Snapshots - Microsandbox

A snapshot is a portable, on-disk capture of a sandbox's writable filesystem. Move it with scp, archive it as .tar.zst, or boot fresh sandboxes from it.

<Note> Snapshots are **disk-only and stopped-only**: the sandbox can't be running when you take one. Memory and CPU state, plus qcow2 backing chains, are future work. </Note>

What gets captured

Captured	Not captured
`upper.ext4`, the sandbox's writable overlay	Memory contents
Pinned image reference + manifest digest	CPU registers / running processes
`fstype`, `format`, optional labels	Network state
Optional content-integrity hash	Open file handles

Booting from a snapshot is a cold boot of a fresh VM that reuses the captured upper layer as its starting filesystem. The image (lower layer) is re-resolved by digest from the local OCI cache, or re-pulled from the registry if missing.

Quick start

You'll usually reach for the CLI first:

bash

# 1. Run something, install state, then stop it
msb run --name baseline --detach python:3.12 -- sleep 3600
msb exec baseline -- pip install requests
msb stop baseline

# 2. Snapshot the stopped sandbox
msb snapshot create after-pip-install --from baseline

# 3. Boot a fresh sandbox from the snapshot
msb run --name worker --snapshot after-pip-install \
    -- python -c "import requests; print(requests.__version__)"

<Tip> The snapshot lives at `~/.microsandbox/snapshots/after-pip-install/`. That whole directory is the snapshot. </Tip>

Snapshot a stopped sandbox

Snapshot under a bare name (resolved to ~/.microsandbox/snapshots/<name>/) or to an explicit path:

<CodeGroup> ```rust Rust use microsandbox::Sandbox;

let h = Sandbox::get("baseline").await?;

// Bare name resolves under ~/.microsandbox/snapshots/<name>/ let snap = h.snapshot("after-pip-install").await?;

// Or write to an explicit path let snap = h.snapshot_to("/tmp/snaps/v1").await?;

println!("{}", snap.digest()); // sha256:...


```typescript TypeScript
import { Sandbox } from "microsandbox";

const h = await Sandbox.get("baseline");

// Bare name resolves under ~/.microsandbox/snapshots/<name>/
const snap = await h.snapshot("after-pip-install");

// Or write to an explicit path
const snap2 = await h.snapshotTo("/tmp/snaps/v1");

console.log(snap.digest); // sha256:...

python

from microsandbox import Sandbox

h = await Sandbox.get("baseline")

# Bare name resolves under ~/.microsandbox/snapshots/<name>/
snap = await h.snapshot("after-pip-install")

# Or write to an explicit path
snap2 = await h.snapshot_to("/tmp/snaps/v1")

print(snap.digest)  # sha256:...

bash

msb snapshot create after-pip-install --from baseline
msb snapshot create ./snaps/v1     --from baseline   # Explicit path
msb snapshot create after-pip-install --from baseline --label stage=ready

</CodeGroup>

The sandbox must be stopped or crashed; running sandboxes are rejected.

Boot from a snapshot

A snapshot already pins its image, so booting from one is mutually exclusive with the image source:

<CodeGroup> ```rust Rust use microsandbox::Sandbox;

let sb = Sandbox::builder("worker") .from_snapshot("after-pip-install") .create() .await?;


```typescript TypeScript
import { Sandbox } from "microsandbox";

const sb = await Sandbox.builder("worker")
    .fromSnapshot("after-pip-install")
    .create();

python

from microsandbox import Sandbox

# `snapshot=` is a peer of `image=` and mutually exclusive with it
sb = await Sandbox.create("worker", snapshot="after-pip-install")

bash

msb run --name worker --snapshot after-pip-install -- python -V

</CodeGroup>

Booting validates the snapshot's manifest, re-resolves the pinned image from the local OCI cache (re-pulling and verifying the digest if it's missing), and reflinks the captured upper.ext4 into the new sandbox's directory. Reflinks use clonefile on macOS APFS, FICLONE on btrfs/XFS, with a sparse-aware fallback on ext4, so each new sandbox gets an independent COW copy without paying the bytes again.

List, inspect, and remove

<CodeGroup> ```rust Rust use microsandbox::Snapshot;

let all = Snapshot::list().await?; // Indexed snapshots let h = Snapshot::get("after-pip-install").await?; // By name, digest, or path println!("{} ({})", h.name().unwrap_or("-"), h.digest());

