
                        # 4. Add to /etc/fstab for permanent use
                        echo '/dev/sdb1 none swap sw 0 0' | sudo tee -a /etc/fstab
                        
                        # 5. Verify
                        swapon --show
                    

Complete process to create and enable a swap partition

Creating Swap File

Complete swap file setup

                        # 1. Create swap file (1GB example)
                        sudo fallocate -l 1G /swapfile
                        # or: sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576
                        
                        # 2. Set proper permissions
                        sudo chmod 600 /swapfile
                        
                        # 3. Format as swap
                        sudo mkswap /swapfile
                        
                        # 4. Enable swap
                        sudo swapon /swapfile
                        
                        # 5. Add to /etc/fstab
                        echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab
                        
                        # 6. Verify
                        swapon --show
                    

Complete process to create and enable a swap file

Swap Priorities

Understanding swap priorities

                        # Enable swap with different priorities
                        sudo swapon -p 10 /dev/sdb1  # Higher priority
                        sudo swapon -p 5 /swapfile   # Lower priority
                        
                        # Check priorities
                        swapon --show
                        
                        # In /etc/fstab, set priorities:
                        # /dev/sdb1 none swap sw,pri=10 0 0
                        # /swapfile none swap sw,pri=5 0 0
                    

Manage swap priorities for optimal performance

Practical Use Cases

System Administration

Boot-time swap activation

                        # /etc/fstab entries for automatic swap activation
                        /dev/sdb1 none swap sw 0 0
                        /swapfile none swap sw,pri=5 0 0
                        
                        # Test fstab entries
                        sudo swapon -a
                        
                        # Verify all swap is active
                        swapon --show
                    

Configure automatic swap activation at boot

Emergency swap addition

                        # Quickly add temporary swap file
                        sudo fallocate -l 2G /tmp/emergency_swap
                        sudo chmod 600 /tmp/emergency_swap
                        sudo mkswap /tmp/emergency_swap
                        sudo swapon /tmp/emergency_swap
                        
                        # Check memory situation
                        free -h
                        
                        # Remove when no longer needed
                        sudo swapoff /tmp/emergency_swap
                        sudo rm /tmp/emergency_swap
                    

Add temporary swap space in emergency situations

Performance Optimization

Multi-device swap setup

                        # Enable multiple swap devices with priorities
                        sudo swapon -p 15 /dev/sdb1  # Fast SSD
                        sudo swapon -p 10 /dev/sdc1  # Slower HDD
                        sudo swapon -p 5 /swapfile   # File-based swap
                        
                        # Check configuration
                        swapon --show
                        
                        # Monitor usage
                        watch -n 1 'swapon --show && echo && free -h'
                    

Set up multiple swap devices with different priorities

SSD-optimized swap

                        # Enable swap with discard support for SSD
                        sudo swapon -d /dev/sdb1
                        
                        # Or with specific discard policy
                        sudo swapon -d=once /dev/sdb1
                        
                        # In /etc/fstab for permanent use:
                        # /dev/sdb1 none swap sw,discard 0 0
                        
                        # Verify discard is enabled
                        cat /proc/swaps
                    

Optimize swap for SSD drives with TRIM support

Monitoring and Troubleshooting

Swap usage monitoring

                        # Monitor swap usage
                        watch -n 2 'swapon --show && echo && free -h'
                        
                        # Check swap usage by process
                        for file in /proc/*/status; do
                            awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file
                        done | grep -v " 0" | sort -k2 -nr
                        
                        # Historical swap usage
                        sar -S 1 10
                    

Monitor swap usage and identify processes using swap

Troubleshoot swap issues

                        # Check if swap device exists
                        ls -la /dev/sdb1
                        
                        # Verify swap signature
                        sudo file -s /dev/sdb1
                        
                        # Check for errors in system logs
                        sudo dmesg | grep -i swap
                        sudo journalctl | grep -i swap
                        
                        # Test swap device
                        sudo mkswap -c /dev/sdb1  # Check for bad blocks
                    

Diagnose and troubleshoot swap-related issues

Advanced Configuration

Encrypted Swap

Set up encrypted swap

                        # Create encrypted swap with cryptsetup
                        sudo cryptsetup luksFormat /dev/sdb1
                        sudo cryptsetup luksOpen /dev/sdb1 encrypted_swap
                        
                        # Format and enable
                        sudo mkswap /dev/mapper/encrypted_swap
                        sudo swapon /dev/mapper/encrypted_swap
                        
                        # Add to /etc/crypttab and /etc/fstab
                        echo 'encrypted_swap /dev/sdb1 none luks' | sudo tee -a /etc/crypttab
                        echo '/dev/mapper/encrypted_swap none swap sw 0 0' | sudo tee -a /etc/fstab
                    

Set up encrypted swap for enhanced security

Swap on LVM

Create swap on logical volume

                        # Create logical volume for swap
                        sudo lvcreate -L 2G -n swap_lv vg0
                        
                        # Format and enable
                        sudo mkswap /dev/vg0/swap_lv
                        sudo swapon /dev/vg0/swap_lv
                        
                        # Add to /etc/fstab
                        echo '/dev/vg0/swap_lv none swap sw 0 0' | sudo tee -a /etc/fstab
                        
                        # Verify
                        swapon --show
                    

Create and manage swap on LVM logical volumes

Systemd Integration

Custom swap service

                        # Create custom swap service: /etc/systemd/system/custom-swap.service
                        [Unit]
                        Description=Enable Custom Swap
                        After=local-fs.target
                        
                        [Service]
                        Type=oneshot
                        ExecStart=/usr/sbin/swapon -p 15 /dev/sdb1
                        ExecStop=/usr/sbin/swapoff /dev/sdb1
                        RemainAfterExit=yes
                        
                        [Install]
                        WantedBy=multi-user.target
                        
                        # Enable the service
                        sudo systemctl enable custom-swap.service
                        sudo systemctl start custom-swap.service
                    

Create custom systemd service for swap management

Best Practices

swapon Command Best Practices

Set Appropriate Priorities - Use higher priorities for faster storage devices
Monitor Swap Usage - Regularly check swap usage to optimize system performance
Use SSD Optimization - Enable discard/TRIM for SSD-based swap
Plan Swap Size - Size swap appropriately based on RAM and workload
Update fstab - Always update /etc/fstab for permanent swap configuration
Test Configuration - Test swap configuration before rebooting