Re: Stacked disks

[Joseph Wolff <joe-w0bN8YRwa0o@public.gmane.org>]

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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