Volumes

<Note> **On microsandbox cloud:** Volumes use fast block storage managed by the platform. Every organization automatically gets an undeletable **host volume**: think of it as the disk of the one computer all its sandboxes run on, and it is what bind and disk-image mounts resolve against. </Note>

Volumes give a sandbox persistent or shared storage by mounting host data, managed storage, disk images, or in-memory filesystems into the guest. Mounts provide direct filesystem access and are significantly faster than the filesystem API, which transfers files individually, so they are preferable for anything beyond ad-hoc reads and writes.

microsandbox supports four mount types on both backends: bind mounts, named volumes, disk-image volumes, and tmpfs.

Cloud default volume

<Tooltip tip="Available only on microsandbox cloud; the local backend has no default volume.">Cloud-only <Icon icon="circle-info" size={11} /></Tooltip>

Every cloud organization has an always-present default volume. You can read and write it directly without starting a sandbox:

<CodeGroup> ```typescript TypeScript const volume = await Volume.getDefault(); await volume.fs().write("state.json", JSON.stringify({ ready: true })); ```

rust

let volume = Volume::get_default().await?;
volume.fs().write("state.json", br#"{"ready":true}"#).await?;

python

volume = await Volume.get_default()
await volume.fs().write("state.json", b'{"ready": true}')

volume, err := m.GetDefaultVolume(ctx)
err = volume.FS().WriteString(ctx, "state.json", `{"ready":true}`)

</CodeGroup>

microsandbox isolates the default volume to the authenticated cloud organization, and you cannot remove it. microsandbox deliberately does not support default-volume lookup on the local backend: forgetting an API key cannot expose or modify files from your machine. Named local volumes continue to live under ~/.microsandbox/volumes/ and retain their existing direct filesystem API.

Bind mounts

<Tooltip tip="On microsandbox cloud, the host path resolves against your organization's host volume, not the computer running the SDK or CLI.">Limited on cloud <Icon icon="circle-info" size={11} /></Tooltip>

Mount a directory from the host directly into the sandbox. Changes inside the sandbox are reflected on the host, and vice versa.

<CodeGroup> ```typescript TypeScript import { Sandbox } from "microsandbox";

await using sb = await Sandbox.builder("dev") .image("python") .volume("/app/src", (m) => m.bind("./src").readonly().noexec()) .volume("/app/data", (m) => m.bind("./data")) .create();


```rust Rust
let sb = Sandbox::builder("dev")
    .image("python")
    .volume("/app/src", |v| v.bind("./src").readonly().noexec())
    .volume("/app/data", |v| v.bind("./data"))
    .create()
    .await?;

python

from microsandbox import Sandbox, Volume

sb = await Sandbox.create(
    "dev",
    image="python",
    volumes={
        "/app/src": Volume.bind("./src", readonly=True, noexec=True),
        "/app/data": Volume.bind("./data"),
    },
)

sb, err := m.CreateSandbox(ctx, "dev",
    m.WithImage("python"),
    m.WithMounts(map[string]m.MountConfig{
        "/app/src":  m.Mount.Bind("./src", m.MountOptions{Readonly: true, Noexec: true}),
        "/app/data": m.Mount.Bind("./data", m.MountOptions{}),
    }),
)

bash

msb create python --name dev \
  --mount-dir ./src:/app/src:ro,noexec \
  --mount-dir ./data:/app/data

</CodeGroup>

microsandbox supports nested destinations. It canonicalizes guest paths and always applies enclosing mounts before their children. SDKs that accept unordered maps (such as Go) produce the same result regardless of map iteration order.

Use --mount-file when you want to bind one host file instead of an entire directory.

File mounts expose the selected host inode through an isolated one-entry filesystem. They do not hard-link or copy the source, host changes remain visible, and other files in the source directory are not part of the guest-visible share. Writable file mounts receive the same default quota as directory binds; use quota=<size> to set a smaller or larger guest-growth budget.

bash

msb create alpine --name config-reader \
  --mount-file ./config.toml:/etc/app/config.toml:ro,nodev

Named volumes

<Tooltip tip="On microsandbox cloud, create named volumes before mounting them; create-on-mount, disk-kind volumes, and size are not available, and quota must be a whole number of GiB.">Limited on cloud <Icon icon="circle-info" size={11} /></Tooltip>

microsandbox manages named volumes and stores them by default under ~/.microsandbox/volumes/<name>/. They persist independently of any sandbox, so you can create a volume, populate it, and mount it into different sandboxes over time.

Named volumes can be directory-backed or disk-backed. Directory volumes mount through virtiofs. Disk volumes are raw ext4 disk images that microsandbox manages, and they mount through virtio-blk.

CLI named mounts are idempotent. -v name:/path and --mount-named name:/path create a directory-backed named volume if it does not already exist, then mount it. If the volume already exists, microsandbox reuses it when the requested storage settings are compatible and errors when they are not. Use --mount-named name:/path:kind=disk,size=20G to create or reuse a disk-backed named volume from the mount flag itself.

SDK named(...) mount helpers are existing-only. They fail if the named volume does not already exist. Use each SDK's explicit named-volume mode API when you want sandbox creation to create or ensure the volume.

Create a volume

<CodeGroup> ```typescript TypeScript import { Volume } from "microsandbox";

const cache = await Volume.builder("pip-cache").create();


