#!/bin/bash
# Setup swap space as a safety net for CPU memory during checkpoint saving.
# Run with: sudo bash setup_swap.sh [SIZE_IN_GB]
#
# Default: 128GB swap file at /swapfile

set -euo pipefail

SWAP_SIZE="${1:-400}"
SWAPFILE="/swapfile"

echo "=== Swap Setup Script ==="
echo "Requested swap size: ${SWAP_SIZE}GB"
echo ""

# Check if running as root
if [ "$EUID" -ne 0 ]; then
    echo "ERROR: Must run as root (sudo bash $0)"
    exit 1
fi

# Show current memory state
echo "--- Current Memory ---"
free -h
echo ""

# Check if swap already exists
if swapon --show | grep -q "$SWAPFILE"; then
    echo "Swap already active at $SWAPFILE"
    echo "To resize, first run: sudo swapoff $SWAPFILE && sudo rm $SWAPFILE"
    echo "Then re-run this script."
    exit 0
fi

# Check available disk space
AVAIL_GB=$(df -BG / | awk 'NR==2 {print $4}' | tr -d 'G')
echo "Available disk space: ${AVAIL_GB}GB"
if [ "$AVAIL_GB" -lt "$((SWAP_SIZE + 50))" ]; then
    echo "WARNING: Only ${AVAIL_GB}GB free. Need ${SWAP_SIZE}GB for swap + 50GB headroom."
    echo "Reduce swap size or free disk space."
    exit 1
fi

# Create swap file
echo "--- Creating ${SWAP_SIZE}GB swap file at $SWAPFILE ---"
fallocate -l "${SWAP_SIZE}G" "$SWAPFILE"
chmod 600 "$SWAPFILE"
mkswap "$SWAPFILE"

# Enable swap
swapon "$SWAPFILE"

# Set low swappiness (only swap as a last resort, not proactively)
sysctl vm.swappiness=10

# Make persistent across reboots
if ! grep -q "$SWAPFILE" /etc/fstab; then
    echo "$SWAPFILE none swap sw 0 0" >> /etc/fstab
    echo "Added swap entry to /etc/fstab"
fi

# Make swappiness persistent
if ! grep -q "vm.swappiness" /etc/sysctl.conf; then
    echo "vm.swappiness=10" >> /etc/sysctl.conf
    echo "Added swappiness=10 to /etc/sysctl.conf"
fi

echo ""
echo "--- Setup Complete ---"
free -h
echo ""
echo "Swap is active. swappiness=10 (swap only used under memory pressure)."