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Re: [Xen-users] Memory Ballooning / Overcommitting

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Memory Ballooning / Overcommitting
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Sat, 19 Jul 2008 17:10:29 +0100
Cc: Chris Hane <chrishane@xxxxxxxxx>
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On Friday 18 July 2008, Chris Hane wrote:
> I have a xen server setup that I want to install a lot of vms on if
> possible.  The vms will have no utilization for the most part; and brief
> utilization when active.  The server has 2GB of RAM.
> We were running into a problem were the dom0 would not let us add anymore
> vms because we were out of physical memory.
> I have started to read about memory ballooning and hoping someone could
> point me in the right direction.  If my understanding is correct,
> ballooning will increase the vm memory if needed up to a maxmem setting.
> And can decrease memory if requested by the dom0.
> Is this an automatic process, or do I have to manually issue a command to
> increase/decrease the domU memory?  The command I know about is:
>   > xm mem-set <Domain> <Mem>

Normally you can't grow a Linux guest beyond its initial allocation because it 
won't have sized its internal datastructures to fit the extra memory.  You 
can either start the guest at the largest you'll ever want it, then shrink it 
if required, or you can pass "mem=<something>" on the kernel command line to 
tell it the largest memory it should prepare to cope with (e.g. mem=4G).

mem-max is just a "safety catch" that prevents the domain from resizing itself 
and stealing lots of memory.  It doesn't change what the domain is capable of 


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