```rust Rust
let cache = Volume::builder("pip-cache")
    .create()
    .await?;

python

cache = await Volume.create("pip-cache")

cache, err := m.CreateVolume(ctx, "pip-cache")

bash

msb volume create pip-cache

</CodeGroup>

Create a disk-backed named volume when a workload needs a real guest block filesystem, such as Docker's data root:

<CodeGroup> ```typescript TypeScript const dockerData = await Volume.builder("docker-data") .disk() .size(20 * 1024) .create(); ```

rust

let docker_data = Volume::builder("docker-data")
    .disk()
    .size(20.gib())
    .create()
    .await?;

python

from microsandbox import VolumeKind

docker_data = await Volume.create(
    "docker-data",
    kind=VolumeKind.DISK,
    size_mib=20 * 1024,
)

dockerData, err := m.CreateVolume(ctx, "docker-data",
    m.WithVolumeKind(m.VolumeKindDisk),
    m.WithVolumeSize(20 * 1024),
)

bash

msb volume create docker-data --kind disk --size 20G

</CodeGroup>

Mount in a sandbox

<CodeGroup> ```typescript TypeScript await using sb = await Sandbox.builder("worker") .image("python") .volume("/root/.cache/pip", (m) => m.named(cache.name)) .create(); ```

rust

let sb = Sandbox::builder("worker")
    .image("python")
    .volume("/root/.cache/pip", |v| v.named(cache.name()))
    .create()
    .await?;

python

sb = await Sandbox.create(
    "worker",
    image="python",
    volumes={
        "/root/.cache/pip": Volume.named(cache.name),
    },
)

sb, err := m.CreateSandbox(ctx, "worker",
    m.WithImage("python"),
    m.WithMounts(map[string]m.MountConfig{
        "/root/.cache/pip": m.Mount.Named(cache.Name(), m.MountOptions{}),
    }),
)

bash

msb create python --name worker \
  --mount-named pip-cache:/root/.cache/pip

</CodeGroup>

The CLI command above creates pip-cache as a directory-backed named volume if it is missing. To create or reuse a disk-backed named volume while mounting it:

bash

msb create docker:dind --name docker-demo \
  --mount-named docker-data:/var/lib/docker:kind=disk,size=20G

Mount the same directory-backed named volume into multiple sandboxes when they need to share files. Use readonly on consumers that should not write. microsandbox attaches disk-backed named volumes as block devices; use one writable sandbox at a time, or read-only mounts where sharing is intended.

Directory-backed named volumes are also accessible without a running sandbox, so you can pre-populate a cache or inspect output after the sandbox stops. Local volumes expose their host path directly; cloud default and managed directory volumes use the same volume filesystem API shown above. Disk-backed local volumes store guest data inside disk.raw; host filesystem helpers operate on the managed volume directory, not the filesystem inside the disk image.

