[SATLUG] diskless ramdisk booting

Borries Demeler demeler at biochem.uthscsa.edu
Thu Jul 21 08:53:35 CDT 2016


I am hoping someone on this list can explain how diskless booting works.

I have a bunch of high school kids in my lab this summer and we are
trying to build a diskless linux cluster as a summer project. I have a
bunch of generic boxes with quad-core processors, on board video and 8
GB RAM each. We configured one of the boxes with CentOS 6.8 as a head
node. It has dhcpd, tftpd, nfs working fine now.  We can successfully
pxe boot various iso images served through the tftp server, the nodes
get private IP addresses, tftp boot the kernels and ramdisks on the iso
images, and the nodes can mount nfs shared volumes from the head node.
All of this works fine.

We now wanted to build our own custom image and wanted to compile a 
minimal kernel and build a minimal ramdisk on which we could nfs mount
various shares that hold the software and home directories we want to have
accessible on the slave nodes.

But we so far have been unable to build a proper kernel or ramdisk that
gets past the mounting point where the root ramdisk file system gets 
mounted and cannot figure out where we are going wrong. Any help would
be appreciated. We tried to follow this guide to the T:

http://eduardo-lago.blogspot.com/2012/06/ram-only-pxe-boot-smallest-diskless.html

and searched the internet for all kinds of other documentation on the topic.
But we must be missing something essential, like our init is not getting 
executed or the kernel cannot find the root file system.

Any help for these kids would be greatly appreciated! Can someone explain
how the boot process works exactly step-by-step so we can see where our
newly built image might be failing? THe boot process always fails at the
point where the initrd specified ramdisk does not mount root.

Thanks, -Borries


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