mergerfs 硬碟合體術：天下儲存，分久必合

Prologue: All Storage Under Heaven — United We Store

It is a truth universally acknowledged in the annals of storage that what is divided must eventually be united, and what is united must eventually be divided. So it goes with NAS drives.

You have two NAS boxes. One holds your video archive, the other your documents. Every time you need a file, you must first ask yourself: “Did I stash this in Cao Cao’s granary or Sun Quan’s?” — as if you were fumbling across the map of the Three Kingdoms hunting for supply caches. By the time you find the file, your sanity is already a casualty of war.

What you need is mergerfs.

mergerfs is a Union Filesystem — it never touches the data on your original drives. Instead, it builds a virtual layer on top, unifying scattered storage under a single command structure. In plain terms: it is Zhuge Liang himself. Without relocating a single city, armed with nothing but a map and a feather fan, he makes every granary in the realm respond as one.

Cao Cao's Xuchang Granary (NAS A)  ──┐
                                      ├──→  /media/unified (Headquarters — unified entry point)
Sun Quan's Jiangdong Stores (NAS B) ──┘

This article deploys the art of war from the Three Kingdoms to walk you through the complete mergerfs storage-unification campaign. Every command has been battle-tested — copy and deploy at will. Before the campaign begins, muster your supplies:

# Install the NFS client (courier horses for communicating with remote granaries)
sudo apt install nfs-common

# Install mergerfs (the grand strategist himself)
sudo apt install mergerfs

First Strategy: Setting Up Camp — Mounting Remote Granaries

Strategist’s Maxim: “Before the grain moves, the camps must be in place.”

The first step in any campaign is not to charge — it is to establish forward camps outside each allied city. Mounting remote NFS storage to local directories is the equivalent of pitching your tents outside Cao Cao’s fortress at Xuchang and Sun Quan’s capital at Jianye, ready to requisition supplies at a moment’s notice.

First, review the deployment map:

# Create mount directories (lay the foundations for camp)
sudo mkdir -p /mnt/storage1 /mnt/storage2

# Mount the first NAS (use soft mount to avoid system lockup if the network drops)
sudo mount -t nfs -o soft,timeo=30,retrans=3 SERVER_A_IP:/data1 /mnt/storage1

# Mount the second NAS
sudo mount -t nfs -o defaults SERVER_B_IP:/data2 /mnt/storage2

A tactical note: the first NAS uses soft,timeo=30,retrans=3. In plain language: “If Cao Cao’s granary doesn’t respond within thirty seconds, retry three times and then retreat.” Why? Because the NFS default is a hard mount — once the network goes down, your system will hold its ground like Guan Yu besieged at Mai Castle, refusing to withdraw, locking up the entire machine until the end of time. A soft mount, on the other hand, embraces the ancient wisdom: “He who knows when to retreat is the true hero.”

Second Strategy: The Grand Alliance — mergerfs Unified Command

Strategist’s Maxim: “The lords need not move their capitals; a single pact commands all under heaven.”

The camps are pitched, the supply lines are open. But every time you need something, you still have to send runners separately to /mnt/storage1 and /mnt/storage2 — as if, despite having signed the alliance, you still dispatch separate convoys to Xuchang and Jianye for every sack of grain. Absurd.

Enter our grand strategist, Zhuge Liang — mergerfs. He doesn’t move a single sack of grain, doesn’t alter a single warehouse. He simply lays a unified command layer on top, so that from headquarters at /media/unified, one gate gives you access to every granary in the realm.

