Re: [Xen-users] Xen ballooning problem.

[George Dunlap <dunlapg@xxxxxxxxx>]

On Tue, Dec 19, 2017 at 4:22 PM, Jason Long <hack3rcon@xxxxxxxxx> wrote:
> Thank you for your reply but I mean is "KVM is also the better option for
> memory ballooning according to most users who accuse XEN of not returning
> unused RAM that has been previously shared with other VMs. KVM, unlike XEN
> is close to dedicated servers and users claim that KVM is way faster. Many
> users also admire the fact that KVM is more frequently updated which in turn
> leads to firmer security."

Jason,

A more helpful way of asking your original question would probably
have been something like this:

---
I read a blog post [1] about KVM vs Xen, which claimed that Xen
doesn't return a VM's RAM to dom0 after the VM is destroyed.  Is that
true?

[1] http://blah-blah-blah
---

As someone else has said, that claim is in part based on a
misunderstanding of how Xen works.

Domain 0 is a guest like any other guest.  If you're running in
'autoballoon' mode (which is the default), when you create a guest,
domain 0 may give up RAM to Xen to create the guest.  When the guest
is destroyed, that memory is given back to Xen, but not given back to
domain 0 unless it asks for it back.

You can tell domain 0 to ask for it back by using the following command:
  xl mem-set 0 [target megabytes]

Arguably autoballoon should do that automatically.  But nobody has
ever complained *to the developers* about this before, and so it
hasn't occurred to anybody to fix it.  Most of us disable autoballoon,
and assign dom0 a fixed amount of memory instead (e.g., by adding
dom0_mem=1024M to the Xen command line).

I would agree that if you just want to start a few toy VMs on your
desktop, KVM is a better choice than Xen.

I have no idea what "close to dedicated servers" is supposed to mean,
but the argument about security is completely bogus.  One of the most
important things for security is to be told about security issues so
you can update, and KVM as a project doesn't do this at all.

 -George

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