> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/fs-fill.md).

# FS fill

FS fill is a Kubernetes pod-level chaos fault that writes a configurable amount of data into a specific path inside a target container. Unlike [Disk fill](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/disk-fill.md), which works as a percentage of the container's ephemeral-storage limit, FS fill writes an absolute size (`FILL_SIZE`) at a specific `FILE_PATH` (`/tmp` by default), so you can target a mounted volume, a writable subdirectory, or any specific filesystem path your application depends on.

Use this fault to test how the workload reacts when the directory it writes to runs out of space: whether it surfaces useful errors, whether log rotation or temp cleanup kicks in, and whether the kubelet evicts the pod for breaching its ephemeral-storage budget.

{% hint style="info" %}
**RUN YOUR FIRST EXPERIMENT**

If you have not configured the chaos infrastructure yet, go to [Quickstart](/resilience-testing/chaos-engineering/new-to-chaos-engineering/quickstart.md) to install the chaos infrastructure and run an experiment end to end.
{% endhint %}

***

### Use cases <a href="#use-cases" id="use-cases"></a>

Run this fault when you want to answer concrete questions like:

* **Path-specific capacity testing:** When `/tmp`, a working directory, or a mounted volume fills up, does the application fail cleanly with a clear log line and `ENOSPC`, or does it hang indefinitely on the next write?
* **Mounted-volume back-pressure:** When the fault writes into a path backed by an `emptyDir` or PVC, do consumers downstream of that path (queue producers, log shippers, batch jobs) detect the failure and shed load?
* **Log-rotation effectiveness:** Filling the log directory simulates uncontrolled log growth. Does the rotation policy catch up and reclaim space, or does the pod tip into eviction before rotation runs?
* **Cleanup hooks on shutdown:** When the kubelet evicts the pod for filling its ephemeral budget, does the application clean up temp data on `SIGTERM`, or does the next pod inherit a polluted directory?
* **Failure-mode coverage for batch jobs:** A batch job that writes intermediate output to a path that runs out of space tests whether the job framework retries, marks the job failed, or hangs.

***

### Prerequisites <a href="#prerequisites" id="prerequisites"></a>

* **Kubernetes version:** 1.21 or later. Go to [What's supported](/resilience-testing/chaos-engineering/new-to-chaos-engineering/whats-supported.md) to confirm distribution support.
* **Target pods are Running:** The pods you intend to fill are in the `Running` state before the fault is launched. The fault reports a precheck failure otherwise.
* **Privileged pods allowed:** The cluster lets you schedule privileged pods in the chaos namespace. The fault writes into the target container through the container runtime.
* **Container runtime access:** The chaos infrastructure can reach the container runtime on the target nodes. The default `containerd` socket path is mounted automatically.
* **Writable path inside the container:** The `FILE_PATH` you target is writable from the container. The fault fails immediately if the path is missing or read-only.

***

### Supported environments <a href="#supported-environments" id="supported-environments"></a>

| Platform                                 | Support status                                                                                                 |
| ---------------------------------------- | -------------------------------------------------------------------------------------------------------------- |
| Amazon EKS                               | Supported                                                                                                      |
| Azure AKS                                | Supported                                                                                                      |
| Google GKE                               | Supported                                                                                                      |
| Red Hat OpenShift                        | Supported                                                                                                      |
| Rancher                                  | Supported                                                                                                      |
| VMware Tanzu                             | Supported                                                                                                      |
| Self-managed Kubernetes (CNCF-certified) | Supported                                                                                                      |
| GKE Autopilot                            | Supported with [Autopilot setup](/resilience-testing/chaos-engineering/use-chaos-engineering/gke-autopilot.md) |
| EKS Fargate, ACI virtual nodes           | Not supported (no access to container runtime sockets)                                                         |

***

### Permissions required <a href="#permissions-required" id="permissions-required"></a>

The fault runs under the chaos infrastructure's service account.

