Re: [Qemu-devel] RAM overcomittment

[James Olsen <jamesml@planetolsen.com>]

Hello,

TM> Another question is  : Can we run run many VM using more RAM than the
TM> RAM installed (on the workstation)? For example I'd like to create 5  
TM> VM's who have 256MB (can use 256MB but only for a slice of time) but  
TM> normally one VM use only 50MB. On the workstation I have only 512MB  
TM> RAM. Is it possible to create these VM's or have we to have 5 X 256  
TM> RAM + HOST RAM = 1.5 GB installed on the workstation?

TM> I think we can save ressource by using RAM overcomittment. What do  
TM> you think? Is this implemented on qemu? Or will be implemented?

It seems to me that no change to QEMU is needed for this; it should
already be supported by your host OS. Simply allocate the memory that
you want (256mb, for example) for each virtual machine. Depending on
how many virtual machines you have open at once, as well as the memory
used by the host OS and any other applications you're running, the
host OS will fit the as much as it can in the physical ram, and
automatically swap out the excess allocated memory to disk.

However, swapping (using virtual memory) will be a huge performance
killer. It will affect the performance of your host OS and all
applications, as well as for your QEMU instances.

-- 
James
Funny quotes: "There are 10 types of people in the world. 
Those who understand binary, and those who don't." -- Unknown
"A computer once beat me at chess, but it was no match for me at
kick boxing." -- Emo Philips