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Re: [Xen-users] Dom0 Memory?

  • To: xen-users@xxxxxxxxxxxxx
  • From: <ray@xxxxxxxxx>
  • Date: Thu, 10 May 2012 16:55:33 -0700
  • Delivery-date: Thu, 10 May 2012 23:56:47 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On Wed, 2012-05-09 at 17:52 +0200, Flavio wrote:
> > What does 'xm info' say?
> # xl info
> total_memory : 8107
> free_memory : 6968
So, 8GB of RAM is what Xen knows, ~7 of them being free... What's the

> As you can see, my 8GB of RAM have disappeared.
> What can you say about that?
Uh? :-O

Ok, this might be a bit too technical, but perhaps still of help:

I'm also sure this has been mentioned in this or another thread related
to this "issue":

Also trying to quickly summarize what's happening, no, your memory
_has_not_ disappeared. Actually, it is right there for your VMs to use
it (and xl invocation above shows right that).

With dom0_mem=1G you told Xen that one of your VM (the one that it will
automatically call 'Domain0' and that it will automatically create on
boot) will use 1G of your 8G of RAM. If you now try to create another VM
(say 'VM_1') with 2G of RAM, it will just do that, and you'll have 1G
+2G=3G out of 8G of your RAM dedicated to these two VMs, while the other
5G are still free.

Have you tried that? If you do, try running `xl info' on Dom0 again
after that. Try also SSH-ing in 'VM_1' and asking (e.g., via `free -m')
how many total and free memory it has. As one should expect, it will say
it has 2GB of RAM in total... Does this make sense?

So, again, the reason why you're not seeing 7GB of RAM in Dom0 is
because they're not there! Dom0 is a virtual machine to which you've
given 1GB of RAM via the cmdline... Having only 1GB of RAM is just doing
right what you asked for!

I can understand that it might look a bit tricky, but what you seem to
be missing here is you should consider Dom0 pretty much like any other
VM that Xen is managing as the hypervisor.

Hope of being of some help.

Those were interesting articles.  I'm left with a couple questions.
1)  How does one determine how much memory to allocate?
2)  Is there a way to garauntee that dom0 never has to use a page file?
3)  Sorry for the unrelated component - That wiki page was great.  When
building for the most current version of Xen, what is a good way to find
all the related wiki pages?


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