[SATLUG] diskless ramdisk booting

Igor Gueths igor.gueths at RACKSPACE.COM
Thu Jul 21 10:55:03 CDT 2016


Hello,
based on your initial description of the problem, it sounds as though the kernel is panicking because it cannot find the root filesystem. That being said, at a high level diskless booting works something like this (assuming of course you are in fact PXE booting):
•BIOS passes control to Eprom on the NIC.
•Boot loader on Eprom attempts to get an IP via DHCP, paying special attention to the TFTP-related DHCP options to determine where its TFTP server is.
•Kernel is downloaded from TFTP server, decompressed and executed.

I unfortunately do not recall if one has to hardcode NFS mount points and such into the kernel image, or if as part of passing control to the kernel a separate config file gets downloaded from which further boot options are passed to the kernel. That being said, I would check to see what sort of message (s) you are getting at boot time when the failure occurs and go from there. Hope this helps, and good luck.
> On Jul 21, 2016, at 8:53 AM, Borries Demeler <demeler at biochem.uthscsa.edu> wrote:
> 
> 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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