Support "differencing" disks, or don't delete immutable disk temp file on closedown

[Marc Evans]

I would like to create guests based on a "base" immutable disk image, plus changes. I know VBox already does this with immutable disks, but it then deletes the writes to a given session on closedown. I would like the option to persist these changes between sessions (preferably automatically so I don't have to check "save changes" every time).

The reason is speed and space: If I can create a base OS image and point guests at that, I have an "instant starting point" for a new guest without reinstalling the OS. If that guest then behaves like a normal PC from that point onwards I can install different apps etc without contamination. Plus it all happens for the disk cost of one base image + differences per guest.

[Lapo Luchini]

Also a problem with VirtualBox 2.0.2: I created an immutable "base", then (after installing a few things in the VM) I made "snapshot 1", then I shut off. On the next activation of the VM I had "base" content as opposed as "snapshot 1" content! OTOH if after the shutdown I do a "revert to snapshot" then it correctly boots "snapshot 1".

VM active: base > snap1 > current state (working) After shutdown: base > snap1 > current state (equals to base) After revert: base > snap1 > current state (equals to snap1)

I do agree that current-state of an immutable should be lost, but not if there is a snapshot in between the immutable and that current-state…

[Mildred Ki'Lya]

Well, it seems already possible. Even for normal disks. I was able to get to a state where a disk had several children. Unfortunately I wasn't able to reproduce that :/

See this topic: ​http://forums.virtualbox.org/viewtopic.php?p=43407

I guess adding that feature would be relatively easy. And that would help waiting for the issue #46 to be resolved.

[Florian]

Isn't this already possible? Simply create a hard disk, do your "base installation" and then create a snapshot. Now whenever you try to attach this base vdi to another virtual machine, it will use a new differencing image for each virtual machine. (this is signaled by showning the blue icon next to the selected vdi in the machine settings hard disk selection dialog)

[Technologov]

Now this becomes partially possible with VBox 3.1 and it's Branching Snapshots feature.

I say, partially, because it is not transparent yet, and VMs can'e be exported with all the snapshots yet. But some progress is made.

-Technologov

[Sander van Leeuwen]

Already possible.

[Jon Schewe]

What's the right way to do this then? I tried with immutable and that didn't work. Is it just to create the base and snapshot it, then attach multiple VMs to the base?