gvisor/g3doc/user_guide/filesystem.md

# Filesystem

[TOC]

gVisor accesses the filesystem through a file proxy, called the Gofer. The gofer
runs as a separate process, that is isolated from the sandbox. Gofer instances
communicate with their respective sentry using the 9P protocol. For a more
detailed explanation see [Overview > Gofer](../../architecture_guide/#gofer).

## Sandbox overlay

To isolate the host filesystem from the sandbox, you can set a writable tmpfs
overlay on top of the entire filesystem. All modifications are made to the
overlay, keeping the host filesystem unmodified.

> Note: All created and modified files are stored in memory inside the sandbox.

To use the tmpfs overlay, add the following `runtimeArgs` to your Docker
configuration (`/etc/docker/daemon.json`) and restart the Docker daemon:

```json
{
    "runtimes": {
        "runsc": {
            "path": "/usr/local/bin/runsc",
            "runtimeArgs": [
                "--overlay"
            ]
       }
    }
}
```

## Shared root filesystem

The root filesystem is where the image is extracted and is not generally
modified from outside the sandbox. This allows for some optimizations, like
skipping checks to determine if a directory has changed since the last time it
was cached, thus missing updates that may have happened. If you need to `docker
cp` files inside the root filesystem, you may want to enable shared mode. Just
be aware that file system access will be slower due to the extra checks that are
required.

> Note: External mounts are always shared.

To use set the root filesystem shared, add the following `runtimeArgs` to your
Docker configuration (`/etc/docker/daemon.json`) and restart the Docker daemon:

```json
{
    "runtimes": {
        "runsc": {
            "path": "/usr/local/bin/runsc",
            "runtimeArgs": [
                "--file-access=shared"
            ]
       }
    }
}
```