Re: [Xen-users] Number of domains versus memory usage

[Ernst Bachmann <e.bachmann@xxxxxxxx>]

On Friday 23 September 2005 17:33, Brian Johnson wrote:
> Hey fellow xen'ers -
>
> We are going to be setting up a new Xen server, my first, and I have a
> question.  From what I have gathered reading through the documentation, you
> are limited in the number of domains you can have running by the amount of
> memory you have.  Our server is going to be starting with 4GB of ram.
>
> Now, if we want to have domains with 1GB of ram, does this mean we can only
> have 4 domains? 

Yes.

> Or is there a more dynamic way of doing this?  In other 
> words, something like, "Domain1 can use up to 1GB of ram, Domain2 can use
> up to 2GB of ram, Domain3 can use up to 4GB of ram".  And any time Domain3
> needs 4GB, it can have it if it's free, otherwise it has to swap.
>
> Is that even possible?  Or is there another solution?  Or am I
> understanding the docs totally wrong?  Any help with this is GREATLY
> appreciated.  Thanks!
>
Well, there's the "balloon" driver, which marks memory as "used" inside a 
domain and gives it back to xen, but it doesn't do so dynamic based on the 
current domains needs.

And a assignment scheme like "give up to 4GB" to domainX won't work well with 
linux inside that domain anyways, as linux usually tries to use ALL available 
ram in some way (disk cache...) so after some time all of your domains would 
always use the max. ram they're allowed to.

if you need to run more domains on your hardware, but don't need the level of 
isolation xen provides (all your planned domains would run linux with the 
same kernel), have a look at http://linux-vserver.org/.

/Ernst

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