[SATLUG] diskless ramdisk booting

Borries Demeler demeler at biochem.uthscsa.edu
Thu Jul 21 11:45:15 CDT 2016

On Thu, Jul 21, 2016 at 11:29:16AM -0500, Mark Mayfield wrote:
> Depending on how you want to do it there are some kernel compile
> options you need to set.
> NFS capability in the kernel, NFSRoot, dhcp in the kernel and make
> sure you get your network card drivers in the kernel also.
> Pretty much all these will not be enabled by default in the kernel
> config. If you do everything right you don't even need a ramdisk.
> You can do some of this in the ramdisk but I had better luck just
> doing it in the kernel. The biggest problem I faced was making sure
> your network drivers are in the kernel or ramdisk. If you know you
> will be using one type of nic card it isn't a big problem but if you
> will use different machines it is more problematic. I've used
> ramdisk creation options in both fedora (dracut) and debian
> (mkinitrd I think) that tell the system to put all (or at least
> nearly all) modules in the ramdisk. Also you have set up your fstab
> right and if your using a ramdisk make sure the fstab configured for
> netboot makes it in the ramdisk. Hope the info helps, I haven't
> messed with it in a while but I worked with net boot systems quite a
> bit a few years back. At A&M I had a pxe server with a bunch of
> distro's it, and a custom boot CD for machines outside of the dhcp
> scope that could still hit the NFS servers.

Sounds great, thanks for the pointers, we'll look into it.
I believe we have all the drivers in there, but maybe we missed a critical
one. It is also possible our initramfs system, or the init script is not correct.

Thanks again, well keep you posted.
> On 07/21/2016 10:55 AM, Bruce Dubbs wrote:
> >Borries Demeler 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.
> >
> >Have you made a custom initrd?  See if
> >
> >http://www.linuxfromscratch.org/blfs/view/stable/postlfs/initramfs.html
> >
> >helps.  You will, of course, need to modify the init.in file.
> >
> >  -- Bruce
> >
> -- 
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