RS: [Beowulf] Virtualization in head node ?

Tim Cutts tjrc at
Wed Sep 16 02:34:48 PDT 2009

On 16 Sep 2009, at 8:23 am, Alan Ward wrote:

> I have been working quite a lot with VBox, mostly for server stuff.  
> I agree it can be quite impressive, and has some nice features (e.g.  
> do not stop a machine, sleep it - and wake up pretty fast).
> On the other hand, we found that anything that has to do with disk  
> access is pretty slow, specially when working with a local disk  
> image file.

I think that's pretty standard for most virtualisation, whichever  
vendor it comes from.  The I/O is fairly sub-optimal.  I've had a fair  
bit of experience now of various VMware flavours.  The I/O performance  
of the desktop versions is fairly shocking; this is presumably largely  
down to the fact that desktops and laptops tend to have fairly slow I/ 
O to start with, and the virtualisation penalty is very noticeable.

Our production virtualisation system uses dual-fabric SAN-attached  
storage (EVA5000), ESX 4.0 as the hypervisor, and we're running about  
20 virtual machines per physical host.  Most of these applications are  
not I/O heavy, but really trivial benchmarking using hdparm indicates  
I/O bandwidth within the VM of about half that if the machine were  
physical.  Very unscientific test, though.  I should do some proper  
testing with bonnie++...

Virtual disk performance in ESX 4.0 definitely feels better than ESX  
3.5, but that's largely because they've got rid of some fairly serious  
brokenness in memory handling in the hypervisor which was leading to  
unnecessary swapping of the VMs.

ESX 4.0 also has a new guest paravirtual SCSI driver which is supposed  
to improve virtual disk performance by about 20% but I have yet to  
test that.


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