Creating a High-Speed RAM Disk on macOS (APFS)

How to Create a RAM Disk on macOS (APFS)

Developer Guide · macOS · Performance

A RAM disk is one of the simplest ways to unlock extreme performance on macOS. This guide explains what a RAM disk is, why you might want one, and walks through a Bash script that creates a temporary APFS RAM disk in seconds.

What Is a RAM Disk?

A RAM disk is a virtual storage volume that lives entirely in system memory (RAM) instead of on a physical SSD or hard drive. Because RAM is dramatically faster than any disk, read and write operations are nearly instant.

The tradeoff is simple: data stored on a RAM disk is temporary. When you reboot or shut down your Mac, the disk disappears.

Why Use a RAM Disk?

• Ultra-fast build directories for Xcode or compilers
• Temporary cache or scratch space
• Video, image, or audio processing pipelines
• Reduced SSD wear during heavy write workloads
• Performance testing and benchmarking

The Script

The Bash script below allocates a fixed amount of RAM, formats it as an APFS volume, and mounts it like a normal disk.

#!/usr/bin/env bash
set -euo pipefail

NAME=“RAMDISK”
MB=4096

SECTORS=$(( MB * 2048 ))

echo “Creating ${MB}MB RAM disk...”

DEV=“$(hdiutil attach -nomount ram://${SECTORS} | awk 'END{print $1}')”

if [[ -z “$DEV” ]]; then
  echo “❌ Failed to allocate RAM disk”
  exit 1
fi

diskutil eraseVolume APFS “$NAME” “$DEV” >/dev/null 2>&1

echo “✅ RAM disk mounted at /Volumes/$NAME”

How the Script Works

1. Strict Error Handling

set -euo pipefail ensures the script exits immediately on error, preventing partial or unsafe states.

2. Disk Size Conversion

macOS RAM disks are created using sector counts, so the script converts megabytes into sectors before allocation.

3. Memory Allocation

hdiutil attach -nomount ram:// requests memory from the system and exposes it as a block device.

4. APFS Formatting

The device is formatted as APFS, allowing it to behave like a modern macOS volume.

5. Mounting

The disk is mounted automatically at /Volumes/RAMDISK and is ready for immediate use.

Important Notes

Allocating large RAM disks reduces memory available to macOS and running applications. Choose a size appropriate for your system.

When to Use This

This setup is ideal for developers and power users who want maximum performance for temporary workflows without installing extra tools or modifying system settings.

When you’re done, simply reboot — the RAM disk disappears automatically.