How does MAAS pick which volume to boot from?

[Mark Shuttleworth]

This is super interesting. It seems to me that MAAS commissioning should
establish a unique way to identify each disk (unique MBR serial or GUID
or label) and then we should use those on boot to be very precise.

I really don't like the arbitrary behaviour of GPT-when-over-2TB but I
think that was studied extensively and proved the only sane approach
because of old BIOS's. At some stage perhaps we can move to GPT-only but
I suspect that's a loooong time away. Or perhaps we could move to a
smarter approach, of using GPT everywhere when we know the BIOS will
work with it, which should box the arbitrary behaviour to a shrinking
group of older servers.

Mark

On 12/05/2017 11:40 PM, Daniel K wrote:
> Digging into chain.c32, it looks like several options could be passed
> for the boot drive.
>
> > Usage:
> > chain.c32 hd<disk#> [<partition>] [options]
> > chain.c32 fd<disk#> [options]
> > chain.c32 mbr:<id> [<partition>] [options]
> > chain.c32 boot [<partition>] [options]
> > chain.c32 fs [options]
> > chain.c32 label=<label> [options]
> > chain.c32 guid=<label> [options]
>
> It would seem that label= or guid= would be the most sure-fire way to
> boot the drive you want to boot, but that requires a GPT partition
> instead of MBR. Fallback method for mbr could be use the mbr:<id> option:
>
> > The mbr: syntax means search all the hard disks until one with a
> specific MBR serial number (bytes 440-443) is found.
> > You can get the MBR serial number, by running the following command
> (change /dev/sda to the correct device):
> > $ hexdump -s 440 -n 4 -e '"0x%08x\n"' /dev/sda
> > 0x0ec8694c
> > Or by running:
> > $ fdisk -l /dev/sda
> > ...
> > Disk identifier: 0x0ec8694c
> > Example:
> > LABEL mbr_serial
> > COM32 chain.c32
> > APPEND mbr:0x0ec8694c