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

On Wed, May 21, 2014 at 10:18 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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
> tools.
>> 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.

It's probably worth pointing out that this was designed so that you
can plug in proprietary sharing modules; and indeed, there is a
company selling such a module, if you're interested.

>> 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.

Didn't Shriram implement an option to compress pages as they were sent
over the wire?  One could imagine doing something similar for the save
file (or indeed, zipping the entire file).  That would turn 0 pages
into almost 0 bytes.

But at the moment, ballooning a VM down before saving it is t he only
option; and in the long option that's probably the best thing to do


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