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To keep Rails Active Storage uploads after a Dokku deploy, mount a persistent host directory at the container path Rails uses for its Disk service, then restart and verify the app. A typical setup is:
- Create an app-specific host directory such as
/var/lib/dokku/data/storage/my-rails-app. - Set ownership and permissions for the UID/GID that runs the deployed Rails process.
- Mount it at the absolute container path
/app/storage. - Configure Active Storage to use
Rails.root.join("storage")when the image’s working directory is/app. - Restart the app so Dokku recreates its containers, and check the mount with
dokku storage:listordokku storage:report.
The mount protects files from replacement of an individual container on that Dokku host. It is not a backup, does not make data portable to another host, and is unavailable during the image build.
Why Rails uploads disappear after a Dokku deploy
Dokku applications run in containers that can be replaced during a deploy, restart, scaling operation, or recovery. Files written only inside a container’s writable layer therefore should be treated as ephemeral. Rails Active Storage’s local Disk service writes blobs into a directory in the application filesystem; without a persistent mount, those blobs disappear when the container holding them is replaced.
A Dokku storage mount keeps the data on the host and presents that host directory inside each newly created runtime container. The solution is deliberately simple, but it depends on three paths agreeing:
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- Host path: a directory on the Dokku server, normally under
/var/lib/dokku/data/storage. - Container path: the absolute directory mounted into the running app, commonly
/app/storage. - Rails root: the directory returned by
Rails.root.join("storage")when the deployed image works from/app.
If any of these differs, Rails may write somewhere other than the mounted volume, or the process may be unable to read and write the mounted files.
Decide whether a local Dokku volume fits the application
A local mount is appropriate when the app runs on one Dokku host and you want straightforward persistence without adding an object-storage service. It keeps files across container replacement on that host and has no network hop for each file access.
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It does not replicate data, move it to a second host, or create an independent backup. If the server, disk, or directory is lost, the mounted files are lost unless you have copied them elsewhere. Applications that must move between hosts, run on multiple hosts, or retain independently managed durability should evaluate an S3-compatible service or another Active Storage service instead.
Check the image working directory first
/app/storage is a convention, not a Dokku requirement. It works when the deployed image’s application root is /app and Rails resolves Rails.root there. A custom Docker image may use a different working directory. Confirm the image configuration and the runtime value of Rails.root before selecting the container target. Mount the host directory at the path Rails actually uses, rather than assuming every image uses /app.
Identify the scheduler and Dokku version
Legacy host:container mount syntax is supported for Docker-local applications, but it is rejected for k3s applications. Newer Dokku installations provide named storage entries that are better suited to scheduler-aware configuration. Use the storage command syntax supported by the installed Dokku version and scheduler; do not copy a Docker-local command unchanged to a k3s app.
Prepare the host directory
1. Choose an app-specific location
Keep each application’s persistent data in its own directory. A conventional choice is:
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/var/lib/dokku/data/storage/my-rails-appUsing an application-specific directory prevents unrelated apps from sharing files accidentally and makes backup, restoration, and migration boundaries clear.
2. Create the directory before mounting
Dokku requires an explicit host path to exist before the mount is created; it does not create that path automatically. On the Dokku server, create the directory with the appropriate administrative privileges:
sudo mkdir -p /var/lib/dokku/data/storage/my-rails-appCheck that the path is on the intended filesystem and has enough capacity for the expected upload workload. A successful mount command does not by itself prove that the directory is writable by the Rails process.
3. Set runtime ownership and permissions
The process inside the container must be able to create directories, write new blobs, and read existing blobs. Buildpack deployments use Dokku’s documented directory-preparation ownership defaults. For a custom Docker image, determine the effective UID and GID of the application process and assign ownership or group permissions on the host directory accordingly.
Do not guess a username from the host operating system: the relevant identity is the numeric user and group seen by the container at runtime. After setting ownership, test both a write and a read from the deployed application. Avoid making the directory world-writable as a shortcut; broad permissions can expose uploaded data and hide an incorrect runtime identity.
