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

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Subject: RE: [Xen-users] How many guest hosts per physical host
From: svarghese <vargh@xxxxxxxxx>
Date: Mon, 21 Dec 2009 08:55:42 -0800 (PST)
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Thanks a lots for all the valuable answers.

I guess I should keep the numbers below 16 so that I don't overrun the
existing 16GB physical RAM.


Nathan Eisenberg wrote:
>> 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 cheap.
> 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.
> Nathan
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