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Modal volumes explained

Q: Modal volumes explained

In Modal, volumes are persistent storage units that allow you to save and share data across function invocations and containers. They are mounted as directories inside your container, enabling durable file storage beyond ephemeral runtime environments.

Quick answer

In Modal, volumes are persistent storage units that allow you to save and share data across function invocations and containers. They are mounted as directories inside your container, enabling durable file storage beyond ephemeral runtime environments.

PREREQUISITES

Python 3.8+
Modal account and CLI installed
pip install modal

Setup

Install the modal Python package and log in to your Modal account using the CLI. Define a volume in your Modal app to enable persistent storage.

bash

pip install modal
modal login

output

Collecting modal
  Downloading modal-1.0.0-py3-none-any.whl (50 kB)
Installing collected packages: modal
Successfully installed modal-1.0.0

Logging in to Modal...
Successfully logged in as user@example.com

Step by step

This example shows how to create a Modal volume, write a file to it, and read the file back in a function. The volume persists data across runs.

python

import modal

# Define a volume named 'my-volume'
volume = modal.Volume("my-volume")

app = modal.App()

@modal.function(
    volumes={"/data": volume}
)
def write_and_read():
    path = "/data/example.txt"
    # Write to the volume
    with open(path, "w") as f:
        f.write("Hello from Modal volume!")
    # Read from the volume
    with open(path, "r") as f:
        return f.read()

if __name__ == "__main__":
    with app.run():
        result = write_and_read()
        print("Read from volume:", result)

output

Read from volume: Hello from Modal volume!

Common variations

You can mount multiple volumes, use volumes with different paths, or share volumes between multiple functions. Volumes can also be used with modal.Stub for local development.

python

import modal

volume1 = modal.Volume("volume1")
volume2 = modal.Volume("volume2")

app = modal.App()

@modal.function(
    volumes={"/data1": volume1, "/data2": volume2}
)
def multi_volume_access():
    with open("/data1/file1.txt", "w") as f:
        f.write("Data in volume1")
    with open("/data2/file2.txt", "w") as f:
        f.write("Data in volume2")
    with open("/data1/file1.txt", "r") as f1, open("/data2/file2.txt", "r") as f2:
        return f1.read(), f2.read()

if __name__ == "__main__":
    with app.run():
        data1, data2 = multi_volume_access()
        print(f"Volume1: {data1}")
        print(f"Volume2: {data2}")

output

Volume1: Data in volume1
Volume2: Data in volume2

Troubleshooting

If you get a VolumeNotFoundError, ensure the volume name is created in your Modal dashboard or CLI before running your app.
Check that the volume mount path inside the container matches the path used in your code.
Remember volumes persist data beyond container lifetimes, but deleting a volume from Modal dashboard will remove stored data.

✅

Key Takeaways

Use modal.Volume to create persistent storage shared across function runs.
Mount volumes inside your function with the volumes parameter to access files.
Volumes persist data beyond ephemeral container lifetimes, enabling durable state.
Always create volumes in Modal dashboard or CLI before referencing them in code.
Multiple volumes can be mounted simultaneously at different paths for complex apps.

Verified 2026-04

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