Stacked disks

Hello,

I would like to set up stacked disk images as follows:

1) Base-image of Ubuntu, with ubuntu-minimal installed.

2) Multiple other images all based on the single image above, with various single-purpose packages installed; eg Apache, Exim, etc.

Now, comes the hard part:

Q2: What happens if I change the underlying ubuntu-minimal image (For a good example, to apply security updates), while several of the other "upper" images (Apache Exim, etc) are running?

Q1: Easier: If everything is down, can I change the underlying image

will the changes be properly reflected in the "upper" (dependent) images?

I see problems with simply a file-based algorithm which looks first in the upper image, and then in the base image..

It's really a reconciliation problem..

FYI: I was a KVM forum attendee; I'm building a hosting infrastructure based on KVM, and so clearly saving / consolidating space & updates

as in the example above is of keen interest to me.

And Thanks again to the Qumranet team for an informative and successful conference!

Thanks,
Joe

Quoted 9 lines

Hello,

I would like to set up stacked disk images as follows:

1) Base-image of Ubuntu, with ubuntu-minimal installed.

2) Multiple other images all based on the single image above, with various single-purpose packages installed; eg Apache, Exim, etc.

Quoted 7 lines

Now, comes the hard part:

Q2: What happens if I change the underlying ubuntu-minimal image (For a good example, to apply security updates), while several of the other "upper" images (Apache Exim, etc) are running?

Q1: Easier: If everything is down, can I change the underlying image

and

Quoted 8 lines

will the changes be properly reflected in the "upper" (dependent) images?

I see problems with simply a file-based algorithm which looks first in the upper image, and then in the base image..

It's really a reconciliation problem..

The quick answer:
You can't do that (both 1, 2).
This disks are block devices that represent a physical disk. Doing something
to a block in the base image can erase information that the next image relays on.
Actually it is good practice to chmod a-w BASE_IMG if there are children.

So you shouldn't mess with the base image, you can replicate it and update all of them.

A better answer:
There is unionFS that might help here: All VMs will mount ro the base image while
having another drive for their private applications. UnionFs work in the file level
above and do the merge between the base and private images so basically you can
update the base image once in a while (when unionFS is down/inactive).

Quoted 3 lines

FYI: I was a KVM forum attendee; I'm building a hosting infrastructure based on KVM, and so clearly saving / consolidating space & updates

such

Quoted 6 lines

as in the example above is of keen interest to me.

And Thanks again to the Qumranet team for an informative and successful conference!

Quoted 2 lines

Thanks,
Joe

Thanks for the participation and for choosing KVM. We enjoyed the forum too.
-Dor