| Resource (`apiGroup`)                                                 | Verbs                                                                    | Why it is needed                                            |
| --------------------------------------------------------------------- | ------------------------------------------------------------------------ | ----------------------------------------------------------- |
| `pods` (`""`)                                                         | `get`, `list`, `create`, `delete`, `deletecollection`, `patch`, `update` | Discover target pods and run the chaos pod on the same node |
| `pods/log` (`""`)                                                     | `get`, `list`, `watch`                                                   | Stream chaos pod logs for status and debugging              |
| `events` (`""`)                                                       | `get`, `list`, `create`, `patch`, `update`                               | Record fault progress as Kubernetes events                  |
| `configmaps` (`""`)                                                   | `get`, `list`                                                            | Mount configuration into the chaos pod when specified       |
| `deployments`, `statefulsets`, `replicasets`, `daemonsets` (`apps`)   | `get`, `list`                                                            | Resolve the parent controller for each target pod           |
| `replicationcontrollers` (`""`)                                       | `get`, `list`                                                            | Resolve the parent controller for legacy workloads          |
| `deploymentconfigs` (`apps.openshift.io`)                             | `get`, `list`                                                            | Resolve the parent controller on OpenShift                  |
| `rollouts` (`argoproj.io`)                                            | `get`, `list`                                                            | Resolve the parent controller for Argo Rollouts             |
| `jobs` (`batch`)                                                      | `get`, `list`, `create`, `delete`, `deletecollection`                    | Run the chaos job that drives the fault                     |
| `chaosengines`, `chaosexperiments`, `chaosresults` (`litmuschaos.io`) | `get`, `list`, `create`, `patch`, `update`, `delete`                     | Manage the chaos engine, experiment, and result CRDs        |

The default Harness chaos infrastructure service account already includes these permissions. You only need to extend it if you are running with a restricted scope.

***

### Fault tunables <a href="#fault-tunables" id="fault-tunables"></a>

Configure the following fault parameters when you add FS fill to an experiment in Chaos Studio. Defaults are shown for reference.

**Chaos parameters**

| Tunable                | Description                                                                                      | Default |
| ---------------------- | ------------------------------------------------------------------------------------------------ | ------- |
| `FILL_SIZE`            | Amount of data to write at `FILE_PATH`. Accepts unit suffixes `K`, `KB`, `M`, `MB`, `G`, `GB`.   | `100M`  |
| `FILE_PATH`            | Absolute path inside the target container where the data is written. Must exist and be writable. | `/tmp`  |
| `TOTAL_CHAOS_DURATION` | Duration of the fault in seconds. The data is held at `FILE_PATH` for the duration.              | `60`    |

**Targeting**

| Tunable                   | Description                                                                                                          | Default    |
| ------------------------- | -------------------------------------------------------------------------------------------------------------------- | ---------- |
| `TARGET_PODS`             | Comma-separated list of pod names to target. Empty selects from the workload's pods using `POD_AFFECTED_PERCENTAGE`. | `""`       |
| `TARGET_CONTAINER`        | Container in the pod whose filesystem is filled. Empty targets the first container in the pod spec.                  | `""`       |
| `NODE_LABEL`              | Label selector to filter target pods by the node they run on. Empty disables node-based filtering.                   | `""`       |
| `POD_AFFECTED_PERCENTAGE` | Percentage of the workload's pods to target. `0` means one pod.                                                      | `0`        |
| `SEQUENCE`                | When multiple pods are targeted, fill `parallel` (all at once) or `serial` (one after another).                      | `parallel` |

**Runtime and helper**

| Tunable             | Description                                                                                                                                                                                                      | Default                           |
| ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------- |
| `CONTAINER_RUNTIME` | Container runtime on the target nodes. One of `containerd`, `docker`, `crio`.                                                                                                                                    | `containerd`                      |
| `SOCKET_PATH`       | Path to the container runtime socket on the target node. Set to match `CONTAINER_RUNTIME`.                                                                                                                       | `/run/containerd/containerd.sock` |
| `RAMP_TIME`         | Wait period in seconds before and after the fault. Go to [ramp time](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/common-tunables-for-all-faults.md#ramp-time) to read how it is applied. | `0`                               |

Tunables that apply to every chaos fault are documented in [common tunables for all faults](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/common-tunables-for-all-faults.md).

