Swap — Extra Memory on Disk

When RAM runs out, Linux can move memory it hasn't touched lately out to disk: swap. Disk is far slower than RAM — but instead of the out-of-memory killer ending your database, the server only slows down for a while. Most cloud servers start without any:

free -h

              total        used        free      shared  buff/cache   available
Mem:          2.0Gi       305Mi       1.4Gi        18Mi       307Mi       1.5Gi
Swap:            0B          0B          0B

A swap file is the easy way — no partition needed:

sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

1. fallocate makes a 2 GiB file and reserves all its space at once.
2. chmod 600 — only root may read it: it can hold anyone's memory, passwords included.
3. mkswap writes the swap header.
4. swapon puts it to use.

swapon --show
free -h

NAME      TYPE SIZE USED PRIO
/swapfile file   2G   0B   -2

Every boot

swapon lasts until the next reboot. One line in /etc/fstab makes it permanent:

/swapfile none swap sw 0 0

sudo swapon -a turns on every swap in fstab — the test, like mount -a.

💡 truncate -s 2G makes a file with holes — mkswap warns about it and swapon refuses it. That's why fallocate.