Re: [Xen-users] 'xl reboot domid' ignores all in-memory changes AND the domu's config file changes

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-09-21 08:13, Simon Hobson wrote:
> I wrote:
>
> > When you do a live memory change, you are **NOT** changing the guest config. 
> > You're sending a message to the guest to use the balloon driver to release 
> > memory (the memory is still there but the guest is tricked into not using it so 
> > it can be re-allocated elsewhere), so if you reboot then the system is starting 
> > afresh and will use all the memory it sees available.
> To expand on that ...
> The guest still has the full amount of memory allocated to it - so if you 
> started with 2G or RAM, it's still got 2G of RAM even after issuing your memset 
> command to turn it down to 1G. All that's happened is that the ballon driver 
> has demanded that the guest OS allocated 1G to it (it's a real memory hog !) - 
> and when that memory is allocated, the ballon driver can hand it back to the 
> host.
>
> Note that you can configure a guest with two memory values - a maximum it can 
> have (what the guest gets to see as the installed memory) and what it should 
> start with (a lower value, with the ballon driver taking care of eating up the 
> difference).
>
>
> The key thing is to realise that when you change the memory in a running guest, 
> you are **NOT** hot-plugging memory in/out of the system. I vaguely recall talk 
> of such a facility, but I don't know if it's implemented. *Adding* memory to a 
> guest by hotplugging is relatively easy - just add it and trigger whatever 
> signals the guest needs in order to use it - IIRC that is supported in at least 
> some OSs. Unplugging memory is a bit harder as you need OS tools to physically 
> stop using it before you unplug it.
xl mem-max is what you're thinking of probably, it lets you adjust the 
max-mem value dynamically, although the OS has to have hotplug support 
in the Xen balloon driver (I think most Linux distros do, but I'm not 
certain as I run custom kernels for all my Linux VM's).  Unlike the 
regular xl mem-set command, this will persist through an xl reboot.
> There's a good writeup on ballooning here :
>
> https://blog.xenproject.org/2014/02/14/ballooning-rebooting-and-the-feature-youve-never-heard-of/

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