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A Docker container is an isolated set of Linux processes, not a complete guest operating system. Docker combines kernel namespaces for separate resource views, cgroups for resource control, and layered filesystems for efficient image sharing; an OCI runtime such as runc, launched through containerd, connects the daemon’s configuration to those kernel features.
What a Docker container actually is
Running docker run does not boot a second Linux kernel. The Docker daemon asks the host kernel to start ordinary processes with restricted, rearranged views of the host. The result looks like a small machine because the process sees its own filesystem root, process tree, hostname, network interfaces and other resources, but those resources are still provided by the host kernel.
Docker’s own overview describes namespaces as the technology that provides the container’s isolated workspace: Docker’s “What is Docker?” documentation. Isolation is configurable rather than absolute. Containers can communicate through networks you connect, and the security boundary also depends on daemon privileges, Linux capabilities, the container configuration and kernel hardening. Docker’s security guidance covers those qualifications at Docker Engine security.
The startup path: from Docker command to kernel process
- The client sends a request. The Docker CLI turns your image, command, environment, mounts, network and resource settings into an API request to the Docker daemon (
dockerd). - The daemon prepares the container. It resolves the image, creates or selects writable storage, sets up mounts and networking, and records the requested limits and security settings.
- containerd manages execution. Docker invokes an OCI-compliant runtime through containerd. Docker’s daemon reference documents
runcas the default runtime and explains this handoff at the dockerd reference. - The runtime configures the kernel. The runtime creates namespaces, joins the process to cgroups, applies capabilities and mounts the container root filesystem, then starts the configured entrypoint.
- The application becomes the container’s process. The process runs directly on the host kernel. If that process exits, the container normally stops, although its image and any separately stored volume data remain available.
This division matters when troubleshooting. A failure can originate in image retrieval, daemon configuration, containerd, the OCI runtime, a kernel feature, a storage driver or the application itself; “Docker” is the user-facing control plane, not the component implementing every isolation primitive.
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Namespaces: separate views of shared kernel resources
A namespace changes what a process is allowed to see. Processes in one namespace can have a different view from processes outside it, even though the same kernel services the entire host.
Process (PID) view
A container can have its own process-numbering view. The application may see itself as PID 1 and see only processes in its container, while the host retains a broader process view. PID 1 also has special responsibilities for signal handling and reaping child processes, so an entrypoint that does not handle those jobs can produce confusing shutdown behavior.
Mount and filesystem view
The mount namespace gives the container its own arrangement of mounted filesystems. Docker can make an image-derived root appear at /, add bind mounts or volumes, and hide host paths that are not explicitly exposed. A mount namespace is not the same thing as data persistence: it controls visibility, while the storage layers and mounts determine where bytes are actually written.
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Network view
Network namespaces provide interfaces, routes and firewall context distinct from the host’s default network namespace. Docker then connects that namespace to a bridge, host network or another configured network. Two containers can communicate when Docker’s networking configuration permits it; namespace isolation does not automatically mean network isolation.
UTS, IPC and user views
UTS settings allow a container-specific hostname. IPC namespaces separate mechanisms such as shared memory and semaphores. User namespaces can map container user IDs to different host IDs, reducing the impact of processes that run as root inside the container when the host and daemon configuration support that mode.
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Namespaces answer the question “what can this process see?” They do not by themselves decide how much CPU time or memory it may consume, nor do they guarantee that a privileged container cannot affect the host. Those concerns require cgroups and security controls.
Cgroups: controlling consumption rather than visibility
Control groups, usually called cgroups, place the container’s processes in a kernel-managed hierarchy. Docker uses them as the resource-control mechanism associated with a container, as described in Docker Engine security. A cgroup can account for usage and enforce limits or priorities for resources such as CPU and memory; the exact controls available depend on the host kernel and Docker configuration.
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- A CPU limit can constrain how much processor time a busy container receives instead of allowing it to consume every available core.
