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RE: [Xen-users] How many guest hosts per physical host

To: svarghese <vargh@xxxxxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] How many guest hosts per physical host
From: Nathan Eisenberg <nathan@xxxxxxxxxxxxxxxx>
Date: Fri, 18 Dec 2009 15:02:21 -0800
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Thread-topic: [Xen-users] How many guest hosts per physical host
> Hi,
> I have a Dell machine, duel core xeon, with 16GB RAM. We are planning
> to run
> Xen on it. The storage can be increased as required. For every guest
> host
> the memory that is going to be allocated is 1 GB. These guest hosts are
> going to run web applications (apache/tomcat) for which 1 GB Ram should
> be
> more than sufficient.
> Can I create more than 16 guest hosts in my physical host? If the total
> number of host with 1GB RAM each exceeds 16, does XEN have any feature
> to
> mange the memory efficiently so that  memory will be allocated to guest
> host
> based on the load on guest hosts at a particular point of time?

The short answer is no.  You cannot overprovision RAM in XEN.
The slightly longer answer is that overprovisioning RAM is a bad idea.  RAM is 
The long answer is yes.  You can provision PV linux domUs with starting memory, 
maximum memory, etc, and they can balloon as needed.

There are real, significant issues with the way ballooning works - or at least, 
there were when I last tinkered with it.  But I personally question the idea of 
over-committing a cheap resource and then introducing some black magic to 
manage it.  

Just buy another box when you fill this one up; or better yet, do a shared 
storage solution so that when you buy another box, you can move domU's to it so 
that you can take the original box down for maintenance.  Scheduling 15 
simultaneous outages to replace one hard drive is a PITA.


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