[vbox-dev] [hello, the Virtualbox snapshot is incremental??

Klaus Espenlaub klaus.espenlaub at oracle.com
Mon Jul 19 14:12:59 PDT 2010

  On 19.07.2010 12:08, 박은병 wrote:
> Hello, I am the virtual box newbie :)
> I want to know the mechanism of the virtual box snapthos feature.
> I have tried to take several snapshot, and the snapshot files for
> state saving are .vdi .sav file
> I already have known that virtual disk is saved incrementally, but how
> about the guest OS's memory??
> I set 1GB the guest memory, where is it??
> is it also incremental snapshot??,
> I suggest that the .sav file has the memory contents, but the size of
> the file is small than memory available to the guest.
VirtualBox only stores the used part of the memory (i.e. if you take the 
snapshot *really* early this means only the first 1MB will be stored). 
Depending on the guest OS it depends how long it takes to touch the 
entire memory. With Linux guests it takes quite some time, essentially 
until the file system cache has grown to fill all memory not used by 

That's the first mechanism. The second mechanism to save space with the 
.sav file is to compress the memory contents. This works nicely as the 
RAM contents are usually very easy to compress, so the fast compression 
code we use (LZO at the moment) achieves quite good results.

The .sav file contains much more information than the guest memory 
contents actually - it's also a full copy of every device state and 
other VirtualBox running state.


> --
> Eunbyung Park

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