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EC2 IO stress

Generate sustained filesystem read and write load on a target EC2 instance via AWS Systems Manager so you can test how the workload behaves under disk pressure or near-full storage.

EC2 IO stress is an AWS chaos fault that generates sustained filesystem read and write load on a target EC2 instance for a configurable duration. The fault writes a configurable number of bytes (or fills a configurable percentage of the filesystem) at VOLUME_MOUNT_PATH and runs NUMBER_OF_WORKERS worker threads. The workload is dispatched via AWS Systems Manager Run Command.

Use this fault to test how a workload reacts when its disk is saturated: do writes block, does the database fall behind on WAL flushes, does an ephemeral-storage limit kick in, does log rotation fail?

RUN YOUR FIRST EXPERIMENT

If you have not configured the chaos infrastructure yet, go to Quickstart to install the chaos infrastructure and run an experiment end to end.

Use cases

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

  • Disk-bound latency: When the filesystem is saturated, how much does p99 application latency rise?

  • Near-full disk handling: When FILESYSTEM_UTILIZATION_PERCENTAGE pushes the disk past 90%, does the application detect it and fall back gracefully, or does it crash on the first failed write?

  • WAL flush stalls: For a database, does the WAL writer keep up under disk pressure, or does transaction throughput collapse?

  • Log rotation failure: Does the logger handle write failures without crashing the application?

  • Ephemeral-storage limits: On EKS, does kubelet evict pods that exceed their ephemeral-storage limits?

Prerequisites

  • Kubernetes version: 1.21 or later for the chaos infrastructure cluster.

  • Target instance is reachable via SSM: The instance has the SSM Agent running and an instance profile with AmazonSSMManagedInstanceCore.

  • Selector provided: Either EC2_INSTANCE_ID or EC2_INSTANCE_TAG is set.

  • Volume mount path is writable: VOLUME_MOUNT_PATH is a directory the stress workload can write to.

  • AWS credentials available: Either an AWS credentials file uploaded as a File Secret in Harness Secret Manager (see Authentication below) or IRSA on the chaos infrastructure service account.

Supported environments

Platform
Support status

Amazon EC2 (Linux instances with SSM Agent)

Supported

Amazon EKS managed worker nodes

Supported (if SSM Agent is installed)

Amazon EKS self-managed worker nodes

Supported (if SSM Agent is installed)

Targeting by tag

Supported via EC2_INSTANCE_TAG

Targeting by ID

Supported via EC2_INSTANCE_ID

Windows instances

Not supported (Linux-only stress payload)

Permissions required

Go to common policy for all AWS faults to use a single superset IAM policy.

Authentication

The fault supports three credential delivery models. Pick one based on how your chaos infrastructure is deployed.

Method
When to use it
How to configure

Harness Secret Manager file secret

Chaos infrastructure runs outside EKS, or you want explicit static credentials

Upload the AWS credentials file as a File Secret in Harness Secret Manager and reference its identifier via AWS_AUTHENTICATION_SECRET

IAM Roles for Service Accounts (IRSA)

Chaos infrastructure runs in EKS and uses an OIDC-bound service account

No tunable changes; the chaos pod inherits the role automatically. Go to AWS IAM integration to set it up

Assume role

The fault needs to act in a different account or with elevated permissions

Set ASSUME_ROLE_ARN to the role ARN; the chaos pod assumes the role on top of its base credentials

When using the Harness Secret Manager method, the File Secret should contain an AWS credentials file in the standard ~/.aws/credentials format:

Upload this file as a File Secret in Harness Secret Manager (Project Setup → Secrets → New File Secret), and pass the secret identifier in AWS_AUTHENTICATION_SECRET.

Fault tunables

Required parameters

Tunable
Description
Default

REGION

AWS region that hosts the target instance.

(required)

EC2_INSTANCE_ID or EC2_INSTANCE_TAG

One of these must be set to select the target instance(s).

""

Chaos parameters

Tunable
Description
Default

FILESYSTEM_UTILIZATION_BYTES

Number of bytes to write to the filesystem. Mutually exclusive with FILESYSTEM_UTILIZATION_PERCENTAGE.

0

FILESYSTEM_UTILIZATION_PERCENTAGE

Percentage of the target filesystem to fill (1-100).

10

VOLUME_MOUNT_PATH

Path on the target instance where the stress workload writes.

/tmp

NUMBER_OF_WORKERS

Number of worker threads to use for IO stress.

1

TOTAL_CHAOS_DURATION

Duration of the fault in seconds.

60

INSTANCE_AFFECTED_PERC

Percentage of matching instances to target (only with EC2_INSTANCE_TAG). 0 targets one instance.

0

INSTALL_DEPENDENCIES

Install the in-instance stress tool if missing. Set to False to skip.

