EDD 4-byte MBR disk signature for the bootdi sk
Quoted 6 lines · Jamie Lokier
Quoted 2 lines · Matt Domsch
it isn't important exactly what value is used, as long as only the boot disk has it, and your code knows what to look for.
How can you be sure what's on other disks the code doesn't know about at the time it writes to the boot disk?
If we rely on only a int13 tool to write the signature, you're absolutely right. Here I was corrected by Michael E. Brown on how we actually use this - entirely from within Linux which can see all connected disks.
Here is our algorithm:
1) look up number in /proc/bios/disk80_sig
2) scan /dev for this sig
3) iff *only* one disk with this sig found, that is the bios
boot disk, we are done, use this disk
4) if multiple disks with this sig found, then write unique sig
to *only* the drives with duplicate signatures, reboot, goto step 1.
Quoted 2 lines
If we encourage its use, so that it's basically assumed to work and boot processes use it by default
EDD/disksig is not a "use by all boot processes by default", it's really a "used by OS installers", not a normal boot runtime thing. Starting from a blank system, you use the EDD/disksig information to determine which disk should get the boot loader and your /boot or / file systems, and then if you want, other file systems. On subsequent boots, the information is collected, but isn't necessarily used. File system labels and/or file system UUIDs, once they exist, provide the needed uniqueness to mount your file systems appropriately.
Quoted 3 lines
then people will be upset if things
like adding another disk to a system just to read data off it cause the boot process to get confused.
Indeed, adding disks between when you've guaranteed you've got a unique signature on each disk, and when you make use of this information (in your OS installer) would be bad - you may break the uniqueness guarantee. But I suggest that is unlikely, since the information should really only be used by the OS installer, and most folks don't change their hardware config mid-install.
Thanks,
Matt
Signature · 4 lines
Matt Domsch Sr. Software Engineer, Lead Engineer Dell Linux Solutions www.dell.com/linux Linux on Dell mailing lists @ http://lists.us.dell.com