Swap Management on Linux — Create, Extend & Make Swap Persistent

[Murali Krishna]

Swap Management on Linux — Create, Extend & Make Swap Persistent
A complete hands-on guide — swap files, swap partitions and LVM swap, with tuning tips (AlmaLinux 9 / RHEL 9)

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What is swap and why do you need it?
Swap is disk space the kernel uses as overflow when physical RAM fills up. It keeps the system alive under memory pressure instead of killing processes, and it's also where memory pages go during hibernation. It's slower than RAM, so it's a safety net — not a replacement for real memory.

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Check Current Swap First

See what swap is active

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swapon --show         # lists active swap devices/files
free -h               # RAM and swap usage overview
cat /proc/swaps       # raw kernel view

Tip: A common starting rule: swap = equal to RAM for systems up to 2 GB, then half of RAM above that. For hibernation you need swap at least equal to RAM.

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1 Create Swap

There are three common ways to add swap. Pick one based on your setup.

Method A — Swap file (most flexible)

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# Create a 2 GB file.
# On XFS (AlmaLinux 9 default) use dd, NOT fallocate — see warning below:
dd if=/dev/zero of=/swapfile bs=1M count=2048 status=progress

# Lock down permissions (required, or swapon refuses it)
chmod 600 /swapfile

# Format as swap and enable
mkswap /swapfile
swapon /swapfile

WARNING — XFS users: Do not use fallocate for swap files on XFS. fallocate can leave holes in the file, and swapon will fail with "Invalid argument". Always use dd for contiguous allocation. On ext4, fallocate -l 2G /swapfile is fine.

Method B — Swap partition

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# Mark the partition type as Linux swap, then:
mkswap /dev/sdb2
swapon /dev/sdb2

Method C — LVM swap volume (clean on LVM systems)

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lvcreate -L 2G -n lv_swap vg_data
mkswap /dev/vg_data/lv_swap
swapon /dev/vg_data/lv_swap

Verify it's live

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swapon --show
free -h

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2 Extend Swap

You can't resize an active swap file in place — you disable it, grow/recreate it, then re-enable. The cleanest approach is to add more, or rebuild bigger.

Option A — Just add a second swap file

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dd if=/dev/zero of=/swapfile2 bs=1M count=2048 status=progress
chmod 600 /swapfile2
mkswap /swapfile2
swapon /swapfile2

The kernel uses multiple swap areas together, so total swap simply grows.

Option B — Rebuild a swap file bigger

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swapoff /swapfile                     # disable first
dd if=/dev/zero of=/swapfile bs=1M count=4096 status=progress
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile

Option C — Extend an LVM swap volume

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swapoff /dev/vg_data/lv_swap          # must be off to resize
lvextend -L +2G /dev/vg_data/lv_swap  # grow the LV
mkswap /dev/vg_data/lv_swap           # re-format (swap has no resize tool)
swapon /dev/vg_data/lv_swap           # re-enable

Tip: Swap has no online resize. Whatever the method, the pattern is always: swapoff -> change size -> mkswap -> swapon.

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3 Persistent Swap Configuration

swapon only lasts until reboot. To make swap survive restarts, add it to /etc/fstab.

Add the right line to /etc/fstab

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# Swap file:
/swapfile            none    swap    defaults    0 0

# Swap partition or LVM swap (by device path):
/dev/vg_data/lv_swap none    swap    defaults    0 0

Better: reference a partition/LV by UUID (survives device renaming)

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blkid /dev/vg_data/lv_swap            # get the UUID
# then in /etc/fstab:
UUID=xxxx-xxxx       none    swap    defaults    0 0

Test the fstab entry without rebooting

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swapoff -a
swapon -a                             # reads /etc/fstab; errors show here
swapon --show

WARNING: A bad /etc/fstab swap line can stall or break boot. Always test with swapon -a after editing, and double-check the path/UUID before you reboot.

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Tuning — Swappiness

swappiness (0–100) controls how aggressively the kernel uses swap. Lower = prefer RAM, swap only when needed. Servers often use 10.

Check and set temporarily

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cat /proc/sys/vm/swappiness       # current value (default often 30 or 60)
sysctl vm.swappiness=10           # apply now (resets on reboot)

Make swappiness persistent

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echo 'vm.swappiness=10' > /etc/sysctl.d/99-swappiness.conf
sysctl --system                   # reload

Tip: Set swap priority when running multiple swap areas so faster storage is used first: swapon -p 10 /dev/vg_data/lv_swap (higher number = used first), or add pri=10 in the fstab options column.

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Remove Swap (when needed)

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swapoff /swapfile                 # disable
# remove its line from /etc/fstab
rm -f /swapfile                   # delete the file
# for LVM: swapoff then  lvremove /dev/vg_data/lv_swap

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Quick Reference Cheat Sheet

• Show swap — swapon --show ; free -h
• Make swap file (XFS) — dd if=/dev/zero of=/swapfile bs=1M count=2048
• Permissions — chmod 600 /swapfile
• Format — mkswap /swapfile
• Enable — swapon /swapfile
• Extend (LVM) — swapoff -> lvextend -> mkswap -> swapon
• Persistent — add line to /etc/fstab, test with swapon -a
• Swappiness — sysctl vm.swappiness=10
• Disable — swapoff /swapfile

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How much swap do you run on your servers, and what swappiness do you set? Share your setup below.