To persist Rails Active Storage uploads on Dokku, mount an app-specific host directory into the container at the directory Rails uses for its local Disk service. A typical setup creates /var/lib/dokku/data/storage/my-rails-app, mounts it at /app/storage, restarts the app, and verifies the mount. The host directory must exist before the mount command runs, and a mount protects files from container replacement on that host but is not a backup.
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The instructions below assume the deployed image runs the Rails application from /app. If your image uses another working directory, use the matching absolute container path and set Rails’ Disk root accordingly.
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What Dokku persistent storage changes
Dokku containers are replaceable during deploys, restarts, and releases. Files written only inside a container’s writable layer can therefore disappear when that container is replaced. A persistent-volume mount places those writes in a directory on the Dokku host, outside the container lifecycle.
For a Rails application using Active Storage’s local Disk service, the important directory is normally the application’s storage directory. With a working directory of /app, that directory is /app/storage. Mounting a host directory there makes uploaded blobs survive a new container on the same Dokku host.
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The mount is available to containers created for deployment and runtime, not to the image build process. It also masks whatever was already present at the target path inside the image. The container’s original files are not merged with the host directory; they are hidden while the mount is attached.
Choose the host and container paths
Use an app-specific host directory
Keep each application’s data in its own directory, conventionally below /var/lib/dokku/data/storage. For an app named my-rails-app, use:
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/var/lib/dokku/data/storage/my-rails-appAn app-specific directory prevents uploads from different applications from sharing a namespace and makes backup jobs easier to scope. Do not point several unrelated apps at the same directory unless shared data is an intentional design.
Mount to an absolute path in the container
Dokku storage targets must be absolute container paths. The common mapping for a Rails image whose working directory is /app is:
/var/lib/dokku/data/storage/my-rails-app:/app/storageConfirm the image’s working directory before adopting this exact target. A custom image might run from another directory, in which case Rails.root.join('storage') resolves elsewhere. Mounting /app/storage in that situation can leave Active Storage writing to an unmounted location.
Prepare the host directory and permissions
Create the directory first
Dokku does not create an explicit host path for you. Create it before invoking storage:mount:
sudo mkdir -p /var/lib/dokku/data/storage/my-rails-appIf the parent directories are absent, the mount command fails instead of provisioning them automatically.
Give the runtime user access
The process running Rails must be able to create directories and files below the mount. For buildpack-based apps, follow Dokku’s documented directory-preparation ownership defaults. For a custom Docker image, determine the effective UID and GID used by the application process and make the host directory writable by that identity. Do not assume that the host’s login user, the Dockerfile’s build user, and the runtime user are the same.
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Check both directory traversal and write permission. A directory can appear correctly mounted while uploads fail because the process can read the mount point but cannot write into it. Apply the least privilege that permits the Rails process to create its Active Storage files; avoid making an upload directory world-writable as a shortcut.
Mount the volume with Dokku
Legacy host-to-container syntax
On Docker-local installations, the established command is:
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Restart after every storage change
Adding, changing, or removing a mount requires a restart so Dokku recreates the app’s containers with the new configuration:
dokku ps:restart my-rails-appA restart is not optional cleanup. An already-running container cannot retroactively acquire a changed mount definition.
Verify Dokku’s recorded configuration
After the restart, inspect the app’s storage configuration:
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dokku storage:list my-rails-appYou can also request the detailed report:
dokku storage:report my-rails-appThese checks confirm that Dokku knows about the mount. They do not, by themselves, prove that Rails can write to it, so perform an application-level upload test as well.
Configure Rails Active Storage for local disk
Define the Disk service
Rails’ conventional local service in config/storage.yml is:
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local:
service: Disk
root: <%= Rails.root.join("storage") %>This configuration resolves the root relative to the application directory. When Rails.root is /app, the resulting root is /app/storage, which is the mount target used above.
Select the service in production
In the production environment configuration, select the local service only if storing files on the Dokku host is deliberate:
config.active_storage.service = :localRestart or redeploy after changing Rails configuration so the running process loads the new setting. If production already uses an object-storage service, mounting /app/storage will not redirect those uploads; the selected Active Storage service determines where blobs are written.
Check the working-directory assumption
Do not infer the path from a local development checkout. Inspect the deployed image and Rails configuration to confirm the runtime value of Rails.root. A custom Dockerfile, an alternate entrypoint, or a different release layout can change that value. The mount target, Disk root, and actual runtime directory must all refer to the same location.
Test persistence before relying on it
- Deploy the app with the mount configured and restart it.
- Upload a small file through the normal Active Storage code path.
- Confirm the application can download or display that file.
- Use
dokku storage:list my-rails-appordokku storage:report my-rails-appto confirm the mount remains registered. - Restart the app again and repeat the download. A successful second request demonstrates survival across container recreation, not merely a successful write to the container layer.
For a stronger check, record the blob’s key, restart, and verify the same record still resolves to the same file. Test both a new upload and a read of an existing upload; permissions that allow one operation can still block the other.
Understand deploy-time behavior and hidden files
Builds cannot use the runtime mount
Dokku makes mounts available to containers created by the deploy and run processes, not during the build process. A buildpack step or Docker build step cannot depend on files that will appear only after the runtime mount is attached. Put application code and build assets in the image or another build-time input.
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If the image already contains files at /app/storage, attaching the host directory hides those files. Dokku describes this as the container files being overwritten at the mounted path; in practical terms, the directories are not merged. Do not place seed files, default uploads, or required runtime assets under the path that will become a mount unless you copy them into the host directory separately.
