Stacked disks

Stacked disks

[Joseph Wolff]

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 and 
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 such 
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



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Re: Stacked disks

[Dor Laor]

>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
and
>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).

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

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

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Re: Stacked disks

[Joseph Wolff]

Thanks, Dor, we'll plan accordingly, and make the base images read-only.

Cheers,
joe



Dor Laor wrote:
>> 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
> and
>> 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).
> 
>>
>> FYI: I was a KVM forum attendee; I'm building a hosting infrastructure
>> based on KVM, and so clearly saving / consolidating space & updates
> such
>> 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
> 
> Thanks for the participation and for choosing KVM.
> We enjoyed the forum too.
> -Dor

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