Manage volumes

<CodeGroup> ```typescript TypeScript const volumes = await Volume.list(); await Volume.remove("old-cache"); ```

rust

let volumes = Volume::list().await?;
Volume::remove("old-cache").await?;

python

volumes = await Volume.list()
await Volume.remove("old-cache")

volumes, err := m.ListVolumes(ctx)
err = m.RemoveVolume(ctx, "old-cache")

bash

msb volume ls
msb volume rm old-cache

</CodeGroup>

Disk-image volumes

Disk-image volumes attach a host disk image as a virtio-blk device and mount its inner filesystem at a guest path. Use them when you want persistent state isolated from ordinary host filesystem metadata, or when distributing a prebuilt filesystem image. The disk format defaults from the file extension (.qcow2, .raw, .vmdk; otherwise raw). microsandbox auto-detects the inner filesystem type unless you set fstype.

<CodeGroup> ```typescript TypeScript await using sb = await Sandbox.builder("seeded") .image("alpine") .volume("/seed", (m) => m.disk("./seed.raw").readonly().noexec()) .create(); ```

rust

let sb = Sandbox::builder("seeded")
    .image("alpine")
    .volume("/seed", |v| v.disk("./seed.raw").readonly().noexec())
    .create()
    .await?;

python

sb = await Sandbox.create(
    "seeded",
    image="alpine",
    volumes={"/seed": Volume.disk("./seed.raw", readonly=True, noexec=True)},
)

sb, err := m.CreateSandbox(ctx, "seeded",
    m.WithImage("alpine"),
    m.WithMounts(map[string]m.MountConfig{
        "/seed": m.Mount.Disk("./seed.raw", m.DiskOptions{Readonly: true, Noexec: true}),
    }),
)

bash

msb create alpine --name seeded \
  --mount-disk ./seed.raw:/seed:ro,noexec,fstype=ext4

</CodeGroup>

Tmpfs

An in-memory filesystem that disappears when the sandbox stops. Useful for scratch space, build artifacts, or anything that doesn't need to outlive the sandbox.

<CodeGroup> ```typescript TypeScript import { Sandbox } from "microsandbox";

