Re: [Xen-users] cluster - basic understanding

[Chris de Vidal <chris@xxxxxxxxxx>]

--- Mathias Diehl <md@xxxxxxxxxxxxx> wrote:
> now that I get closer to setup my first productive XEN machine I 
> wonder how a cluster will work with XEN.

So did I:
http://lists.xensource.com/archives/html/xen-users/2005-03/msg00010.html


> So one Cluster is build by 5 nodes here. Involving XEN - does 
> that mean following a) or b)?
(...)
> b)
> 
> Application        x    y    
> Cluster           ---  ---
> XENdomU        U  U U  U U
> XEN dom0       xxxx xxxx xxxx
> Hardware        HH   HH   HH   
> 
> Here three physical servers host a xen dom0 each with two 
> domU's. Two of these domU's on diffrent HH now build a cluster.


The correct answer is b.


The Xen hypervisor kernel which runs dom0 and domU must always be right on the 
hardware.  It runs
in kernel-level space, not user level.  That means nothing can exist between it 
and the hardware,
not even a cluster kernel.

So you can't, for example, create a highly available Xen hypervisor.  You can 
create several Xen
hypervisor machines and thus dozens of Xen domUs with which to run your cluster 
upon (plan B
above).

You could also create a cluster and use a virtualization product that runs in 
user space such as
VMware ($$), UML (free), or FreeVPS (also free).  FreeVPS has the most 
limitations but should be
the fastest of the three.  If you can accept the limitations, that's what I 
would choose.

UML has two modes for running processes, one in which the processes are all 
visible to the host
(tt mode) and one in which all processes are hidden in a seperate kernel space 
and only one thread
is visible to the host (skas).  Skas is supposed to be faster but I would think 
tt mode would lend
itself better to SSI clusters such as OpenSSI in which the threads of an app 
are spread across the
cluster.  In these clusters a multi-threaded app (such as a tt mode UML 
machine) performs better
than a single-threaded app (such as a skas UML machine).  But if you're leaning 
more toward high
availability and you need UML's capabilities then perhaps skas would be the way 
to go.

My needs are mostly high-availability with some load balancing.  I would buy 
three VT-enabled
servers, install Xen on each, create three domUs and cluster them with OpenSSI. 
 That way I've got
high availability with reasonably decent load balancing.  If I need more 
horsepower it's so very
easy to scale up; just add another Xen node+domU and add it to the cluster 
(even mid-day).  If I
need to do maintenance, the high-availability features take over (even mid-day).

Disclaimer:
All of my comments could be poppycock, especially the bit about UML's skas 
mode.  I haven't tested
*any* of it, I just have been reading alot about clustering lately.

Hope this helps!

CD

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