RE: [Xen-users] Automatically provisioning IP addresses on a new VM

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Dustin Henning wrote:
>         Couldn't it be even simpler?  Couldn't these providers have one or
> two spare DomUs for each OS configured and available, making only the
> assignment process automated (if even it is)?  I'm not saying that's how
> they do it, but 5 minutes seems like an awfully quick deployment for
> something that doesn't already exist (I would think copying the image could
> take longer than that unless it was pretty basic).
It's common to use a COW (Copy on Write) file for this sort of thing. 
You might have (say) a 5GB base file, but initially the customers 
file will be negligible in size. Once the customer starts modifying 
the filesystem, then his file starts growing accordingly - but 
anything not updated will still not take up any space.
Long term, each customers file will keep growing, but it will 
probably still share some parts in common with the original. Apart 
from the space saving, the main advantage is that you can make large 
files (ie telmplate filesystems) available without the time required 
to actually copy all the data.
One way to see something similar very easily is to boot up a Knoppix 
disk. That uses unionfs to combine the read-only contents of the 
CD/DVD with the writable space of an in memory filesystem (tempfs 
IIRC). It's different since it works at the file level, but the 
principal is the same and you get to see what looks like a large 
writable filesystem, while nearly all of it is in fact on read-only 
medium.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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