swap-inflater

#!/bin/bash

# A QAD script to ensure there is sufficient swap on a server. If there is not
# enough, and there is sufficient free disk, a swapfile is created under /.

set -eo pipefail
err_report() {
    echo "errexit on line $(caller)" >&2
}
trap err_report ERR

# Percentage of memory to allocate as swap.
maxmempercent=50
# Filename to create. In the event of a name clash, we will append digits.
swapbase="/swapfile"
# Maximum % of free space in the root FS that we are allowed to consume.
# Use the command line flag '--force' to override this.
maxdiskpercent=20

while [[ $1 ]]; do
    if [[ $1 == "--force" ]]; then
        force=1
    else
        maxmempercent=$1
    fi
    shift
done

memtotal=$(awk '/^MemTotal:/ {print $2}' /proc/meminfo)
swaptotal=$(awk '/^SwapTotal:/ {print $2}' /proc/meminfo)
let minswap=memtotal*maxmempercent/100
let shortfall=minswap-swaptotal

# Only make changes if the shortfall is significant, i.e. >20%
let leeway=minswap/5

if [[ $shortfall -gt $leeway ]]; then
    rootfree=$(df -k / | awk '/^\// {print $4}')
    let maxfilesize=rootfree*maxdiskpercent/100
    if [[ $shortfall -ge $rootfree ]]; then
        echo "Not enough free space in /. Aborting."
        exit 1
    elif [[ $shortfall -ge $maxfilesize && ! $force ]]; then
        echo "Refusing to create swapfile larger than $maxdiskpercent% of free space on /"
        echo "Incant '$0 --force' to override."
        exit 2
    else
        swapfile=$swapbase
        i=0
        while [[ -f $swapfile ]]; do
            let i+=1
            swapfile="${swapbase}$i"
        done
        dd if=/dev/zero of=$swapfile bs=1024 count=$shortfall
        chmod og= $swapfile
        mkswap $swapfile
        cat <<EOF >>/etc/fstab
$swapfile	none		swap	swap	0	0
EOF
        swapon -a
    fi
fi