On Tuesday, June 7, 2011, at 4:54:26 AM, Holger Parplies wrote:

> There are probably people on this list who can tell you from experience
> how VMware virtual disks behave (or VirtualBox or whatever), and it might
> even be very likely that they all behave in similar ways (such as
> changing roughly the same amount of the virtual disk file for the same
> amount of changes within the virtual machine), but there's really no
> guarantee for that. You should try it out and see what happens in your
> case.

I'm one of those people; I had to dig into the structure of VirtualBox's
VDI file structure after a problem truncated one of them and rendered my
Email files useless.

They have a metadata area at the beginning, of several thousand bytes, but
following that are exact images of the structure you would see from a
physical disk. There's one exception: they may be sparse. However, a 20-GB
virtual disk will actually be 20 GB in size unless it's created as a
"dynamic disk" and it's not clear to me whether the unused portions will be
in the image, or whether the sparse-array approach will be handled by the
metadata.

Were I attempting to back up the images I would assume that the unused
areas would be included. My solution to backing up my VMs was to install
backuppc for each of them and treat them the same as physical machines on
my net. This did lead to problems backing up Win2K and WinXP VMs, but only
those already fully addressed for physical systems.

-- 
Jim Kyle
mailto: jim AT jimkyle DOT com


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