Mount the directory with Dokku
Legacy Docker-local syntax
For a Docker-local application using the traditional syntax, mount the host directory at the absolute container path:
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dokku storage:mount my-rails-app /var/lib/dokku/data/storage/my-rails-app:/app/storageThe segment before the colon is the host path. The segment after it is the path inside the container. Both must be intentional: changing the container target changes where Rails must look for its Disk root.
Named storage entries on newer installations
Newer Dokku versions can create a named storage entry and then mount that entry with an explicit container directory. The pattern is:
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dokku storage:mount my-rails-app uploads --container-dir /app/storageUse the exact command syntax exposed by your Dokku version. Named entries support scheduler-aware configuration and can carry options such as process-type scoping, read-only mounts, volume options, and replacement with --replace. These options are useful when only selected process types should see a volume or when a process must not modify it.
Do not use the legacy host:container form for a k3s application; Dokku rejects that form there. Select the named-entry workflow and follow the k3s-compatible syntax installed on the server.
Restart after every storage change
Adding, changing, or removing a mount requires a container recreation. Restart the app after the mount operation:
dokku ps:restart my-rails-appA restart is also required after changing the target path or mount options. Without it, existing containers may continue running with their previous filesystem configuration.
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Verify Dokku’s recorded configuration
Inspect the app’s storage configuration:
dokku storage:list my-rails-app
dokku storage:report my-rails-appConfirm that the expected host entry and container target are shown. Then exercise the application by uploading a small file, downloading it, restarting the app again, and downloading the same file. This checks the complete path from Rails to the mounted directory rather than merely checking Dokku’s metadata.
Configure Rails Active Storage to use the mount
Set the local Disk root
Rails’ local Active Storage service can use a Disk root based on the application root. In config/storage.yml:
local:
service: Disk
root: <%= Rails.root.join("storage") %>This makes the storage directory follow the deployed application root instead of embedding a host-specific path in Rails configuration.
Select the service in production
Use the local service in the production environment only when local disk is an intentional architecture choice:
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Confirm the path alignment
When Rails.root is /app, the Disk root resolves to /app/storage, matching the Dokku mount. If the image uses another root, either mount the host directory at the resulting path or deliberately configure a different Disk root. Validate the effective setting in the running production container; a correctly mounted directory is irrelevant if Rails writes to another location.
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Understand what Dokku mounts do during deployment
Mounts are runtime resources, not build resources
Dokku documents that mounts are available to containers created by run and by the deploy process, not during the build process. A build step cannot rely on the mounted directory being present. Do not place required build artifacts, generated assets, or migration inputs in the runtime volume and expect the image build to read them.
If a build must create an application directory, create it in the image or during the build independently. The runtime mount is attached only when Dokku creates the running container.
The mount hides image content at its target
A mount does not merge host files with files already present in the image. Dokku notes that if the target path contains pre-existing content, the container files are overwritten from the mounted view. Consequently, any files baked into the image under /app/storage become inaccessible while the mount is attached.
Do not depend on seed files, fixture uploads, or generated content under the target directory surviving the transition to a mount. Put durable data in the host directory before deployment, or copy it through a deliberate migration procedure before enabling the mount.