- A memory limit can cause allocation failures or an out-of-memory termination when the process exceeds its allowance. The application may report an ordinary allocation error, while the host remains protected from unlimited growth.
- Accounting data helps explain why a container is throttled or terminated, but it does not make an inefficient application efficient.
Namespaces and cgroups solve different problems. Namespaces provide a process with a restricted perspective; cgroups constrain the quantity of shared resources it can use. A production configuration normally needs both, plus least-privilege capabilities, an appropriate user, a read-only root where practical and a carefully selected set of mounts.
Images, layers and the writable container layer
A Docker image is assembled from filesystem layers. Each Dockerfile instruction generally contributes a layer, and unchanged layers can be shared by many containers. This sharing reduces duplicate storage and lets Docker reuse already-downloaded content.
When Docker creates a container, it adds a container-specific writable layer above the image’s read-only layers. The process sees one merged directory tree, but Docker can keep the common image content unchanged underneath. The storage-driver overview explains this model at Docker storage drivers.
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What happens on a read
If a requested file exists in a lower image layer and has not been replaced, the merged filesystem returns that lower-layer content. Multiple containers can read the same immutable image data concurrently.
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What happens on a write
A write cannot alter a shared read-only image layer. The storage implementation creates a container-owned version in the writable layer, then applies the change there. This is commonly called copy-on-write, although the copy occurs when a lower-layer object first needs modification rather than when the container starts.
Deleting files in a container
Deleting a file from an image does not physically remove it from the lower layer. The writable layer records a deletion marker so the merged view hides the lower copy. The original image remains reusable by other containers.
OverlayFS and the classic overlay2 model
On Linux systems using Docker’s classic overlay2 storage driver, OverlayFS combines read-only lower directories with a container-specific upper directory and presents a merged mount. Docker’s implementation details, including copy-up behavior, are documented in the OverlayFS storage driver guide.
Suppose an image contains /etc/app.conf. A container that only reads it can use the lower image layer. If the application edits it, overlay2 copies the file into the upper directory, and subsequent reads and writes use that upper copy. The image’s version is unchanged, so another container created from the same image still starts with the original file.
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Copy-up can be expensive for large files or directories, and repeated writes increase the container layer’s size. Docker therefore advises using volumes for write-heavy application data rather than treating the writable layer as a database or durable workspace. The storage-driver guidance is at Docker storage drivers.
The current storage-driver caveat: Engine 29.0 and later
Do not assume every current installation stores image data under /var/lib/docker/overlay2. Docker’s selection guidance says fresh Docker Engine 29.0 and later installations use the containerd image store by default. An installation upgraded from an earlier setup can continue using a classic storage driver instead.
With the containerd image store, Docker documents image contents and container snapshots under /var/lib/containerd; other daemon data remains under /var/lib/docker. The daemon data-directory behavior is described in Docker daemon configuration overview. The directory you inspect is therefore evidence of the active configuration, not a universal rule.
How the main storage choices differ
| Option | What it provides | When it fits | Important qualification |
|---|---|---|---|
| Containerd snapshotters | Snapshot and image storage integrated with the containerd image store. | Fresh Docker Engine 29.0+ installations using the default image store. | Exact behavior depends on the selected snapshotter and host configuration. |
| overlay2 | OverlayFS lower layers, an upper writable layer and a merged view. | Classic Linux Docker installations needing a broadly compatible copy-on-write driver. | Upgraded installations may retain it; it is not the default for every fresh 29.0+ installation. |
| fuse-overlayfs | An OverlayFS-compatible approach implemented through FUSE. | Certain rootless Linux hosts where kernel OverlayFS support or permissions are limiting. | Suitability depends on kernel, distribution and rootless configuration. |
| btrfs or zfs | Filesystem-native snapshot and copy-on-write capabilities. | Hosts already designed and maintained around those filesystems. | They require additional filesystem setup and operational maintenance. |
| vfs | A compatibility-oriented storage approach without copy-on-write efficiency. | Testing or environments where copy-on-write support is unavailable. | It can use more storage and is generally not the first choice for production workloads. |
Docker’s current selection documentation covers these choices and their compatibility considerations at Select a storage driver. Choose based on Engine version, whether the containerd image store is enabled, Linux kernel and backing-filesystem support, workload write patterns, and operational stability—not on a directory name copied from another host.
