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Re: [Xen-users] over-allocation of memory

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: George Shuklin <george.shuklin@xxxxxxxxx>
  • Date: Fri, 22 Oct 2010 01:17:46 +0400
  • Delivery-date: Thu, 21 Oct 2010 14:18:46 -0700
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This can not be happen. There is no oversell in Xen. Ever. All
technology used by all experiments still coming to single conclusion: no

The Xen is driven by idea of page allocation. So, if we allocate page to
some VM we can not allocate it to other machine. 

All Xen memory controls works around idea of xenballoon - special
service (part of guest kernel) wich take memory from guest and returns
to hypervisor. And vice versa. 

Xen works with idea 'max memory' (theoretical limit to take memory from
hypervisor and return it to certain DomU). If maxmem is more than
available free memory for Xen, that means 'no more memory for DomU', and
this means 'ok, this is end line', MemoryErorr, OOM, etc as if domU was
normal comuter with non-rubber silicon DRAM modules with limited

All Xen can do: xen summ of maxmem > real_mem_for_host. But it could not
give more real pages (physical memory) to guests than have.

So, asked condition can not occur.

Ð ÐÑÐ, 21/10/2010 Ð 15:00 -0600, Greg Woods ÐÐÑÐÑ:
> This is probably a really stupid question, but what happens if you have
> more memory allocated to domU's than your physical RAM? Will that cause
> dom0 to crash? DomU's to crash? Or just slow performance due to swap use
> on dom0?
> I ask this because we've got high availability clusters running domU's,
> and I want to know how much memory is safe to allocate to domU's given
> that a failure of one server could suddenly cause twice as many domU's
> to be started on the other server. I'd like to know if I really have to
> keep half the RAM reserved for a fairly rare occurrence.
> --Greg
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