Troubleshoot common failures
| Symptom | Likely cause | What to check or change |
|---|---|---|
| Uploads vanish after redeploy | Rails wrote to the container layer or to a path other than the mount target. | Check the production Active Storage service, the runtime Rails.root, and the target shown by storage:report. Upload a test file, restart, and verify it remains. |
storage:mount fails immediately |
The explicit host directory does not exist. | Create the full directory on the Dokku host, then run the mount command again. |
| Uploads fail with permission errors | The container’s effective UID/GID cannot write the host directory. | Determine the runtime identity for the image and adjust ownership or group permissions; do not solve it with unrestricted permissions. |
| The mounted directory appears empty | The mount hides files that were baked into the image, or the host directory is newly empty. | Inspect the host directory itself and copy any required data there through an explicit migration. A mount is not a merge operation. |
| The build cannot see uploaded files | Build containers do not receive runtime mounts. | Move build-time inputs into the source tree, image, or a build-specific mechanism. Keep user uploads as runtime data. |
| Changes do not take effect | Existing containers were not recreated. | Run dokku ps:restart my-rails-app after adding, changing, or removing the mount. |
| A Docker-local command is rejected by k3s | Legacy host:container syntax is unsupported for that scheduler. |
Use a named storage entry and the k3s-compatible command syntax for the installed Dokku version. |
| One instance sees files another cannot | The architecture uses separate hosts or isolated local volumes. | Use shared object storage or another shared filesystem when multiple instances or hosts must access the same blobs. |
Back up and restore the persistent directory
The host mount prevents loss caused by replacement of a container on the same server; it does not provide an independent copy. Treat /var/lib/dokku/data/storage/my-rails-app as production data and include it in a backup plan that stores copies separately from the Dokku host or disk.
Back up consistently
- Record the exact host path, app name, and container target in the backup inventory.
- Back up the files with metadata and directory structure intact.
- Protect backups with access controls appropriate for user uploads, which may contain private or regulated information.
- Test restoration to a separate location rather than assuming that a successful backup job is usable.
Restore without confusing image files and volume files
Restore the data into the host directory used by the mount, not into the image’s /app/storage layer. Verify ownership for the runtime UID/GID, restart the app, and perform an upload and download test. If you restore while a container is running, coordinate the operation so Rails does not observe a partially restored directory.
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Local Dokku storage versus S3-compatible object storage
Rails Active Storage supports cloud services such as Amazon S3, Google Cloud Storage, and Azure Storage, as well as mirror configurations. The right choice depends on where the app runs, how many instances need the files, and who operates backups.
| Decision factor | Local Dokku mount | S3-compatible object storage |
|---|---|---|
| Durability and backup model | Files live on one Dokku host and require a separate backup system. | Durability, replication, and versioning depend on the provider and bucket policy; backups still need an explicit retention plan. |
| Portability across hosts | Low unless the directory is copied or the disk is moved. | High: a new app host can use the same bucket or a controlled migration. |
| Operational complexity | Simple mount and filesystem permissions, plus host-disk monitoring and backups. | Requires credentials, bucket or container policy, lifecycle rules, and network configuration. |
| Latency | Usually local filesystem access on the Dokku host. | Each uncached operation crosses the network and depends on service and connection latency. |
| Cost | Uses Dokku host disk and whatever backup storage you add. | Uses provider storage, request, transfer, and possibly retrieval charges. |
| Access control | Unix ownership and permissions on the host, plus host access controls. | Service credentials, bucket policies, and application-level authorization. |
| Shared access | Works naturally for processes on the same host and mounted volume; separate hosts need another shared layer. | Designed for access from multiple app instances and hosts. |
Use a local mount when single-host operation and simple filesystem semantics outweigh portability. Prefer object storage when independent durability, multi-host access, or migration between servers is a core requirement. Keep production buckets, prefixes, or equivalent namespaces separate from development and test data.
Operational checklist
- Verify the image’s working directory and the resulting
Rails.root. - Create an app-specific host directory before invoking
storage:mount. - Set ownership and permissions for the actual runtime UID/GID.
- Mount the host directory at an absolute container path that matches Rails’ Disk root.
- Use named storage entries when the Dokku version or scheduler requires them.
- Restart the app after every storage configuration change.
- Confirm the configuration with
dokku storage:listordokku storage:report. - Test upload, download, restart, and download again.
- Back up the host directory independently and test a restore.
- Reconsider local disk if the app will run across hosts, require shared access, or need independently managed durability.
Final recommendation
For a single-host Rails app using local Active Storage, create /var/lib/dokku/data/storage/my-rails-app, give it the deployed process’s effective ownership, mount it at the Rails storage path, restart the app, and verify persistence with a real upload cycle. Treat that mount as durable runtime storage—not as build input, a merge with image files, or a backup—and move to object storage when portability, multi-host access, or independent durability matters.
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