Snapshot::remove("after-pip-install", false).await?; Snapshot::reindex("~/.microsandbox/snapshots").await?;


```typescript TypeScript
import { Snapshot } from "microsandbox";

const all = await Snapshot.list();                       // Indexed snapshots
const h = await Snapshot.get("after-pip-install");       // By name, digest, or path
console.log(`${h.name ?? "-"} (${h.digest})`);

await Snapshot.remove("after-pip-install");
await Snapshot.reindex("~/.microsandbox/snapshots");

python

from microsandbox import Snapshot

all = await Snapshot.list()                              # Indexed snapshots
h = await Snapshot.get("after-pip-install")              # By name, digest, or path
print(f"{h.name or '-'} ({h.digest})")

await Snapshot.remove("after-pip-install")
await Snapshot.reindex("~/.microsandbox/snapshots")

bash

msb snapshot ls
msb snapshot inspect after-pip-install
msb snapshot rm after-pip-install

# Also if it has indexed children
msb snapshot rm after-pip-install --force

# Rebuild the index from artifacts on disk
msb snapshot reindex

</CodeGroup>

Opening a snapshot is cheap: it validates the manifest and checks the upper file's size, but does not read the file's contents. The same path powers inspect and the boot flow; the load-bearing local operation has to stay fast.

list and get are backed by a small local index DB. The index is just a cache for fast lookups, not a source of truth; if it ever gets out of sync (or you delete it), reindex rebuilds it from the artifacts on disk.

Move snapshots between machines

The artifact is a directory containing manifest.json (canonical JSON; identity is the SHA-256 of these bytes) plus the captured upper.ext4. The directory is the snapshot; there's no hidden daemon state, so any of these three transports work:

bash

# Copy the directory directly with scp (image must be cached or pullable on the target)
scp -r ~/.microsandbox/snapshots/after-pip-install \
    other-host:~/.microsandbox/snapshots/

# Bundle into a .tar.zst, transport, then import
msb snapshot export after-pip-install /tmp/snap.tar.zst
scp /tmp/snap.tar.zst other-host:
ssh other-host msb snapshot import /tmp/snap.tar.zst

# Fully offline: include the OCI image cache so the target needs no network
msb snapshot export after-pip-install /tmp/snap.tar.zst --with-image
ssh other-host msb snapshot import /tmp/snap.tar.zst

Archives default to .tar.zst since zstd collapses sparse zero runs cheaply. Pass --plain-tar for a plain .tar; both are accepted on import via magic-byte detection.

Or drive export and import from code:

<CodeGroup> ```rust Rust use std::path::Path; use microsandbox::Snapshot; use microsandbox::snapshot::ExportOpts;

Snapshot::export( "after-pip-install", Path::new("/tmp/snap.tar.zst"), ExportOpts { with_image: true, ..Default::default() }, ).await?;

let snap = Snapshot::import( Path::new("/tmp/snap.tar.zst"), None, ).await?;


```typescript TypeScript
import { Snapshot } from "microsandbox";

await Snapshot.export("after-pip-install", "/tmp/snap.tar.zst", {
  withImage: true,
});

const snap = await Snapshot.import("/tmp/snap.tar.zst");

python

from microsandbox import Snapshot

await Snapshot.export(
    "after-pip-install",
    "/tmp/snap.tar.zst",
    with_image=True,
)

snap = await Snapshot.import_("/tmp/snap.tar.zst")  # Trailing _ avoids the keyword

bash

msb snapshot export after-pip-install /tmp/snap.tar.zst --with-image
msb snapshot import /tmp/snap.tar.zst

</CodeGroup>

Integrity verification

By default, snapshot creation does not hash the upper file. The artifact carries the upper's size and the pinned image's manifest digest, which together catch any corruption that would stop it from booting. Skipping the hash keeps inspect fast on multi-GB sparse upper layers, where the cost would otherwise dominate.

Opt in to a content-integrity hash when crossing a trust boundary:

<CodeGroup> ```rust Rust use microsandbox::Snapshot;

// Compute and record an integrity hash at create time let snap = Snapshot::builder("baseline") .name("after-pip-install") .record_integrity() .create() .await?;

// Verify a snapshot's recorded integrity on demand let report = snap.verify().await?;


```typescript TypeScript
import { Snapshot } from "microsandbox";

// Compute and record an integrity hash at create time
const snap = await Snapshot.builder("baseline")
  .name("after-pip-install")
  .recordIntegrity()
  .create();

// Verify a snapshot's recorded integrity on demand
const report = await snap.verify();

python

from microsandbox import Snapshot

# Compute and record an integrity hash at create time
snap = await Snapshot.create(
    "baseline",
    name="after-pip-install",
    record_integrity=True,
)

# Verify a snapshot's recorded integrity on demand
report = await snap.verify()

bash

# Compute and record an integrity hash at create time
msb snapshot create after-pip-install --from baseline --integrity

# Verify a snapshot's recorded integrity on demand
msb snapshot verify after-pip-install
msb snapshot inspect after-pip-install --verify

</CodeGroup>

msb snapshot export automatically populates the integrity hash before bundling, and msb snapshot import verifies it on the way in. The local index path stays fast; the cross-machine path stays trustworthy.

Use cases

Reusable build state. Install dependencies once, snapshot, then msb run --snapshot ... repeatedly without paying the install cost. Common pattern for CI, agent workloads, and reproducible dev environments.
Portable scratch state. Capture a sandbox after a long setup, hand the artifact to a teammate or push it to shared storage, and let them boot from the same starting point.
Local fork-by-copy. Multiple sandboxes from one snapshot are independent; each copy of the upper layer diverges on its own.
Disaster recovery. Snapshot a sandbox before a risky migration; if it goes wrong, msb rm the broken one and msb run --snapshot from the pre-migration artifact.

What's not yet supported

Live (running) snapshots. The sandbox must be stopped first. Live capture requires an in-guest filesystem freeze and memory checkpointing that's tracked as future work.
qcow2 backing chains. Snapshots always copy the upper layer today. The manifest's format and parent fields are reserved so qcow2 chains can land additively without a schema break.
Push/pull from a registry. The artifact format is registry-shaped on purpose, but transport tooling beyond scp and tar.zst is a separate piece of work.
Snapshotting DiskImage volumes. The same mechanism would apply (reflink the disk file), but is currently out of scope.