Configure and Manage Swap Space¶

What is Swap Space?¶

Swap is space on your hard drive that acts as extra (but slower) RAM. When your physical RAM fills up, Linux moves less-used data to swap to free up RAM for active programs.

Think of it like this:

RAM: Your desk - fast and convenient for work you’re doing now
Swap: A filing cabinet - slower to access, but expands your working space

When is Swap Used?¶

Scenario 1: Running out of RAM

You’re running many programs and RAM fills up. Linux moves inactive pages to swap, freeing RAM for active processes.

Scenario 2: Hibernation

When you hibernate, Linux copies all RAM to swap, then powers off. On resume, it reads swap back into RAM.

Scenario 3: Memory Management

Even with free RAM, Linux might swap out rarely-used pages to have more free RAM for file caching (speeds up the system).

Swap Size Recommendations¶

Physical RAM	Swap Size (No Hibernation)	With Hibernation
< 2 GB	2x RAM	3x RAM
2-8 GB	= RAM	2x RAM
8-64 GB	4-8 GB (or 0.5x RAM)	1.5x RAM
> 64 GB	4-8 GB minimum	Not practical

Note: These are guidelines. Your needs depend on your workload.

Creating Swap Space¶

Two Types of Swap¶

1. Swap Partition:

Dedicated disk partition for swap
Slightly faster
Fixed size (harder to change)
Traditional approach

2. Swap File:

Regular file used as swap
Easier to create/resize
More flexible
Modern approach

Both work equally well. Swap files are more convenient.

Creating Swap Files¶

fallocate - Quick File Creation¶

What it does: Instantly creates a file of specific size.

Why use it: Fastest way to create swap file.

Example - Creating 2GB swap file:

# Step 1: Create the file (instant!)
fallocate -l 2G /swapfile

# Step 2: Set correct permissions (CRITICAL for security!)
chmod 600 /swapfile

# Step 3: Format as swap
mkswap /swapfile

# Step 4: Enable it
swapon /swapfile

# Step 5: Verify
swapon --show
free -h

# Step 6: Make permanent
echo '/swapfile none swap sw 0 0' >> /etc/fstab

Real-world scenario - VPS with no swap:

# Many VPS providers don't include swap
# Check current swap
free -h
# Shows: Swap: 0B

# Create 4GB swap file
sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

# Verify
free -h
# Now shows: Swap: 4.0Gi

# Make permanent
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

dd - Alternative File Creation¶

What it does: Copies data to create file (slower but more compatible).

Why use it: Works on all systems, shows progress.

Example:

# Create 2GB swap file with progress
dd if=/dev/zero of=/swapfile bs=1M count=2048 status=progress

# Then follow same steps as fallocate
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile

Creating Swap Partitions¶

Using fdisk¶

Real-world scenario - New disk for swap:

# Step 1: Identify disk
lsblk
# /dev/sdb is your new disk

# Step 2: Create partition
fdisk /dev/sdb

# In fdisk:
# Press: n (new partition)
# Press: p (primary)
# Press: 1 (partition number)
# Press: Enter (default start)
# Press: +8G (8GB size)
# Press: t (change type)
# Press: 82 (Linux swap)
# Press: w (write changes)

# Step 3: Format as swap
mkswap /dev/sdb1

# Step 4: Enable
swapon /dev/sdb1

# Step 5: Make permanent
echo '/dev/sdb1 none swap sw 0 0' >> /etc/fstab

Swap Commands¶

mkswap - Format as Swap¶

What it does: Prepares a partition or file to be used as swap space.

Why use it: Required before using any space as swap.

Examples:

# Format partition
mkswap /dev/sdb1

# Format file
mkswap /swapfile

# With label (helpful for identification)
mkswap -L "swap1" /dev/sdb1

Output example:

Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
no label, UUID=1234abcd-5678-...

swapon - Enable Swap¶

What it does: Activates swap space so Linux can use it.

Why use it: Swap isn’t usable until you enable it.

Examples:

# Enable specific swap
swapon /swapfile
swapon /dev/sdb1

# Enable all swap in /etc/fstab
swapon -a

# Enable with priority (higher number = used first)
swapon -p 10 /dev/sdb1
swapon -p 5 /swapfile

# Show what's active
swapon --show

# Verbose mode
swapon -v /swapfile

Understanding priority:

# Fast SSD swap - high priority (used first)
swapon -p 100 /dev/nvme0n1p3

# Slow HDD swap - low priority (used last)
swapon -p 10 /dev/sdb1

Output of swapon –show:

NAME      TYPE SIZE USED PRIO
/swapfile file   2G   0B   -2
/dev/sdb1 partition 4G 512M 5

swapoff - Disable Swap¶

What it does: Deactivates swap space.

Why use it: Before removing swap or making changes.

