[gPXE] Fw: Fw: Booting MS-DOS using gPXE/iSCSI: C: drive is readonly

[Shao Miller]

Good day Adrian,

In regards to your "VERIFY ON" preventing disk writes to a SAN provided 
by gPXE's INTerrupt 13h hook:

I have seen something similar, when the device underlying my SAN is 
read-only.  For example, I thought it might be fun to have a read-only 
SAN with DOS on it that multiple clients could share at the same time (I 
wasn't going to write any files).  In Linux, I did:

$ losetup -r /dev/loop0 dos.hdd
$ vblade 1 1 eth0 /dev/loop0

When I SAN-booted this from gPXE, the clients did not know the the 
medium was read-only.  They could write sectors, but those sectors could 
possibly be lost once 'vblade' (an AoE target program) discarded them 
from its buffers.  Surely enough, turning "VERIFY ON" will report the 
fact that the sectors were not actually written.

It's possible that you could achieve something similar using levels of 
device-mapper trickery and iSCSI.

Note that the FAT filesystem (like so many other filesystems) is not 
intended for simultaneous use by multiple clients; that would be like 
having a hard drive physically attached to multiple computers with a 
spider-web of IDE ribbons spliced together and expecting the OS running 
on each computer to magically co-operate when writing to the filesystem 
without any awareness of the other OSs.  I'm not suggesting that you're 
doing this.

So is it possible that your SAN's underlying storage device/file is 
read-only?

- Shao Miller