[ipxe-devel] HTTP SAN / ISO booting

Shao Miller Shao.Miller at yrdsb.edu.on.ca
Mon Aug 1 19:27:59 UTC 2011

On 7/29/2011 14:03, James A. Peltier wrote:
> But, the raw ISO image does boot on a system with no hard disk 
> indicating that it is booting into a full RAM disk. The only 
> difference is that it's a CD drive attached to my KVM host. This would 
> then seem to indicate that the sanboot of the ISO image is not 
> presenting as a physical disk of any sort to the underlying operating 
> system and thereby dying. Would this be correct?

Yes, you are correct.  As Andrew described, there isn't a standard way 
to tell an OS about an HTTP SAN disk, like the iBFT for iSCSI and the 
aBFT for AoE.

Roughly speaking, you have pre-kernel and post-kernel times in your OS 
booting process.  Pre-kernel, iPXE hooks INTerrupt 0x13 and emulates the 
disk/disc.  But post-kernel, iPXE is gone, so the OS must re-establish 
an HTTP SAN; there's no standard way to do this, as far as I know.

DOS uses INT 0x13 throughout, so will always have access to the SAN, via 
iPXE's hook.

Also, that your VM works without an HDD doesn't exactly suggest that the 
OS is all loaded to RAM...  You have said that there's a virtual optical 
disc drive for the VM, and you might find interesting results if you 
were to detach that ODD at the precise moment that the kernel is booted; 
that is essentially what happens when SAN-booting "live CDs" and if the 
SAN connection disappears.

- Shao Miller

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