marionnet-dev team mailing list archive

[Alexey Eromenko <al4321@xxxxxxxxx>]

> About the VirtualBox integration, it seems to us, with our limited knowledge
> of VirtualBox, that there could be a problem. Actually, Marionnet takes
> advantages of the UML ability to deal with cow filesystems. When a Marionnet
> virtual machine is created, its virtual disk is a couple (B,F) where B is a
> (big) cow "Backend" and F is a (small, sparse file) cow "Frontend". In this
> way, when the user defines a project with some machines M1,..,Mn, we store
> in the project file (which is a tar archive) only the set of frontends
> F1,..,Fn. The project can be sent and re-used in another host only if the
> set of backends B1,..,Bn are also installed in the new host. Backends are a
> sort of "prerequisite" to run a project in a host.
>
> VirtualBox provides a snapshot mechanism that seems equivalent: as far as I
> can see, the backend is the virtual disk (.vdi) and the frontend is the
> snapshot (again a .vdi). But what about the disk space?

> Could a snapshot be
> a sparse file, i.e. a small piece of data that we could put in the project
> file, as we do with cow files? This is our main doubt about the introduction
> of VirtualBox in Marionnet. Maybe, you will dissipate it immediately...

VirtualBox has several types of images:

-normal images (read-write, those can be assigned to only one running
VM) -- this is what VirtualBox GUI offers for newbies, but probably
not suitable for your project.
-immutable images (read-only, and can be assigned to many running VMs)
-- this is what you need, it seems. This is the equivalent of UML's
'Backend' images.
-differencing images - (read-write) this is the cow images, that are
roughly equivalent to UMLs 'frontend' images. You will need to use
them. Differencing images can be done over normal images, or over
immutable images.

VDI is a container disk format, used by VirtualBox, but it can have
different types.

NOTE: VirtualBox has built-in management layer. This is quite a huge
difference from UML or Qemu engines, from programmer's point of view
-- it changes the programming paradigms in a big way.

This management layer has several consequences:
1. You don't have to store a lot of things in your projects (Hard Disk
attachments, MAC addresses, RAM size, etc...) - a VM in VirtualBox is
really an object, that has everything attached and pre-configured.
You can rely on VirtualBox GUI for basic VM settings (this is what GNS3 does)

2. Management layer is a protection layer against user's mistakes: In
UML or Qemu if you attach the same 'Backend' image in two VMs and
mount them both as 'read-write', it will corrupt the filesystem in the
image.
No such thing possible in VirtualBox.

With immutable image as base, VirtualBox enables 'cow' mode
automatically, while with 'normal' image as base, it will refuse to
start second VM.

This management layer is a singleton, per user. (called 'VBoxSVC').

Practical Example:
1. in GUI, create new VM, called "Linux-Motherimage" -- install Linux into it.
2. convert it to immutable:
2.a. in GUI, detach "Linux-Motherimage.vhd" from "Linux-Motherimage" VM.
2.b:
$ VBoxManage modifyhd Linux-Motherimage.vhd --type immutable

3. create children VMs:
3.a. In GUI, create new VM "Linux-child-VM01", choose "Use Existing
Disk" = "Linux-Motherimage.vhd"
Now it will use it as 'cow', and create 'frontend' differencing image
automatically.
Look in folder:
./Linux-child-VM01/Snapshots/{UUID}.vdi
NOTE: This is not a standard snapshot, and therefore it is invisible from GUI.

This file is a differencing image COW image, based on immutable
"Linux-Motherimage.vhd".

3.b. By default, VirtualBox discards all changes on VM shutdown, and
reverts back to original state. (equals to "Linux-Motherimage.vhd")
To prevent this from happening, use:
VBoxManage modifyhd ./Linux-child-VM01/Snapshots/{UUID}.vdi --autoreset off

3.c. Create more children:
"Linux-child-VM02", choose "Use Existing Disk" = "Linux-Motherimage.vhd"
VBoxManage modifyhd ./Linux-child-VM02/Snapshots/{UUID}.vdi --autoreset off
...

4. Differencing images are linked to the base image (to either
'normal' or 'immutable' image).
You can look on this images hierarchy in GUI->File->Virtual Media Manager

-- 
-Alexey Eromenko "Technologov"