[SATLUG] diskless ramdisk booting

Mark Mayfield mayfield_mark at gvtc.com
Thu Jul 21 13:33:21 CDT 2016


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On 07/21/2016 11:41 AM, Borries Demeler wrote:
> On Thu, Jul 21, 2016 at 03:55:03PM +0000, Igor Gueths wrote:
>> 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.
> Yes, that's exactly how far we get with the pxe boot.
>
>> 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.

When I did it with nfsroot in the kernel, the root argument was passed 
from the bootloader
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grub 1 example

title pxebootclient
     kernel path/from/tftp/to/distboot/vmlinuz root=/dev/nfs 
nfsroot=192.168.1.2:/nfsshare ip=dhcp rw
     initrd path/from/tftp/to/distboot/initrd.img
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> Our first try is to boot a small self-contained initramfs we built by hand that has a
> init script as well as busybox built into it. The nsf mount points can be put into /etc/fstab
> on the ram image and can be loaded as long as the kernel supports nfs. When we boot an
> image from, say CentOS installation media, this part works perfectly well, and we can
> access nfs mounted dirs just fine, but our own initramfs doesn't seem to work, or
> maybe it is the init.
>
> -b.
>
>
>>> 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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