Snapshots

A snapshot keeps a machine exactly as it is, so you can come back to it later: before an upgrade, or before trying something risky.

Taking one#

Choose Take Snapshot in the Virtual Machine menu (Cmd+Shift+S), or open Snapshots… in the machine's menu to name it first.

swiftvm snapshot web-server "Before upgrade"
swiftvm snapshots web-server

A machine that is shut down is saved as it is on disk.
A running machine pauses for a moment and comes back exactly where it was. Its memory is part of the snapshot.

Revert… puts the machine back as it was when the snapshot was taken.

swiftvm revert web-server "Before upgrade"

Everything that happened in the machine since is lost. Take another snapshot first if you want to keep it.
A snapshot of a running machine resumes at the moment it was taken, with the same programs running.

swiftvm rename-snapshot web-server "Before upgrade" "Before 2.0"
swiftvm delete-snapshot web-server "Before 2.0"

Deleting a snapshot doesn't change the machine or its other snapshots.

Snapshots are instant and share disk space with the machine, because APFS shares what hasn't changed. On another file system they are full copies.
A snapshot of a running machine also keeps its memory, which can take several gigabytes.
A snapshot covers the machine's own disk, not the disk images attached to it.
A running machine can't be snapshotted while it has a writable disk image or a USB device attached, or while your Mac is locked. Eject or detach it, or shut the machine down first.
After going back, the guest's clock is behind. The guest tools set it right away.