Bulletproof Docker + Nextcloud Startup: Surviving Power Cuts Without a UPS 🛡️

Running a self-hosted Nextcloud instance on your NAS without a UPS? You’ve probably run into this dreaded error after a sudden power loss or OS reinstall:

Your data directory is invalid
Ensure there is a file called .ncdata in the root of the data directory.

Or maybe:

“Permissions are not writable”
“Missing .ncdata”
“Docker container won’t start”

These problems typically happen because Docker starts before your data disk is mounted, especially if you're using /srv/personal symlinks that point to a UUID-based disk path.

This guide combines multiple powerful solutions into one bulletproof setup — ensuring your Nextcloud container remains healthy across power cuts, OS reinstalls, UUID changes, and mount delays.

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✅ What You'll Achieve

• 🧷 Stable symlinks for persistent Docker volumes
• ⏳ Make Docker wait for your disk and .ncdata file
• 🔐 Auto-fix Nextcloud folder permissions on boot
• 🔁 Auto-run fsck after power loss

📁 Step 1: Use a Stable Symlink for Your Data Disk

1. Mount the Disk via OMV

Mount your data disk in OMV → File Systems. It will be mounted under a UUID path like:

/srv/dev-disk-by-uuid-1234abcd/

2. Create a Stable Symlink

sudo rm /srv/personal
sudo ln -s /srv/dev-disk-by-uuid-1234abcd /srv/personal

This allows Docker to always use /srv/personal/nextcloud even if the UUID changes — just update the symlink later.

3. Update Docker Volume Path

volumes:
  - /srv/personal/nextcloud:/var/www/html/data

⏱️ Step 2: Make Docker Wait for .ncdata File

Prevent startup errors by delaying Docker until the file is accessible.

Create the Wait Service:

sudo nano /etc/systemd/system/docker-wait-for-ncdata.service

Paste this:

[Unit]
Description=Wait for /srv/personal/nextcloud/.ncdata before starting Docker
DefaultDependencies=no
After=local-fs.target
Before=docker.service

[Service]
Type=oneshot
ExecStart=/bin/bash -c 'while [ ! -f /srv/personal/nextcloud/.ncdata ]; do echo "Waiting for .ncdata..."; sleep 3; done'
RemainAfterExit=true

[Install]
WantedBy=multi-user.target

Enable it:

sudo systemctl daemon-reload
sudo systemctl enable docker-wait-for-ncdata.service

✅ What to Do Next

• 🔹 Step 1: Manually Create .ncdata

You only need to do this once — unless you erase the data folder again.

sudo touch /srv/personal/nextcloud/.ncdata
sudo chown 33:33 /srv/personal/nextcloud/.ncdata

This creates the marker file that your wait service is looking for.

✅ After this, your systemd logic will work as intended every time, including after power loss.

⚙️ Step 3: Create Docker Service Override

Ensure Docker respects the wait service.

sudo mkdir -p /etc/systemd/system/docker.service.d
sudo nano /etc/systemd/system/docker.service.d/override.conf

Paste this:

[Unit]
Requires=docker-wait-for-ncdata.service
After=docker-wait-for-ncdata.service

Save and reload:

sudo systemctl daemon-reload

Verify:

systemctl cat docker.service

🧼 Step 4: Force Filesystem Check on Boot

1. Identify the Disk

df -h /srv/personal/nextcloud

2. Enable fsck Auto-Check

sudo tune2fs -c 1 /dev/sdX

Replace /dev/sdX with the actual device (e.g. /dev/sdb1).

🔐 Step 5: Auto-Fix Permissions on Boot

1. Create the Script

sudo nano /usr/local/bin/fix-nextcloud-perms.sh

Paste:

#!/bin/bash

# Set ownership of the local Nextcloud bind mount directory
chown -R 33:33 /srv/personal/nextcloud

# Set ownership of the Docker volume data directory
chown -R 33:33 /var/lib/docker/volumes/nextcloud_nextcloud_html/_data

Make it executable:

sudo chmod +x /usr/local/bin/fix-nextcloud-perms.sh

2. Create the Service

sudo nano /etc/systemd/system/fix-nextcloud-perms.service

Paste:

[Unit]
Description=Fix Nextcloud Permissions
After=local-fs.target

[Service]
ExecStart=/usr/local/bin/fix-nextcloud-perms.sh
Type=oneshot

[Install]
WantedBy=multi-user.target

Enable it:

sudo systemctl daemon-reload
sudo systemctl enable fix-nextcloud-perms.service

🔁 Step 6: Recreate Symlink After UUID Change

1. Re-mount via OMV

Then recreate the symlink:

sudo rm /srv/personal
sudo ln -s /srv/dev-disk-by-uuid-NEWUUID /srv/personal

📋 Summary

Step	Description
1	Create a symlink for persistent Docker volumes
2	Make Docker wait for .ncdata before starting
3	Auto-run fsck on every boot
4	Auto-fix Nextcloud folder permissions
5	Update symlink if UUID changes

🎉 Conclusion

By combining symlinks, systemd wait services, fsck, and auto permission fixes, your NAS setup becomes resilient and ready for real-world chaos.

This is the ideal solution for homelabbers without a UPS who still want reliable, self-hosted services like Nextcloud, Immich, or Jellyfin.