# Create and Tune a Linux Swap File on CentOS, RHEL & Rocky Linux

> How to allocate, secure, persist, and tune a swap file on CentOS, RHEL, Rocky Linux, and small AWS EC2 instances to prevent OOM crashes.

- Canonical URL: <https://carlossura.com/blog/swap-file-centos/>
- Author: Carlos Sura
- Published: 2019-09-06
- Updated: 2026-10-20
- Tags: swap, swap file, centos, linux, aws, rhel

Small cloud instances (such as AWS EC2 `t2.micro` or `t3.micro` nodes with 1 GB of RAM) commonly ship without a swap partition enabled. When memory spikes during package builds, database queries, or cron jobs, the Linux **Out-Of-Memory (OOM) killer** abruptly terminates critical daemons like MySQL, Nginx, or Docker containers.

Adding a dedicated swap file provides a safety buffer against sudden memory exhaustion and allows the kernel to page out idle anonymous memory.

---

## Allocate and Enable the Swap File

Run the following commands as `root` (or with `sudo`) to allocate a 2 GB swap file, lock down permissions to `0600` (so unprivileged local users cannot read swapped memory pages), format it as swap, and activate it immediately:

```bash
# Allocate a 2GB file (note: on older XFS filesystems on RHEL/CentOS 7, use dd if fallocate creates holes)
sudo fallocate -l 2G /swapfile

# If running on an older XFS filesystem where swapon reports "swapon failed: Invalid argument", use dd instead:
# sudo dd if=/dev/zero of=/swapfile bs=1M count=2048 status=progress

# Restrict read/write permissions strictly to root
sudo chmod 600 /swapfile

# Format the file as Linux swap space and enable it
sudo mkswap /swapfile
sudo swapon /swapfile
```

---

## Persist the Swap File Across Reboots in `/etc/fstab`

Append the swap entry to `/etc/fstab` so the operating system mounts `/swapfile` automatically at boot:

```fstab
/swapfile swap swap defaults 0 0
```

You can append this idempotently from the CLI with:

```bash
grep -q '^/swapfile ' /etc/fstab || echo '/swapfile swap swap defaults 0 0' | sudo tee -a /etc/fstab
```

---

## Tune `vm.swappiness` and `vm.vfs_cache_pressure`

By default, many Linux distributions set `vm.swappiness = 60`, which causes the kernel to swap out process memory aggressively even when plenty of free RAM remains. On cloud servers backed by EBS/NVMe storage, lowering `vm.swappiness` to `10` instructs the kernel to keep active application memory in RAM and only swap under genuine memory pressure:

```bash
cat << 'EOF' | sudo tee /etc/sysctl.d/99-swap-tuning.conf
vm.swappiness = 10
vm.vfs_cache_pressure = 50
EOF

sudo sysctl --system
```

---

## Verify Swap Status

Confirm that the swap file is active and inspect memory utilization:

```bash
sudo swapon --show
free -h
```