MergerFS: Combine Multiple Drives into One Pool

Have a bunch of different-sized drives? Want to use them as one big storage pool without RAID complexity? MergerFS is the answer. Here's how to pool your drives while maintaining flexibility.

What is MergerFS?

MergerFS is a union filesystem that:

• Combines multiple drives into a single mount point
• No reformatting required - Use existing filesystems
• No redundancy overhead - Full capacity available
• Drive independence - Each drive remains accessible individually
• Flexible policies - Choose how files are distributed

MergerFS vs RAID vs ZFS

Feature	MergerFS	RAID	ZFS
Mixed drive sizes	✅	❌	⚠️
No parity overhead	✅	❌	❌
Drive independence	✅	❌	❌
Redundancy	❌*	✅	✅
Performance	Good	Great	Great
Complexity	Low	Medium	High

*Use SnapRAID alongside MergerFS for parity protection!

Installation

# Ubuntu/Debian
sudo apt install mergerfs

# Or latest from GitHub
wget https://github.com/trapexit/mergerfs/releases/download/2.38.0/mergerfs_2.38.0.ubuntu-focal_amd64.deb
sudo dpkg -i mergerfs_2.38.0.ubuntu-focal_amd64.deb

Basic Setup

Prepare Your Drives

Assuming drives at /mnt/disk1, /mnt/disk2, /mnt/disk3:

# Create mount point for pool
sudo mkdir -p /mnt/storage

# Mount drives (add to fstab for persistence)
sudo mount /dev/sdb1 /mnt/disk1
sudo mount /dev/sdc1 /mnt/disk2
sudo mount /dev/sdd1 /mnt/disk3

Create MergerFS Pool

# Basic merge
sudo mergerfs /mnt/disk1:/mnt/disk2:/mnt/disk3 /mnt/storage

# With recommended options
sudo mergerfs -o defaults,allow_other,use_ino,cache.files=off,moveonenospc=true,category.create=mfs,dropcacheonclose=true,minfreespace=50G,fsname=mergerfs /mnt/disk*:/mnt/storage

Persistent Mount (fstab)

/etc/fstab

# Individual drives
/dev/sdb1 /mnt/disk1 ext4 defaults 0 2
/dev/sdc1 /mnt/disk2 ext4 defaults 0 2
/dev/sdd1 /mnt/disk3 ext4 defaults 0 2

# MergerFS pool
/mnt/disk* /mnt/storage fuse.mergerfs defaults,allow_other,use_ino,cache.files=off,moveonenospc=true,category.create=mfs,dropcacheonclose=true,minfreespace=50G,fsname=mergerfs 0 0

Key Options Explained

category.create (File Placement Policy)

Policy	Description
mfs	Most free space - New files go to drive with most room
lfs	Least free space - Fill drives evenly
epmfs	Existing path, most free space - Keep files together
rand	Random placement
ff	First found - Fill drives sequentially

For media libraries: mfs or epmfs For general use: mfs

minfreespace

Stop writing to a drive when it has less than specified free space:

minfreespace=50G  # Leave 50GB free on each drive

moveonenospc

If a drive fills up during a write, move the file to another drive:

moveonenospc=true

Use with Docker

Perfect for media stacks!

docker-compose.yml

services:
  jellyfin:
    image: jellyfin/jellyfin
    volumes:
      - /mnt/storage/media:/media:ro
      
  radarr:
    image: lscr.io/linuxserver/radarr
    volumes:
      - /mnt/storage/media/movies:/movies
      - /mnt/storage/downloads:/downloads

All containers see one unified /mnt/storage path.

Monitoring Pool Status

# Check pool status
df -h /mnt/storage

# See individual drive usage
df -h /mnt/disk*

# Find which drive a file is on
getfattr -n user.mergerfs.fullpath /mnt/storage/somefile

Adding New Drives

1. Format and mount new drive
2. Add to mergerfs mount options
3. Remount or restart

# Add /mnt/disk4
sudo umount /mnt/storage
sudo mergerfs /mnt/disk*:/mnt/storage -o ...options...

Tips from Experience

1. Label your drives - Use /dev/disk/by-id/ in fstab
2. Test failover - Unplug a drive, verify others work
3. Monitor health - Use Scrutiny or smartd
4. Regular SnapRAID sync - Cron job daily
5. Backup critical data - SnapRAID isn't instant recovery

My Storage Setup

Current configuration:

• 3 x 26TB drives in MergerFS pool
• 1 x 24TB drive in MergerFS pool
• 1 x 22TB drive in MergerFS pool
• 1 x 18TB drive in MergerFS pool
• minfreespace: 500GB (per drive)
• Used for Jellyfin media library (Without Parity)

Learn More

• MergerFS Documentation
• Storage Options Comparison
• NFS Shares

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