Examples:

# Disable specific swap
swapoff /swapfile

# Disable all swap
swapoff -a

# Verbose
swapoff -v /swapfile

Real-world scenario - Resizing swap file:

# Current 2GB swap is too small, need 4GB

# Step 1: Disable swap
swapoff /swapfile

# Step 2: Delete old file
rm /swapfile

# Step 3: Create new 4GB file
fallocate -l 4G /swapfile
chmod 600 /swapfile

# Step 4: Format and enable
mkswap /swapfile
swapon /swapfile

# Step 5: Verify
swapon --show

free - Memory and Swap Status¶

What it does: Shows RAM and swap usage.

Why use it: Quick check of memory situation.

Examples:

# Human-readable
free -h

# In megabytes
free -m

# In gigabytes  
free -g

# Continuous updates (every 2 seconds)
free -h -s 2

# With totals
free -ht

# Wide mode (more detailed)
free -hw

Output example:

free -h
              total        used        free      shared  buff/cache   available
Mem:           16Gi       8.0Gi       2.0Gi       100Mi       6.0Gi       7.5Gi
Swap:          4.0Gi       512Mi       3.5Gi

What it means:

total: Total RAM/swap installed
used: Currently in use by programs
free: Completely unused
shared: Used by tmpfs/shared memory
buff/cache: Used for caching (can be freed if needed)
available: How much can be used by programs (free + reclaimable cache)

vmstat - Virtual Memory Statistics¶

What it does: Shows system activity including swap in/out.

Why use it: Monitor if system is swapping heavily (performance problem).

Examples:

# Single snapshot
vmstat

# Update every 2 seconds
vmstat 2

# 10 updates, 2 seconds apart
vmstat 2 10

# Memory statistics
vmstat -s

# Active/inactive memory
vmstat -a

# Disk statistics
vmstat -d

Output example:

vmstat 2
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0  10240 204800 102400 512000    0    0    10    50  100  200  5  2 90  3  0
 0  1  10240 204800 102400 512000    5   10    20   100  150  250  8  4 85  3  0

Critical columns:

si (swap in): KB/s reading from swap (from disk to RAM)
so (swap out): KB/s writing to swap (from RAM to disk)
wa: % CPU waiting for I/O

What to watch for:

si  so  ← Both zero: No swapping (good!)
0   0

si  so  ← Occasional swapping (normal)
5   10

si   so  ← Heavy swapping (performance problem!)
100  200

If you see high si/so consistently, you need more RAM!

Persistent Swap Configuration¶

/etc/fstab Entries¶

Format:

device/file  none  swap  options  0  0

Examples:

# Swap file
/swapfile none swap sw 0 0

# Swap partition
/dev/sdb1 none swap sw 0 0

# With priority
/dev/sdb1 none swap pri=10 0 0
/swapfile none swap pri=5 0 0

# Using UUID (more reliable)
UUID=1234abcd-... none swap sw 0 0

# Using label
LABEL=swap1 none swap sw 0 0

Multiple swap spaces:

# Fast SSD swap (priority 100)
/dev/nvme0n1p3 none swap pri=100 0 0

# Slower SSD swap (priority 50)
/dev/sda2 none swap pri=50 0 0

# Emergency HDD swap (priority 10)
/swapfile none swap pri=10 0 0

Priority rules:

Higher number = used first
Same priority = used in parallel (striped for performance)
Default priority = -2

Tuning Swap Behavior¶

vm.swappiness - Control Swap Tendency¶

What it is: Kernel parameter controlling how aggressively Linux swaps.

Why tune it: Balance between RAM usage and swap usage.

Value range: 0-100

0: Avoid swapping except emergency
1: Minimum swapping (recommended for servers)
10: Very little swapping (good for servers)
60: Default (balanced)
100: Swap aggressively

Examples:

# Check current value
cat /proc/sys/vm/swappiness
sysctl vm.swappiness

# Temporary change (until reboot)
sysctl -w vm.swappiness=10
echo 10 > /proc/sys/vm/swappiness

# Permanent change
echo "vm.swappiness=10" >> /etc/sysctl.d/99-swappiness.conf
sysctl -p /etc/sysctl.d/99-swappiness.conf

# Or edit /etc/sysctl.conf
vi /etc/sysctl.conf
# Add: vm.swappiness=10
sysctl -p

Recommendations:

# Desktop/Laptop (more responsive)
vm.swappiness=60

# Server with plenty of RAM
vm.swappiness=10

# Server with limited RAM
vm.swappiness=30

# Database server
vm.swappiness=1

Real-world scenario - Web server optimization:

# Server has 32GB RAM, barely uses swap
# Lower swappiness to keep more in RAM

# Check current setting
cat /proc/sys/vm/swappiness
# 60 (default)

# Set to minimal swapping
echo "vm.swappiness=10" >> /etc/sysctl.d/99-swappiness.conf
sysctl -p /etc/sysctl.d/99-swappiness.conf

# Monitor results
vmstat 2
# Watch si/so values (should be mostly 0)

Swap on LVM¶

Why Use LVM for Swap?¶

Benefits:

