What you must know
This section contains basic information about concepts and objects used when managing Virtual Machines.
QEMU modes: KVM vs emulation
When using Reven with QEMU VMs, you will have to switch between two virtual machine mode, because recording is restricted to full emulation mode:
| Mode | Technology | Speed | What we use it for |
|---|---|---|---|
| KVM | Virtualization with hardware support | Fast | Installation, heavy configuration |
| Emulation (TCG) | Full software: code for each instruction | Slower | Recording, simple configuration |
From the point of view of the guest OS running, these two modes are completely different hardware. Going from one to the other is akin to moving a physical hard disk between two machines, and as a consequence requires properly shutting down the VM before making the switch.
Note: the same is true when changing most other hardware options, you must do it while the VM is shut down.
Snapshots
Reven uses the native disk / live snapshot mechanism from QEMU which might differ from what users expect. Two different objects are called "snapshots", so to avoid any confusion we need to differentiate:
| Name | What it is | Support switching VM modes |
|---|---|---|
| Disk snapshot | The living VM's hard disk | Yes, by shutting the VM down |
| Live snapshot | A frozen state of a running VM, to be restored anytime | No: must be restored using the same mode it has been created with |
Live snapshots are very handy, because they provide a very quick way to restore a VM from a known good VM state, instead of having to wait for the VM to boot up. This is even more true in Emulation mode, where booting a VM takes a few minutes.
Finally, note that the above table is a simplification. Notably, live snapshots are stored within the qcow disk snapshot file itself, and this is reflected in the Project Manager's organisation.
You can now head over to the Adding VMs section.