await using sb = await Sandbox.builder("worker") .image("alpine") .volume("/tmp/scratch", (m) => m.tmpfs().size(100).noexec()) .create();


```rust Rust
let sb = Sandbox::builder("worker")
    .image("alpine")
    .volume("/tmp/scratch", |v| v.tmpfs().size(100).noexec())
    .create()
    .await?;

python

sb = await Sandbox.create(
    "worker",
    image="alpine",
    volumes={
        "/tmp/scratch": Volume.tmpfs(size_mib=100, noexec=True),
    },
)

sb, err := m.CreateSandbox(ctx, "worker",
    m.WithImage("alpine"),
    m.WithMounts(map[string]m.MountConfig{
        "/tmp/scratch": m.Mount.Tmpfs(m.TmpfsOptions{SizeMiB: 100, Noexec: true}),
    }),
)

bash

msb create alpine --name worker \
  --tmpfs /tmp/scratch:100:noexec

</CodeGroup>

Mount options

Mounts use normal Linux behavior by default: writable, executable, suid-enabled, and device-enabled. Adjust with:

readonly / ro when the guest should not write to the mount.
noexec when files on the mount should not be executed directly.
nosuid when setuid/setgid bits should be ignored.
nodev when device files should be ignored.

noexec does not stop interpreters from reading scripts from the mount, for example sh /app/src/script.sh.

For stronger in-guest hardening, create the sandbox with the restricted security profile. Restricted sandboxes set no_new_privs, drop mount-admin capability from user commands, and force nosuid,nodev on user mounts. This profile is incompatible with workloads such as sudo and Docker-in-Docker.

Fallback guest ownership

Bind mounts and directory-backed named volumes normally present host files without a per-file metadata override using the sandbox user's fallback identity. Set a mount owner when those files should consistently appear as another guest (uid, gid) pair. This changes only the guest view: it does not chown the host files or create per-file metadata overrides, and an existing per-file override still takes precedence.

Both IDs are required together and may range from 0 through 4294967295. Mount owners require stat virtualization, so they cannot be combined with stat-virt=off, tmpfs, host disk images, or disk-backed named volumes. The local backend supports this policy; the cloud backend currently rejects it until ownership capability negotiation is available.

CLI syntax and flags

CLI mount flags use SOURCE:DEST[:OPTIONS]. The : before OPTIONS starts the option block, and commas separate options inside that block.

Flag	Source	Options
`--mount-dir`	Host directory	`ro`, `rw`, `noexec`, `nosuid`, `nodev`, `stat-virt=strict\|relaxed\|off`, `host-perms=private\|mirror`, `quota=<size>`, paired `uid=<N>,gid=<N>`
`--mount-file`	Host file	`ro`, `rw`, `noexec`, `nosuid`, `nodev`, `stat-virt=strict\|relaxed\|off`, `host-perms=private\|mirror`, `quota=<size>`, paired `uid=<N>,gid=<N>`
`--mount-named`	Named volume	`ro`, `rw`, `noexec`, `nosuid`, `nodev`, `kind=dir\|disk`, `size=<size>` for `kind=disk`, `quota=<size>` for directory volumes; directory-backed named volumes also support `stat-virt=strict\|relaxed\|off`, `host-perms=private\|mirror`, paired `uid=<N>,gid=<N>`
`--mount-disk`	Host disk image	`ro`, `rw`, `noexec`, `nosuid`, `nodev`, `format=raw\|qcow2\|vmdk`, `fstype=<type>`
`--tmpfs`	Guest path	`ro`, `rw`, `noexec`, `nosuid`, `nodev`; size may be passed as `PATH:SIZE[:OPTIONS]`

Combining mounts

You can repeat mount flags, so a sandbox can combine host directories, individual files, named volumes, disk images, and tmpfs scratch space.

<CodeGroup> ```typescript TypeScript await using sb = await Sandbox.builder("full") .image("python") .volume("/app/src", (v) => v.bind("./src").readonly().noexec()) .volume("/app/pyproject.toml", (v) => v.bind("./pyproject.toml").readonly()) .volume("/root/.cache/pip", (v) => v.named("pip-cache").statVirtualization("relaxed"), ) .volume("/dataset", (v) => v.disk("./dataset.qcow2").format("qcow2").fstype("ext4").readonly(), ) .volume("/tmp", (v) => v.tmpfs().size(1024).noexec().nosuid().nodev()) .create(); ```

rust

let sb = Sandbox::builder("full")
    .image("python")
    .volume("/app/src", |v| v.bind("./src").readonly().noexec())
    .volume("/app/pyproject.toml", |v| v.bind("./pyproject.toml").readonly())
    .volume("/root/.cache/pip", |v| {
        v.named("pip-cache")
            .stat_virtualization(StatVirtualization::Relaxed)
    })
    .volume("/dataset", |v| {
        v.disk("./dataset.qcow2")
            .format(DiskImageFormat::Qcow2)
            .fstype("ext4")
            .readonly()
    })
    .volume("/tmp", |v| v.tmpfs().size(1024).noexec().nosuid().nodev())
    .create()
    .await?;

bash

msb create python --name full \
  --mount-dir ./src:/app/src:ro,noexec \
  --mount-file ./pyproject.toml:/app/pyproject.toml:ro \
  --mount-named pip-cache:/root/.cache/pip:stat-virt=relaxed \
  --mount-disk ./dataset.qcow2:/dataset:ro,format=qcow2,fstype=ext4 \
  --tmpfs /tmp:1G:noexec,nosuid,nodev

</CodeGroup>

Reference

For exact volume APIs, see TypeScript, Rust, Python, or Go. For local volume management, see Volume commands.

Cloud default volume

Bind mounts

Named volumes

Create a volume

Mount in a sandbox

Sharing and host-side access

Manage volumes

Disk-image volumes

Tmpfs

Mount options

Fallback guest ownership

CLI syntax and flags

Combining mounts

Reference