Easy to resize
Can be on multiple disks
Snapshots possible (though not common for swap)

Example - Create LVM swap:

# Step 1: Create logical volume
lvcreate -L 8G -n lv_swap vg01

# Step 2: Format as swap
mkswap /dev/vg01/lv_swap

# Step 3: Enable
swapon /dev/vg01/lv_swap

# Step 4: Make permanent
echo '/dev/vg01/lv_swap none swap sw 0 0' >> /etc/fstab

Resizing LVM swap:

# Need more swap space

# Step 1: Disable swap
swapoff /dev/vg01/lv_swap

# Step 2: Extend logical volume
lvextend -L +4G /dev/vg01/lv_swap

# Step 3: Reformat (required for swap!)
mkswap /dev/vg01/lv_swap

# Step 4: Re-enable
swapon /dev/vg01/lv_swap

# Step 5: Verify
swapon --show
free -h

Monitoring Swap Usage¶

Check What’s Using Swap¶

Find swap usage by process:

# Quick check
for file in /proc/*/status; do
    awk '/VmSwap|Name/{printf $2 " " $3}END{print ""}' $file
done | sort -k 2 -n -r | head

# Output shows:
# firefox 512000 (512MB)
# chrome 256000 (256MB)
# mysql 128000 (128MB)

Detailed swap analysis:

# For each process, show swap usage
for pid in $(ls /proc | grep -E '^[0-9]+$'); do
    if [ -f /proc/$pid/smaps ]; then
        swap=$(grep Swap /proc/$pid/smaps 2>/dev/null | awk '{sum+=$2} END {print sum}')
        if [ ! -z "$swap" ] && [ "$swap" -gt 0 ] 2>/dev/null; then
            name=$(cat /proc/$pid/comm 2>/dev/null)
            echo "$swap KB - $name (PID: $pid)"
        fi
    fi
done | sort -n -r | head -20

Troubleshooting¶

Problem: System Running Out of Memory¶

Symptoms: System very slow, heavy swapping.

Solutions:

# Check situation
free -h
vmstat 2 5

# If swap is full or nearly full:
# Option 1: Add more swap (temporary fix)
fallocate -l 4G /emergency-swap
chmod 600 /emergency-swap
mkswap /emergency-swap
swapon /emergency-swap

# Option 2: Find memory hogs
ps aux --sort=-%mem | head
top -o %MEM

# Option 3: Kill memory-hungry processes (carefully!)
pkill firefox
pkill chrome

# Long-term solution: Add more RAM!

Problem: Heavy Swapping (System Slow)¶

Symptoms: High si/so in vmstat, system sluggish.

Solutions:

# Monitor swapping
vmstat 2

# If heavy swapping (si/so > 100):
# Solution 1: Lower swappiness
sysctl -w vm.swappiness=10

# Solution 2: Find what's swapped out
# (See "Check What's Using Swap" above)

# Solution 3: Clear swap and reload to RAM
# WARNING: Only if you have enough free RAM!
swapoff -a
swapon -a

Problem: Cannot Enable Swap¶

Symptoms: swapon fails with error.

Solutions:

# Check if formatted as swap
file -s /swapfile
# Should show: swap file

# If not formatted:
mkswap /swapfile

# Check permissions (swap files must be 600)
ls -l /swapfile
chmod 600 /swapfile

# Check logs
dmesg | grep swap
journalctl | grep swap

Problem: Swap Not Activating at Boot¶

Symptoms: After reboot, swap isn’t active.

Solutions:

# Check fstab entry
cat /etc/fstab | grep swap

# Test manual activation
swapon -a

# Check for errors
systemctl status swap.target
systemctl list-units | grep swap

# Verify file/partition exists
ls -l /swapfile

Best Practices¶

1. Secure swap files:

# Always set 600 permissions
chmod 600 /swapfile

2. Use appropriate size:

# Server with 16GB RAM
# 8GB swap is plenty

3. Tune swappiness:

# Servers: 10
# Desktops: 60 (default)

4. Monitor regularly:

# Check weekly
free -h
vmstat 2 5

5. Multiple swap for performance:

# Equal priority for striping
/dev/sda2 none swap pri=10 0 0
/dev/sdb2 none swap pri=10 0 0

Quick Reference¶

Creating Swap¶

# Swap file (recommended)
fallocate -l 4G /swapfile
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile
echo '/swapfile none swap sw 0 0' >> /etc/fstab

Managing Swap¶

swapon --show              # Show active swap
swapon -a                  # Enable all
swapoff -a                 # Disable all
free -h                    # Check usage
vmstat 2                   # Monitor activity

Tuning¶

# Check swappiness
cat /proc/sys/vm/swappiness

# Set permanently
echo "vm.swappiness=10" >> /etc/sysctl.d/99-swap.conf
sysctl -p

Monitoring¶

# Current status
free -h
swapon --show

# Watch for swapping
vmstat 2 10

# Find what's using swap
grep VmSwap /proc/*/status | grep -v "0 kB"