Plan migrations deliberately
Before introducing a mount to an existing deployment, identify any files currently inside the container’s storage directory. They will not automatically appear in the new host directory. Copy data through a controlled migration process before or during the change, then restart and verify the application against the mounted location. The same caution applies when changing the host path or removing a mount.
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Use named storage entries on newer Dokku installations
Newer Dokku versions support named storage entries, which are useful when scheduler-aware settings or more explicit options are needed. The exact command form depends on the installed Dokku version and scheduler, so check the local command help before applying it.
Create and attach an entry
The supported workflow includes creating a storage entry and attaching it with a container directory:
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dokku storage:mount my-rails-app my-rails-storage --container-dir /app/storageUse the actual entry name you create. Named entries can carry additional configuration that is cumbersome in a single host-to-container string.
Scope and options
Dokku’s named-storage features can include process-type scoping, read-only mounts, volume options, and a --replace operation. Process-type scoping matters when only selected processes should see a volume; a web process and a background worker that both handle Active Storage files generally need consistent access to the same data. A read-only mount is appropriate for consumers that must not modify files, not for the process receiving uploads.
Match the scheduler
Legacy host:container syntax remains supported on Docker-local installations but is rejected for k3s apps. A command that works on one scheduler can therefore fail on another even when the paths are correct. Identify whether the app runs under Docker-local or k3s, use the storage syntax supported by that scheduler, and confirm the installed Dokku version’s option names before scripting the setup.
Troubleshoot the common failure modes
The mount command says the host path is missing
Create the complete directory tree with mkdir -p, then rerun the mount. Dokku’s explicit host-path behavior does not auto-create the directory.
Uploads disappear after a redeploy
Check whether Active Storage is actually using :local, whether its Disk root resolves to the mounted directory, and whether the mount is listed by Dokku. If the app writes to another path, the volume is protecting the wrong directory. If no mount is present, the files were written to the ephemeral container layer.
Rails reports permission denied
Compare the runtime process UID and GID with the ownership and mode of the host directory. Correct the host directory’s ownership according to Dokku’s buildpack defaults or your custom image’s runtime identity. Also check execute permission on every parent directory; access to the final directory alone is insufficient.
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Files seem to vanish immediately after attaching the volume
That is the expected masking behavior when the image had pre-existing content at the target. Inspect the host directory, not the hidden image layer, and migrate any required files into the host directory before restarting.
The app still uses a container path after changing storage
Restart the app after changing the mount. Containers created before the change retain their original mount configuration. Verify the registration with storage:list or storage:report, then test an upload and a download.
Docker-local syntax is rejected
If the app is scheduled on k3s, the legacy host:container form is not accepted. Use the scheduler-compatible named-storage workflow and its supported options instead.
Back up the host directory separately
A Dokku mount prevents data loss caused by replacement of a container on the same host. It does not create an independent copy, protect against host-disk failure, or provide disaster recovery. Back up /var/lib/dokku/data/storage/my-rails-app with a system that stores copies outside the live directory, and test restoration.
Keep the backup boundary aligned with the app. Include the Active Storage files and any metadata your recovery plan requires, but do not assume that a database backup alone contains the binary blobs. Conversely, restoring blobs without the corresponding database records can leave unreachable files.
Decide between local Dokku storage and object storage
Rails Active Storage supports cloud services such as Amazon S3 and Google Cloud Storage, as well as Azure Storage and Mirror services. The choice is architectural: local disk is simple and fast to access from one host, while object storage is managed outside the Dokku machine and is designed for portability or shared access.
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|---|---|---|
| Durability and backup model | Survives container replacement on the same host; you must operate separate backups of the host directory. | Durability and replication are provided by the storage service’s design and plan; configure retention and independent backups according to its policy. |
| Portability across hosts | Data is tied to the mounted Dokku host until you copy or restore it elsewhere. | Designed for access from multiple hosts and easier migration between deployment nodes. |
| Operational complexity | Requires directory creation, permissions, mount lifecycle, host monitoring, and backup jobs. | Requires credentials, bucket or container policy, endpoint configuration, and lifecycle management. |
| Latency | Uses local filesystem access from the app host. | Uses network requests to the storage endpoint, so application behavior depends on network and service response time. |
| Cost | Consumes local disk and backup capacity on the Dokku host. | Uses the provider’s storage, request, transfer, and optional replication pricing. |
| Access control | Primarily host filesystem ownership, permissions, and Dokku access. | Uses service credentials, bucket policies, endpoint controls, and application configuration. |
| Shared storage across instances | Requires all instances to reach the same host-backed data, which a single-host mount does not provide by itself. | Multiple app instances can use the same bucket or prefix when configured with compatible credentials and policies. |
Choose the local mount when the app is intentionally tied to one Dokku host and you can operate host backups. Prefer object storage when portability, multiple hosts, independently managed durability, or shared access across instances is a requirement. Rails’ environment-specific configuration lets production use object storage while development or a single-host deployment uses local disk.
Quick Recap
Production checklist
- The host directory is app-specific and exists before the mount command.
- The container target is an absolute path and matches the deployed
Rails.root.join('storage')location. - The Rails process UID and GID can read and write the host directory.
config.active_storage.serviceselects the intended production service.- The app was restarted after every mount change.
dokku storage:listordokku storage:reportshows the expected entry.- An upload survives a restart and can still be downloaded.
- Any files previously present at the target were migrated because the mount masks image content.
- Backups copy the host directory independently and restoration has been tested.
- The storage syntax matches the app’s scheduler and Dokku version.
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