{% hint style="warning" %}
**PICK FS FILL VS DISK FILL DELIBERATELY**

Use **FS fill** when you want to target a specific path (mounted volume, `/tmp`, log directory) with a known absolute size. Use [**Disk fill**](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/disk-fill.md) when you want to consume a percentage of the container's ephemeral-storage limit without caring about the exact location.
{% endhint %}

#### Configure for your container runtime <a href="#configure-for-your-container-runtime" id="configure-for-your-container-runtime"></a>

Set `CONTAINER_RUNTIME` and `SOCKET_PATH` to match the runtime on the target node:

| `CONTAINER_RUNTIME`    | `SOCKET_PATH`                     |
| ---------------------- | --------------------------------- |
| `containerd` (default) | `/run/containerd/containerd.sock` |
| `docker`               | `/var/run/docker.sock`            |
| `crio`                 | `/var/run/crio/crio.sock`         |

***

### Fault execution in brief <a href="#fault-execution-in-brief" id="fault-execution-in-brief"></a>

Writes `FILL_SIZE` of data at `FILE_PATH` inside the target container, holds the data there for the configured duration, and removes the data when the fault ends.

***

### Expected behavior during fault execution <a href="#expected-behavior-during-fault-execution" id="expected-behavior-during-fault-execution"></a>

* `FILE_PATH` accumulates data until `FILL_SIZE` is reached, then the fill is held for the rest of the duration.
* If `FILE_PATH` lives on the container's writable layer and the total writable usage crosses the container's `resources.limits.ephemeral-storage`, the kubelet evicts the pod with an ephemeral-storage breach reason.
* If `FILE_PATH` lives on a mounted volume (`emptyDir`, PVC, `hostPath`), the fill is confined to that volume. Ephemeral-storage limits do not apply, but the volume itself can run out of space, and writes from the application to that path fail with `ENOSPC`.
* Application writes to `FILE_PATH` may fail with `ENOSPC` (no space left on device) before the kubelet acts. The blast radius depends on which subsystem of the application uses that path.
* Reads from `FILE_PATH` are not affected. Files that already existed remain readable.
* The node-level filesystem stays untouched unless `FILE_PATH` resolves to a host-mounted path. The fault is scoped to the target container's view of the filesystem.

{% hint style="info" %}
**WHEN THE FAULT ENDS**

After `TOTAL_CHAOS_DURATION`, the fault removes the data it wrote at `FILE_PATH`. If the pod was evicted before the fault completed, the eviction stands; the replacement pod starts with a fresh filesystem.
{% endhint %}

#### Signals to watch <a href="#signals-to-watch" id="signals-to-watch"></a>

A useful experiment captures signals from three layers. Attach [resilience probes](/resilience-testing/chaos-engineering/use-chaos-engineering/probes.md) to assert each layer automatically:

* **Application error rate and disk-error logs:** Watch for `ENOSPC`, `No space left on device`, and any disk-related stack traces in your logs. Use an [HTTP probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/http-probe.md) to assert direct endpoint health.
* **Eviction events:** Track `kube_pod_status_reason{reason="Evicted"}` for the target workload's namespace if you expect the fill to breach the ephemeral-storage limit. Use a [Prometheus probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/apm-probes.md) for an automatic check.
* **Volume usage (if `FILE_PATH` is a mounted volume):** Use the appropriate volume-level metric (PVC usage, node filesystem free space) to confirm the fill is hitting the intended path. Use a [Kubernetes probe](/resilience-testing/chaos-testing/probes/k8s-probe.md) when you have a relevant Kubernetes object to watch.

***

### Verify the fault execution effect <a href="#verify-the-fault-execution-effect" id="verify-the-fault-execution-effect"></a>

While the experiment is running, confirm that the target path is actually filling up:

1. **List disk usage at the target path inside the container.**

   ```bash
   kubectl exec -n <namespace> <pod-name> -c <container> -- df -h <FILE_PATH>
   ```

   The usage on the filesystem backing `FILE_PATH` should increase by approximately `FILL_SIZE`.
2. **List the largest files at the target path.**

   ```bash
   kubectl exec -n <namespace> <pod-name> -c <container> -- ls -lhS <FILE_PATH>
   ```

   You should see the file the fault wrote (named by the fault, with a size near `FILL_SIZE`).
3. **Look for eviction or `ENOSPC` events.**

   ```bash
   kubectl get events -n <namespace> --sort-by='.lastTimestamp' | tail -20
   ```

   Expect `Evicted` events only if the fill exceeds the container's ephemeral-storage limit. Otherwise, expect application-level errors instead.

