[Beowulf] SATA II - PXE+NFS - diskless compute nodes

Greg Kurtzer gmk at runlevelzero.net
Fri Dec 15 11:49:20 PST 2006

On Dec 14, 2006, at 2:33 PM, Donald Becker wrote:

> On Sat, 9 Dec 2006, Joe Landman wrote:
>> Guy Coates wrote:
>>> At what  node count does the nfs-root model start to break down?   
>>> Does anyone
>>> have any rough numbers with the number of clients you can support  
>>> with a generic
>>> linux NFS server vs a dedicated NAS filer?
>> If you use warewulf or the new perceus variant, it creates a ram disk
>> which is populated upon boot.  Thats one of the larger  
>> transients.  Then
>> you nfs mount applications, and home directories.  I haven't  
>> looked at
>> Scyld for a while, but I seem to remember them doing something  
>> like this.
> I forgot to finish my reply to this message earlier this week.   
> Since I'm
> in the writing mood today, I've finished it.
> Just when were getting past "diskless" being being misinterpreted as
> "NFS root"...

I prefer the term "stateless" to describe Warewulf and Perceus  
provisioning model (stateless installs may have local disks for swap  
and scratch/data space).

> RAMdisk Inventory
> We actually have five (!) different types of ramdisks over the system
> (see the descriptions below).  But it's the opposite of the Warewulf
> approach.  Our architecture is a consistent system model, so we
> dynamically build and update the environment on nodes.  Warewulf-like
> ramdisk system only catch part of what we are doing:

The stateless provisioning model has a very different goal then  
Scyld's Bproc implementation and thus a comparison is misleading.

> The Warewulf approach
>   - Uses a manually selected subset distribution on the compute node
> ramdisk.
>     While still very large, it's never quite complete.  No matter how
> useless
>     you think some utility is, there is probably some application out
> there
>     that depends on it.
>   - The ramdisk image is very large and it has to be completely  
> downloaded
> at
>     boot time just when the server is extremely.
>   - Supplements the ramdisk with NFS, combining the problems of  
> both.(*)
> The
>     administrator and users to learn and think about how both fail.

I suppose that under some circumstances these observations maybe  
applicable, but with that said... I have not heard of any of the  
Warewulf or stateless Perceus *users* sharing these opinions.

Regarding the various cluster implementations: there is not one size  
fits all, and all of the toolkits and implementation methods have  
tradeoffs. Rather then point out the problems in the various cluster  
solutions, I would just like to reiterate that people should evaluate  
what fits their needs best and utilize what works best for them in  
their environment.

> (*1) That said, combining a ramdisk root with NFS is still far more
> scalable and somewhat more robust than using solely NFS.  With careful
> administration most of the executables will be on the ramdisk,  
> allowing
> the server to support more nodes and reducing the likelihood of
> failures.

Well said, I agree. There are also some general policies that will  
work reasonably well and doesn't require much system specific tuning  
(if any).

> The phrase "careful administration" should be read as "great for  
> demos,
> and when the system is first configured, but degrades over time".  The
> type of people that leap to configure the ramdisk properly the first
> time are generally not the same type that will be there for long-term
> manual tuning.

What an odd way of looking at it.

Great for demos but not for a long term solution because it degrades  
over time???

If you are referring to careless people or admins mucking up the  
virtual node file systems I think the physical muckage would be the  
least of the concerns when these people have root. Not to mention  
blaming the cluster toolkit or provisioning model for allowing the  
users the freedom and flexibility to do what they want is  
misidentifying the problem.

> Either they figure out why we designed around dynamic,
> consistent caching and re-write, or the system will degrade over time.

Why would a system not built around "dynamic consistent caching and  
re-write" degrade over time?

Many thanks!
Greg Kurtzer
gmk at runlevelzero.net

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