Beowulf 1 and 2 (Was: CLIC = Mandrake Cluster Distribution)

[Jim Matthews]

I have found downsides to the Beowulf 1 and 2 model so I am doing 
something in between...

The reliability/upgrade solution I have found is to have multiple single 
system images, mirrored disks and perhaps multiple boot image and file 
servers.  I have found several benefits of a single system image (well 
almost single; I actually use a separate NFS mounted root fs, minus /usr 
which is shared, for each system.  This ends up being ~70MB per node). 
 In my environments we have the need to have several clusters (or a 
cluster of subclusters).  Using netbooting I can easily maintain 
multiple subclusters of various types/arch from a single boot server. 
 On one of my cluster of clusters I am booting 2 alpha subclusters and 5 
intel subclusters from the same boot server.  Each subcluster can be 
served different kernels and be running different versions of linux. 
 The largest of these cluster of clusters is managing/booting 104 intel 
and alpha nodes booting from a single boot server.

The solution I have found to upgrading the boot server is to have 2 boot 
servers.  When it is time to upgrade, I reconfigure the new boot server 
with the new OS and get everything setup for the client images.  Then it 
is just a case of swapping the boot server and rebooting all of the 
nodes on the cluster (if you have a cluster where it never can be all 
rebooted then having 2 or more active boot servers is the solution).  I 
maintain the cluster consistency and configuration using a set of custom 
scripts.  I have also found that in large clusters it is best to 
separate the boot server from the file server so that the load on the 
boot server does not get too high affecting it's ability to share root 
file systems.  It is also very easy to setup multiple file servers for 
various subclusters.

Another concern which people sometimes have is that netbooting will 
create too much traffic on the network.  I have found that the traffic 
is very low once the nodes have booted, considerably less then 1% of the 
network is used for sending NFS messages from the boot server to the 
clients.  I have not seen any measurable loss of system/network 
performance using this netboot model (compared to a Beowulf 1 model) 
over a single 100mbs switched network connection to each node.

This model also easily supports the use of 3rd party drivers/modules on 
an individual node or subcluster basis.  This is simply configured on 
the boot server via subcluster management scripts.  In one of my 
subclusters I am running Myrinet on all of the nodes.  The servers are 
connected via Gigabit to a hierarchy of subcluster switches.

Here is my pro/con list for netbooting with a single system image:

Pros:
* All configuration and setup is maintained on the boot server
* Client node setup is much faster
* Client nodes can be run diskless or with just swap/scratch space. 
 Node disk failures do not require OS reinstall.  In a diskless 
environment reliability is greatly improved, especially with large 
clusters (100s of nodes), were disk failure becomes more common.
* OS and configuration for all clients can be mirrored on the boot server.

Cons:
* The boot server is a single point of failure (one solution is multiple 
servers)
* Setup is more technically complex (in the long run it is much easier 
to manage I have found)


My $0.04


--JIM

Josip Loncaric wrote:

>~snip~
>  
>
>>                            The Beo 2 model also provides single system
>>image with regard to process space, and methods to manage remote processes
>>from a central point of control.
>>    
>>
>
>This is an attractive feature, but in my personal view, not essential.  I'm
>reasonably happy with separate process spaces and periodic central collection
>of machine status data.  We operate a heterogeneous cluster built in several
>phases with different hardware and different networks, so it makes more sense
>for me to consider each class of machines separately.  On the other hand, a
>central point of control and unified view of the entire cluster open the
>possibility of rather sophisticated management.
>
>
>In my experience, the most time consuming task in operating a Beowulf 1
>cluster is the initial node installation or reinstallation after suffering
>disk damage.  The Beowulf 2 model makes this much easier.  The second most
>annoying problem is loss of network connectivity (usually due to some hardware
>glitch or network driver flakiness under heavy load).  The Beowulf 2 model
>depends on network connectivity more than the Beowulf 1 model, where one can
>operate the problematic node in stand-alone mode and debug the problem.  The
>third issue on my list is specialized hardware with its own drivers and
>daemons, which are usually designed for Beowulf 1 operation but may be
>difficult to convert to the Beowulf 2 model.  Finally, the issue of process
>startup time may concern some people: the Beowulf 1 model usually goes through
>standard Linux login, which is typically slower than the Beowulf 2 model,
>where all processes are started on the head node and then immediately migrated
>to the appropriate compute node.
>
>Sincerely,
>Josip
>
>P.S.  Our cluster (almost) never goes down, even during system upgrades. 
>Robustness comes from redundancy and compartmentalization.  We typically
>upgrade one section of the system, monitor new software for bugs it
>(re)introduced, find fixes, then complete the cluster upgrade.  Multiple
>system images allow this kind of experimentation.  The single system image
>approach is much more dependent on this single image working reliably on all
>of your hardware immediately.  One of the following two approaches may appeal
>to you:
>
>1) Don't keep all of your eggs in one basket 
>   -->multiple images, multiple versions
>
>2) Keep all of your eggs in one basket and WATCH that basket 
>   -->thoroughly debugged single image, no version skew
>
>P.P.S.  Version skew can begin before Linux gets loaded, in BIOS.  Diagnosing
>that one machine in a hundred becomes flaky under heavy load because its BIOS
>settings are subtly different is a very time consuming process...  By
>comparison, detecting and fixing Linux version skew is easy (start by checking
>/var/log/rpmpkgs, automatically generated daily on each machine).
>
>  
>

-- 

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 James W. Matthews - UNIX System Administration / Beowulf Cluster Design
 Raytheon Technical Services Company - NASA Langley Research Center
 MS 128 - 18E West Taylor Street - Hampton, VA 23681
 E-Mail: J.W.Matthews at LaRC.NASA.GOV - Phone: (757) 864-5259
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