***

### Recovery and cleanup <a href="#recovery-and-cleanup" id="recovery-and-cleanup"></a>

* **End of duration:** When `TOTAL_CHAOS_DURATION` elapses, the data written at `FILE_PATH` is removed and disk usage at the path returns to baseline within seconds.
* **Evicted pods reschedule:** If the fault tripped an ephemeral-storage eviction, the workload's controller schedules a replacement pod on any Ready node with capacity.
* **Mounted volumes retain their state:** If `FILE_PATH` lived on a PVC and the pod was evicted, the next pod that mounts the same PVC sees an empty path (because the fault removed its data), not a fresh volume.
* **Stale application state:** Applications that wrote real data to `FILE_PATH` during the fault may have lost data due to `ENOSPC`. Check the application's recovery logic before assuming the workload is healthy.
* **Abort the experiment early:** Stopping the experiment from Chaos Studio triggers cleanup of the data at `FILE_PATH`. Any eviction that has already happened cannot be undone.

***

### Limitations <a href="#limitations" id="limitations"></a>

This fault is not appropriate in the following scenarios:

* **Serverless Kubernetes (EKS Fargate, ACI virtual nodes):** These platforms do not expose container runtime sockets and reject the privileged access the fault needs. GKE Autopilot is supported once the one-time setup in [Chaos on GKE Autopilot](/resilience-testing/chaos-engineering/use-chaos-engineering/gke-autopilot.md) is in place.
* **Read-only paths:** If `FILE_PATH` is read-only (because of `readOnlyRootFilesystem`, `securityContext`, or a volume mounted read-only), the fault fails immediately with a write error.
* **Paths outside the container:** This fault writes inside the target container's filesystem view. It does not write to the host node directly. Use [Node I/O stress](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/node/node-io-stress.md) to fill the node filesystem.
* **Sub-MiB `FILL_SIZE`:** A fill below the noise floor of `df` does not produce a measurable effect. Use a fill of at least a few MiB to see meaningful change.
* **Network-attached volumes with thin-provisioning quirks:** Some cloud storage classes report a different "free" size than the underlying provisioned volume. A fill that succeeds on the file system may still produce inconsistent quota signals from the storage backend.

***

### Troubleshooting <a href="#troubleshooting" id="troubleshooting"></a>

<details>

<summary>FS fill experiment stays Pending or never starts in Harness Chaos Engineering</summary>

Inspect the chaos pods in the experiment namespace with kubectl describe pod -n . The most common causes are taints on the target node, missing RBAC for the chaos service account, or a PodSecurity admission policy blocking privileged pods. Confirm the service account has the permissions listed above and the chaos namespace has the required Pod Security level.

</details>

<details>

<summary>FS fill reports no space at FILE_PATH or fails to write</summary>

The most common causes are: FILE\_PATH does not exist inside the target container, the path is mounted read-only, or the underlying volume is already at capacity. Verify the path with kubectl exec -n -c -- ls -ld . Check securityContext.readOnlyRootFilesystem and the volume's mount options.

</details>

<details>

<summary>FS fill runs but the application does not surface any disk error</summary>

The application may not write to FILE\_PATH at all, or it may write to a different path. Set FILE\_PATH to the directory the application actually uses (log directory, working directory, scratch space), not just /tmp. Verify by listing open file descriptors with kubectl exec -c -- ls -l /proc/1/fd.

</details>

<details>

<summary>Connection to container runtime fails for fs-fill in Harness Chaos Engineering</summary>

The default SOCKET\_PATH is /run/containerd/containerd.sock. If your nodes use Docker, set CONTAINER\_RUNTIME=docker and SOCKET\_PATH=/var/run/docker.sock. For CRI-O, set CONTAINER\_RUNTIME=crio and SOCKET\_PATH=/var/run/crio/crio.sock. Confirm the path by SSHing to the node and running ls -l on each candidate socket file.

</details>

***

### Related faults <a href="#related-faults" id="related-faults"></a>

* [Disk fill](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/disk-fill.md): Fill a percentage of the container's ephemeral-storage limit instead of writing an absolute size at a specific path.
* [Pod CPU hog](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/pod-cpu-hog.md) and [Pod memory hog](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/pod-memory-hog.md): Apply CPU or memory pressure on the target container.
* [Node I/O stress](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/node/node-io-stress.md): Drive disk I/O at the node level rather than against a single container.
* [Common pod fault tunables](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/common-tunables-for-pod-faults.md): Shared environment variables for selecting target pods and workloads.