Subject: Re: [vserver] Pros and Cons of 32/64bit guests on 64bit host
From: Ed W <lists@wildgooses.com>
Date: Fri, 22 Aug 2008 14:52:26 +0100
Fri, 22 Aug 2008 14:52:26 +0100

>> With KVM you can overcommit, as with others I believe, ie for you 8GB
>> machine 
>> you can have 4 VMs with 4GB each; physical RAM will get used
>> according to 
>> each VM's usage.
>>     
>
>
> Sorry I don't follow.  If 4 VMs (not KVM/QEMU) with 4G each then I need
> 16G RAM at least.
>   


Only if they all use 4G at the same time

It's common for a machine to be largely idle, so for example you may 
find that the working set is only 100MB despite nominally using 4G.  
This makes it possible to page out most of the instance and require a 
lot less ram.  You are suddenly stuffed big time though if all your 
machines suddenly come alive and need their ram back...

I think overcommit can be handled slightly more neatly with vservers 
because effectively it's just a bunch of processes which are running in 
one operating system with a bit of a partition between them.

Ed W



With KVM you can overcommit, as with others I believe, ie for you 8GB
machine 
you can have 4 VMs with 4GB each; physical RAM will get used
according to 
each VM's usage.
    


Sorry I don't follow.  If 4 VMs (not KVM/QEMU) with 4G each then I need
16G RAM at least.
  


Only if they all use 4G at the same time

It's common for a machine to be largely idle, so for example you may find that the working set is only 100MB despite nominally using 4G.  This makes it possible to page out most of the instance and require a lot less ram.  You are suddenly stuffed big time though if all your machines suddenly come alive and need their ram back...

I think overcommit can be handled slightly more neatly with vservers because effectively it's just a bunch of processes which are running in one operating system with a bit of a partition between them.

Ed W