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Re: [Xen-devel] question about xen4.4 features

On Wed, 2014-05-21 at 02:51 +0000, Bohai (ricky) wrote:
> I have two questions about xen features.

I was in two minds about whether this was really a development question
or would be better suited for the -users list. I've given it the benefit
of the doubt this time, but for future queries please consider whether
the -users list might be a more appropriate forum.

> 1.
> When I read the feature matrix the below:
> http://wiki.xen.org/wiki/Xen_Project_Release_Features
> I found xen has supported the " Memory Sharing ", which means 
> " allow sharing of identical pages between HVM guests ".
> But I can't find more detail description about it.
> Whether it means all the memory pages of the guests can be shared if they are 
> identified same?
> Or just zero pages can be shared?

I believe this is down to the policy and functionality of the userspace

> Or where I can find the detail description about it?

As you can see it is listed as "tech preview" and as the footnote states
this is due to limited tools support. I'm afraid this pretty much
extends to documentation as well.

There is a simple tool to test things manually in

The xc_memshr_* functions in libxc a documented in xenctrl.h.

The underlying hypercalls seem to be in xen/include/public/domctl.h

> 2. 
> I tried the vm save feature by the command "xl save".
> And I found all the memory of the vm(my vm memory configuration is 4GB )has 
> been saved.
> The saved file size is almost 4GB.
> Whether it's right? Or any option I missed?

No, this is normal and right.

> Can I just save the valid content in the memory of the vm.

If you balloon down only the non-ballooned pages will be stored. But
other than that the hypervisor doesn't know what memory is considered
valid by the guest OS.


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