[Xen-devel] Re: Writing a tool for Shared Persistent Windows Boot Image

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

Jim Burnes wrote:
> > If you start with a single image, and then create "COW" files using 
> > the qcow format, then you can have a shared base image.
> Let me make sure I understand your answer.   COW files are sparse 
> storage for the QEMU environment (which tools you use for Xen).
> We want a static filesystem image that represents a snapshot of a 
> Windows XP system right after boot.  When we activate that image we 
> want to perform a few housekeeping issues (like set the MAC and 
> re-DHCP etc), but we also want to make sure that any writes to the 
> image are redirected to an overlay writable file system.  In other 
> words we want a single shared image of the OS itself with all writes 
> going to the COW / shadow image of that specific VM.
COW == Copy On Write.  It's a separate file that only stores the data 
that has been written since the cow was created.
> Is the COW file you speak of overlayed on top of a single static 
> Windows image or does the COW file contain the entire Windows XP boot 
> image plus any writes.
It just contains the writes after the COW was created.

> I know this depends on your previous answer, but if we delete the COW 
> doesn't that delete the XP boot image also?
If you delete the COW, the underlying base image is unaffected.

> If the COW file is used as an extent-space to the static Windows XP 
> image, then that should work.
> Otherwise I think it would require a recreation of the full Windows XP 
> image.  Ideally that image can be kept static as the continual 
> deletion and re-creation of relatively large COW files would be 
> time-consuming and would tend to fragment hard disk space.
You really shouldn't worry about disk fragmentation but that's a whole 
other thread :-)
Regards,

Anthony Liguori

> Other than that, I'm very grateful for your kind assistance.
>
> Thanks again,
>
> Jim Burnes
> Boulder, CO

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