# Create the unified entry point (the strategist's command tent at headquarters)
sudo mkdir -p /media/unified

# Execute the merger command (Zhuge Liang unfurls the map — the grand alliance begins)
sudo mount -t fuse.mergerfs \
  -o defaults,allow_other,use_ino,nonempty,category.create=mfs,minfreespace=10G \
  /mnt/storage1:/mnt/storage2 \
  /media/unified

This single command is the core pact of the entire campaign. Let us decode the strategist’s deployment, parameter by parameter:

• defaults — Standard marching parameters (rw, suid, dev, etc.). The reliable baseline — no army marches without it.
• allow_other — All soldiers may access the stores, not just the commanding general. Without this, only root can enter; everyone else is turned away at the tent flap.
• category.create=mfs — Most Free Space: the strategist’s most brilliant dispatch policy. New supplies are automatically routed to whichever granary has the most room. Cao Cao’s side still has eighty percent capacity while Sun Quan’s is down to twenty? The next shipment goes straight to Xuchang. The strategist handles logistics so the general doesn’t have to.
• minfreespace=10G — Stop shipping when reserves drop below ten bushels. When any granary falls below 10GB of free space, the strategist automatically stops sending new files there — preventing an overflow crisis.
• use_ino — Every supply sack gets a unique seal (inode), ensuring sacks from different granaries never share the same mark.
• nonempty — Permits garrisoning a camp even if it already holds supplies. Without this, if the headquarters directory already contains files, you’ll be barred at the gate.
• /mnt/storage1:/mnt/storage2 — The colon is the supply road connecting each granary. Need to bring a third lord into the alliance? Just add another colon and path. Expanding the coalition is that simple.

Third Strategy: Enshrining the Law — Permanent fstab Configuration

Strategist’s Maxim: “A field order wins a battle; only codified law secures the realm.”

Everything above is temporary — reboot the machine and the pact dissolves, supply lines collapse, and headquarters is left performing the Empty Fort Strategy for real. To make this deployment permanent, the field orders must be carved into the law of the land: /etc/fstab.

# [NFS Mounts]
SERVER_A_IP:/data1  /mnt/storage1  nfs  soft,timeo=30,retrans=3  0  0
SERVER_B_IP:/data2  /mnt/storage2  nfs  defaults              0  0

# [mergerfs Pool]
/mnt/storage1:/mnt/storage2  /media/unified  fuse.mergerfs  defaults,allow_other,use_ino,category.create=mfs,minfreespace=10G  0  0

Before you inscribe the law, heed this warning: test the formation before carving it into the war manual. Verify everything works with the manual commands first, then write it into fstab. If you get it wrong, the machine won’t boot — and that’s no longer Romance of the Three Kingdoms, it’s a tragedy.

Inspecting the Granaries: Verification

Strategist’s Maxim: “Once the formation is set, inspect the camp before anything else.”

The pact is signed, the law is carved — but a wise commander still inspects the granaries in person to confirm the supplies are truly in place.

Tally the Total Stores

df -h /media/unified

This command shows the combined capacity of both NAS units. If Cao Cao’s side has 2TB and Sun Quan’s has 3TB, you should see roughly 5TB of available space. The feeling of uniting all storage under heaven — there is nothing quite like it.

Test the Supply Dispatch

Create a large file in /media/unified, then check /mnt/storage1 and /mnt/storage2 — which granary received the new shipment? If the strategist’s category.create=mfs policy is working, it should appear on whichever side has more free space.

A Note on Network Bandwidth

Remember, the underlying transport is NFS over the network. Your merged storage performance is bounded by network bandwidth — no matter how wide the supply road, if the courier horses are slow, the grain arrives late. A 1Gbps or faster LAN is recommended. Otherwise, large file transfers will teach you what the ancients meant by “shipping grain a thousand miles by ox cart.”

Luo Guanzhong’s Verdict

Luo Guanzhong writes: “I have surveyed the storage of all under heaven, and its state is like the warring kingdoms. Each NAS holds its own territory, files scattered across the four corners, and the user marches wearily between them, searching without rest. mergerfs is the grand alliance strategy — it moves not a single brick, relocates not a single file, yet through a mere virtual layer it unites all domains as one. Access flows as naturally as the arm commands the fingers. In the art of war, the supreme strategy is to subdue the enemy without battle; in the art of storage, the supreme strategy is to unify without moving a single file. Those who master this art may truly be called united without migration, commanding without movement — storage in concert, responding as one.“

May your storage dominion, from this day forward, be united under heaven — with no more kingdoms at war.