[rhelv6-list] Minimal RAM for a RHEL6 KVM host?

[carlopmart]

On 02/03/2011 07:09 PM, Phil Meyer wrote:
> On 02/03/2011 10:26 AM, carlopmart wrote:
>> On 02/03/2011 06:08 PM, Phil Meyer wrote:
>>> On 02/03/2011 09:00 AM, carlopmart wrote:
>>>> Hi all,
>>>>
>>>> Which is the minmal amount of RAM needed for a RHEL6 KVM host?? I read RedHat's
>>>> docs about this, and it seems 2GB ... Really??
>>>>
>>>> Is it possible to limit RAM used by the KVM host as a Xen does??
>>>>
>>>> Thanks.
>>>
>>> Xen dom0 is a tiny, raw hypervisor using bits of the Linux kernel to operate
>>> hardware.
>>>
>>> KVM is a kernel module based hypervisor. Thus it uses the full kernel and OS with
>>> all that implies.
>>>
>>> VMs utilizing the KVM hypervisor run as applications in the host application space.
>>> This is totally different than Xen.
>>>
>>> It has great advantages, and a few disadvantages.
>>>
>>> Using KVM, you have to think of the VMs as fixed sized applications.
>>>
>>> Want to run 2 4GB KVM VMs? The host needs to have more than 8GB available to it in
>>> order to dedicate RAM to the VMs as well as manage its own resources.
>>>
>>> I am bending the specifics slightly to make the point that KVM based VMs run on top
>>> of a full kernel/OS, and that KVM VMs are treated as applications within that space.
>>>
>>> Hope this helps.
>>>
>>> Good Luck!
>>>
>>
>> Thanks Phil. I will run minimal services on host side (ntp, postfix and kvm). In
>> the example that you explain, does enough with the host had, for example, 8.5 GB
>> of RAM?
>>
>> I don't need HA or Clustering services or to connect to external storage (I will
>> use local disks).
>>
>> Thanks.
>>
>>
>
> Here is a real scenario:
>
> I have a well equipped server with 32GB RAM installed.
>
> I installed RHEL6 with kvm, libvirt, virt-manager, ksm, etc.
>
> I have personally created 60 KVM based VMs running Fedora 14 set to 512MB RAM each
> on this system.
>
> They all functioned within specifications for the hardware they were running on.
>
> The host computer showed almost exactly 30GB used by the guests under load, and as
> low as 17GB when all 60 were initially booted and idle.
>
> Therefore, I can conclude that a 32GB system can manage up to 60 512MB KVM based VMs.
>
> I/O for those guests is another matter entirely! :)
>
> You will drive the host crazy unless you turn of cache for the VMs, and use some
> kind of fast storage array for all those clients.
>
> But that is another plus of using a real host based hypervisor.
>
> In this same scenario I could run 4 2GB KVM based VMs, for example, and leave disk
> cacheing on in the config. This way the host will use its large memory reserve of
> 24GB as mostly disk cache for my VMs. This can produce results of over 500MB/sec
> reads in bonnie++ inside the VMs!
>
> Dangerous, but fun!
>
> Using the host as a disk cache for the VMs can cause a lot of issues on a crash, and
> prevents live migration of the VM, but man, the VMs disk i/o screams!
>
> Good Luck!
>

Many thanks Phil.

-- 
CL Martinez
carlopmart {at} gmail {d0t} com