True

PROXY

HTTP/HTTPS proxy used by the in-instance installer (for example https_proxy=http://proxy.server:3128).

""

SEQUENCE

Order in which multiple instances are processed: parallel or serial.

parallel

RAMP_TIME

Wait period in seconds before and after the fault.

0

Authentication

Tunable
Description
Default

ASSUME_ROLE_ARN

ARN of an IAM role to assume on top of the base credentials.

""

AWS_AUTHENTICATION_SECRET

Identifier of the File Secret in Harness Secret Manager that contains the AWS credentials file. Not required when using IRSA.

""

PICK THE RIGHT MOUNT PATH

The default VOLUME_MOUNT_PATH=/tmp may be on the root filesystem on small instances. Filling the root filesystem can prevent the application (or kubelet) from writing logs and recovering. Point VOLUME_MOUNT_PATH at a separate volume for production-like blast radius.

Fault execution in brief

Sends an SSM Run Command to the selected instance(s) in REGION that runs NUMBER_OF_WORKERS filesystem-stress threads against VOLUME_MOUNT_PATH, writing FILESYSTEM_UTILIZATION_BYTES bytes (or filling FILESYSTEM_UTILIZATION_PERCENTAGE of the filesystem) for TOTAL_CHAOS_DURATION seconds.

Expected behavior during fault execution

  • IO operations per second on the target filesystem climb to the saturation point of the underlying EBS volume.

  • CloudWatch VolumeReadOps / VolumeWriteOps and VolumeQueueLength rise correspondingly.

  • Application latency typically rises for any read or write path that touches the same volume.

  • Free space on VOLUME_MOUNT_PATH decreases according to FILESYSTEM_UTILIZATION_PERCENTAGE; if it reaches 100%, further writes fail with ENOSPC.

  • On EKS, pods writing to ephemeral storage may be evicted if they exceed their ephemeral-storage limit.

WHEN THE FAULT ENDS

The chaos pod stops the stress workload and removes the temporary files it wrote to VOLUME_MOUNT_PATH. Free space returns to the prior level; CloudWatch IO metrics drop within minutes.

Signals to watch

  • EBS IO utilization: Use a Prometheus probe on aws_ebs_volume_queue_length to confirm saturation.

  • Free space: Use a command probe that runs df -h <mount> via SSM to track utilization.

  • Application write errors: Use a Prometheus probe on application-specific write-error counters.

Verify the fault execution effect

While the experiment is running:

  1. Check free space via SSM.

  2. Check IO activity.

Recovery and cleanup

  • End of duration: The chaos pod terminates the stress workload and removes the temporary files it wrote.

  • Abort the experiment: Stopping the experiment from Chaos Studio cancels the in-flight SSM command.

  • Manual cleanup of orphan files: If the experiment exited unexpectedly, log into the instance via SSM Session Manager and remove temporary files under VOLUME_MOUNT_PATH.

Limitations

  • Linux-only payload: This fault stresses Linux instances.

  • SSM Agent required: Instances without the SSM Agent online cannot be targeted.

  • Filesystem may not recover free space immediately: Some filesystems (XFS, ext4) hold deleted-but-still-open file handles. Free space may take a moment to reflect after cleanup.

  • EBS burst credits: Sustained IO consumes EBS burst credits on gp2 / st1 / sc1 volumes. After the fault, IO performance may stay degraded until credits replenish.

  • Disk quotas: If the target uses quotas, the stress workload may stop early when it hits the quota instead of reaching FILESYSTEM_UTILIZATION_PERCENTAGE.

Troubleshooting

EC2 IO stress experiment fails with InvalidInstanceId in Harness Chaos Engineering

The SSM Agent is not online for the target instance. Confirm with aws ssm describe-instance-information --filters 'Key=InstanceIds,Values='. If missing, install the SSM Agent and attach an instance profile that includes AmazonSSMManagedInstanceCore.

EC2 IO stress runs but no IO activity is visible

The most common causes are: VOLUME_MOUNT_PATH does not exist on the target or is read-only; the in-instance stress tool failed to install (set INSTALL_DEPENDENCIES=True and verify network egress, or use PROXY); FILESYSTEM_UTILIZATION_BYTES is set to a tiny value; or you are watching CloudWatch metrics on the wrong volume. Run 'iostat -x 1 5' via SSM during the fault to confirm activity locally.

EC2 IO stress filled the root filesystem and broke the instance

VOLUME_MOUNT_PATH was on the root filesystem, and the fault filled it to 100%. Connect via SSM Session Manager and free space by removing the temporary files under VOLUME_MOUNT_PATH. For future runs, point VOLUME_MOUNT_PATH at a dedicated data volume.

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