Why volumes survive container removal
The writable container layer belongs to a particular container. Removing that container removes its layer, so changes stored only there disappear. A named volume or another persistent mount stores data outside that disposable layer and can be mounted into a replacement container.
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A safe persistence demonstration
- Create a volume:
docker volume create demo-data. - Write data through a temporary container:
docker run --rm -v demo-data:/data alpine sh -c 'echo persistent > /data/example.txt'. - Remove the temporary container. The
--rmflag removes its container layer when the command exits. - Read the volume from another container:
docker run --rm -v demo-data:/data alpine cat /data/example.txt. The output ispersistentbecause the file was written to the volume. - Delete the volume only when its data is no longer needed:
docker volume rm demo-data.
A bind mount can similarly expose a host directory, but it couples the container to a specific host path and host permissions. Volumes are usually easier for Docker to manage and are the normal choice for durable, write-heavy application data.
Inspecting what Docker is using
docker inforeports the active storage driver and other daemon details. Check this before interpreting filesystem paths.docker inspect <container>shows mounts, the image reference, network settings and resource-related configuration for one container.docker image inspect <image>exposes image metadata, including its layer-related information.docker volume inspect <volume>shows the volume’s driver and host-side mount location when Docker can report one.
These commands describe configuration; they do not prove that an application is persisting the files you care about. Verify the mount destination inside the container and test replacement or removal before trusting a deployment.
Common misconceptions and failure modes
“A container is a lightweight virtual machine.”
It may package an application with a familiar root filesystem, but ordinary containers share the host kernel. A kernel-level vulnerability, excessive capability or unsafe daemon configuration can cross the intended boundary, which is why isolation should be treated as a security configuration rather than a magical wall.
“The image is modified when the application edits a file.”
The image layers are read-only. The container gets an upper layer or snapshot for changes. Rebuilding an image is the way to make a new immutable application baseline; editing a running container is not an image-release process.
“Removing a container deletes all application data.”
It deletes data that exists only in that container’s writable layer. Data in a named volume, bind mount or other persistent storage remains until that storage is explicitly removed or cleaned up.
“Every host has /var/lib/docker/overlay2.”
That path is associated with the classic overlay2 arrangement. Fresh Engine 29.0+ installations default to the containerd image store, whose image and snapshot data are documented under /var/lib/containerd; upgraded hosts may differ.
“A storage driver is the right place for database durability.”
Storage drivers manage image and container layers. Databases and other write-intensive services should place their durable data in volumes or an appropriate external storage system, then use backups and recovery tests independent of the container lifecycle.
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- Identify the Docker Engine version and confirm whether the containerd image store is active with
docker info. - Separate disposable image/container-layer data from application data that must survive replacement.
- Use volumes or deliberate bind mounts for persistent writes; keep temporary caches in the container layer only when losing them is acceptable.
- Check Linux kernel, rootless requirements, backing filesystem and distribution support before selecting overlay2, fuse-overlayfs, btrfs, zfs or another option.
- Set cgroup limits appropriate to the workload and monitor the resulting throttling or memory failures.
- Review namespaces, capabilities, user identity, mounts and network attachments together; no single feature supplies complete security.
- Test the actual failure path: stop and remove a container, recreate it from the image, and confirm that required data is still present in its persistent mount.
The useful mental model is a stack: Docker configuration is translated by the daemon, containerd and an OCI runtime; namespaces shape what processes can see; cgroups constrain what they can consume; and layered storage supplies an efficient read-only image plus a per-container change area. Once persistent data is moved to volumes and the active storage backend is verified, container replacement becomes a controlled operation rather than a data-loss gamble.
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