[Beowulf] nfs pivot was: statless compute nodes

[Joe Landman]

On 05/28/2015 07:48 AM, Michael Di Domenico wrote:
> On Wed, May 27, 2015 at 9:56 PM, Joe Landman
> <landman at scalableinformatics.com> wrote:
>> There are some oddities, not the least of which most distributions are
>> decidedly not built for this.  But if you get them to a point where they
>> think they have a  /dev/root and they mount it, life generally gets much
>> easier rather quickly.
>>
>> One of the other cool aspects of our mechanism is that we can pivot to a
>> hybrid or NFS after fully booting.  And if the NFS pivot fails, we can fall
>> back to our ramboot without a reboot.  Its a thing of beauty ... truly ...
> Can you explain this in more detail?  I currently pxe boot into
> busybox and then pivot_root to boot up RHEL.  it works well, but if it
> fails, it gets all screwed up and i have to reboot.  i'd like to know
> more about how boot the os and recover back to the pxe image if it
> fails, that sounds really handy... :)

Our pxe boot environment is not a whittled down thing, its a full blown, 
fully operational stand alone Linux (Debian based) that *just works*.  
Atop this, we included this:

https://scalability.org/2015/01/stateless-booting/

The fallback is a very simple check to see if the mount succeeded or 
not.  If so, we continue, if not, we skip the bind mounts and pivot_root.

We are working on a CentOS/RHEL version of this (though the brain damage 
in the startup scripts there is far worse than in other distros ... see 
https://scalability.org/2009/09/fighting-the-dmraidmdadm-battles-in-initrd-for-rhelcentos-5-x/ 
iand others, its worse in 6.x, and I expect no less pain in 7.x)

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-- 
Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics, Inc.
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