[Beowulf] The Walmart Compute Node?

Robert G. Brown rgb at phy.duke.edu
Sat Nov 10 09:59:21 EST 2007


On Fri, 9 Nov 2007, Mark Hahn wrote:

>>>> Get loads of ram, vmware-server and BINGO! you have a cluster!
>>> 
>>> But this isn't a cluster - it's enterprise masturbation. We're talking 
>> 
>> And as noted, it may be a reasonable way to build a dynamically
>> repurposeable cluster intended to run certain classes of EP or not
>
> do people really do this with jobs so short that it wouldn't make more sense 
> to just netboot a custom image?  then you get full-on native performance, but 
> you probably don't want to try
> repurposing more than a few times/hour.  heck, even if you did,
> just use kexec rather than rebooting, and I think you could come up in a new 
> image close to as fast as a VM would start.
>
> IMO, VM's are mainly about isolation (and, for commercial low-duty-cycle
> applications, overcommittment for better utilization.)

I think that they are just two different ways of accomplishing much the
same thing.  The good thing about vms is that they are extremely
portable, because the local install already has all the devices managed
and vmware just simulates a "generic" device.  So you can move a VM from
one machine to another, heterogeneous machine -- even from a machine
running lin to one running win.  Even from my organization (which may
only be marginally trusted) to your organization.  A VM is also a
sandbox of sorts; this is one of its main reasons for existence in
corporate environment where you can grab a webserver VM image and just
boot it up more or less ready to run, then make it go away, without
having to do much but establish your network boundaries.

Diskless/PXE is great if you have a relatively small set of images you
want to run and don't begrudge the sysadmin time required to create them
and manage them and set up THEIR network and other boundaries.  It's
much more difficult to root restrict a netboot image from accessing a
local disk, though, than it is to restrict a VM, where it is actually
difficult to MAKE it access a local disk (impossible outside the
VMserver console, and it requires a shutdown, reconfiguration, reboot,
and unless the disk is unmounted from the primary OS it ends up dual
mounted without locking, meaning that yes, it will gradually start to
break, leaving tracks a blind sysadmin could follow).

All of this could certainly be managed for diskless images as well --
it's just different.  I just think the sandbox thing and portability
thing required for many COD environments come much more naturally (and
often more cheaply in human time) for VMs than for handcrafted boot
environments.

     rgb

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-- 
Robert G. Brown
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone(cell): 1-919-280-8443
Web: http://www.phy.duke.edu/~rgb
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