External snapshots with virsh

By default, virsh creates internal snapshots, which means base data and snapshot data is stored in the same disk file. With external snapshots, base data and snapshot data is stored separately: base disk file is left intact, and all changes are written into a separate snapshot overlay file. For me, this is the way 😄 Unfortunately, external snapshots support in virsh is limited, virt-manager GUI can't create / use them, and reverting external snapshots involves manual editing config xml to change disk file path.

The manual xml editing part was what irritated me most, so I figured I'd just save VM config with virsh dumpxml before creating a snapshot, and then restore it with virsh define if / when I'd need to revert. Then I decided to wrap this and other routine commands needed to work with external snapshots into a script, virsh-snap.sh.

Installation

Download script:

wget -P ~/.local/bin https://github.com/shvchk/virsh-external-snapshots/raw/main/virsh-snap.sh

Make it executable:
```
chmod +x ~/.local/bin/virsh-snap.sh
```

Usage

Create snapshot named snapshot-1 of vm-42:
```
virsh-snap.sh create vm-42 snapshot-1
```
Snapshot name is optional, by default current date and time is used as a name, e.g. 2022.07.29_23.55.12.

List snapshots of vm-42:
```
virsh-snap.sh list vm-42
```
Note that this is not the same as virsh snapshot-list. Snapshots are created without metadata and won't be shown with virsh snapshot-list.

Revert and delete snapshot-1 of vm-42:
```
virsh-snap.sh revert vm-42 snapshot-1
```

Revert, but don't delete snapshot-1 of vm-42:
```
virsh-snap.sh soft-revert vm-42 snapshot-1
```
This allows to use this snapshot later with unrevert.

Undo revert of a snapshot-1 of vm-42, i.e. make it active again:
```
virsh-snap.sh unrevert vm-42 snapshot-1
```

Delete snapshot snapshot-1 of vm-42:
```
virsh-snap.sh delete vm-42 snapshot-1
```
Might be useful if you change your mind after soft-revert.

Show help:

virsh-snap.sh help

virsh-snap.sh: easy to use external snapshots

Usage: virsh-snap.sh <action> <domain> [snapshot name]


Actions:
--------
create, c           Create snapshot
                    If no snapshot name is provided, current date and time is used

list, ls, l         List snapshots

disk                Show active disk

revert, rev, r      Revert snapshot and delete it (same as soft-revert + delete)

soft-revert,
srev, sr            Revert snapshot without deleting it (allows unrevert)

unrevert,
unrev, ur           Unrevert snapshot, i.e. make soft-reverted snapshot active again

delete, del, rm     Delete snapshot

help, h             This message


Snapshots are created without metadata, so they won't be shown with `virsh snapshot-list`