Swap Expansion

Notes for checking and expanding swap on Linux/RHEL systems.

Swap is disk space used as extra memory when RAM is full. It is slower than RAM, but it can help prevent processes from failing when memory pressure is high.

Safety rule

Before changing swap, check:

1. Am I on the correct server?
2. Is this production or test?
3. Is the system actually running out of memory?
4. Is swap already configured?
5. Is the disk or filesystem where swap will be created healthy?
6. Is there enough free disk space?
7. Is the server using LVM or a swap file?
8. Do I have approval?

Do not use swap expansion as a replacement for fixing a real memory leak.

Check current memory and swap

Show memory and swap usage

free -h

Show active swap devices/files

swapon --show

Alternative:

cat /proc/swaps

Check disk space

df -h

Check block devices

lsblk

Option 1: Create a swap file

This is often the simplest method.

Example: create a 4 GB swap file.

1. Create swap file

sudo fallocate -l 4G /swapfile

If fallocate is not available or does not work:

sudo dd if=/dev/zero of=/swapfile bs=1M count=4096 status=progress

2. Set correct permissions

sudo chmod 600 /swapfile

3. Format as swap

sudo mkswap /swapfile

4. Enable swap

sudo swapon /swapfile

5. Verify

swapon --show
free -h

6. Make swap permanent

Edit /etc/fstab:

sudo nano /etc/fstab

Add:

/swapfile none swap sw 0 0

Test fstab:

sudo mount -a

Then verify swap:

swapon --show
free -h

Option 2: Extend existing LVM swap

Use this if swap is already on an LVM logical volume.

1. Check swap

swapon --show
lsblk
sudo lvs
sudo vgs

Example swap device:

/dev/mapper/vg_root-lv_swap

2. Turn swap off

sudo swapoff /dev/VG_NAME/LV_SWAP_NAME

Example:

sudo swapoff /dev/vg_root/lv_swap

3. Extend the logical volume

Example: add 4 GB:

sudo lvextend -L +4G /dev/VG_NAME/LV_SWAP_NAME

Example:

sudo lvextend -L +4G /dev/vg_root/lv_swap

4. Recreate swap signature

sudo mkswap /dev/VG_NAME/LV_SWAP_NAME

Example:

sudo mkswap /dev/vg_root/lv_swap

5. Turn swap back on

sudo swapon /dev/VG_NAME/LV_SWAP_NAME

Example:

sudo swapon /dev/vg_root/lv_swap

6. Verify

swapon --show
free -h
sudo lvs

Check swappiness

Swappiness controls how aggressively Linux uses swap.

Show current swappiness

cat /proc/sys/vm/swappiness

Alternative:

sysctl vm.swappiness

Temporarily change swappiness

Example:

sudo sysctl vm.swappiness=10

Permanently change swappiness

Create a config file:

sudo nano /etc/sysctl.d/99-swappiness.conf

Add:

vm.swappiness = 10

Apply:

sudo sysctl --system

Common values

vm.swappiness = 60  default on many systems
vm.swappiness = 10  common for servers where swap should be used less aggressively

Do not change swappiness blindly. Follow company standards.

Troubleshooting high swap usage

High swap usage can mean:

not enough RAM
memory leak
application using too much memory
too many processes
wrong application sizing
normal behavior after previous memory pressure

First checks:

free -h
swapon --show
top
ps aux --sort=-%mem | head
journalctl -k | grep -i "out of memory"
dmesg -T | grep -i "killed process"

OOM killer checks

If Linux killed a process because of memory pressure:

journalctl -k | grep -i "out of memory"

or:

dmesg -T | grep -i "killed process"

Disable swap temporarily

sudo swapoff /swapfile

or for a swap LV:

sudo swapoff /dev/VG_NAME/LV_SWAP_NAME

Remove a swap file

Only do this when you are sure it is no longer needed.

sudo swapoff /swapfile
sudo rm /swapfile

Then remove the /swapfile line from:

/etc/fstab

Verify:

swapon --show
free -h

First command set for swap issues

hostnamectl
free -h
swapon --show
cat /proc/swaps
df -h
lsblk
sudo lvs
sudo vgs
top
ps aux --sort=-%mem | head
journalctl -k | grep -i "out of memory"

Dangerous actions

Be careful with:

swapoff on systems under memory pressure
mkswap on the wrong device
editing /etc/fstab
removing swap files
changing LVM volumes
changing swappiness without approval

Running mkswap on the wrong device can destroy existing data.

Personal notes

Add sanitized notes here.

Examples:

- Swap was full because application memory usage increased.
- Swap file was added as a temporary fix.
- Real issue was a memory leak, not missing swap.
- LVM swap was extended after checking free VG space.