Re: [Xen-users] Xen 4.0 memory overcommitment

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Apr 21, 2010 at 10:23:39PM +0200, Antoine Benkemoun wrote:
>    Hello,
> 
>    I am running a functional Xen 4.0 platform. The objective is the project I
>    am currently running is to replace 4 VMWare ESX by 4 Xen 4.0 hypervisors.
>    I have been able to reproduce all the core VMWare features in Xen (but
>    better of course =D) except memory overcommitment.
>

What do you exactly mean with memory overcommitment here? 

Xen supports memory ballooning, but it has never really supported memory 
overcommit,
by design. 

Memory in Xen guests is guaranteed to be real memory, so the guests will perform
well using it. If you need some virtual/overcommitted memory just add more swap 
to the guest.
 
>    I know this feature has been included since Xen 3.3 but I have found
>    almost no information about it.
> 

I don't think Xen 3.3 has any kind of memory overcommit.

>    When I try to start a domain with more RAM then is available on the server
>    I get the following message :
> 
>    VmError: I need 1433600 KiB, but dom0_min_mem is 200704 and shrinking to
>    200704 KiB would leave only 1348320 KiB free.
> 

See: 
http://wiki.xensource.com/xenwiki/XenBestPractices

You should use dom0_mem= parameter for xen.gz to dedicate a fixed amount
of memory for dom0.

>    Well I know it doesn't have enough RAM, that's the whole point. Do I need
>    to enable something specific somewhere ? I am using the Jeremy's 2.6.31
>    kernel (thank you whoever you may be !).
> 
>    Thank you in advance for your help,
> 

Xen 4.0 actually added some forms of memory overcommit, tmem and hvm guest
memory page sharing (de-duplication).

See: http://wiki.xensource.com/xenwiki/Xen4.0

-- Pasi


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