How does MAAS pick which volume to boot from?

[Daniel K]

Mark,

Thank you for the response.

It indeed would seem that chain.c32 supports selecting a disk/partition to
boot based on its MBR serial or GPT GUID, so the steps should be just to
have the provisioningserver script pass that info to chain.c32. That would
certainly ensure that the drive that grub is installed to is the drive that
gets booted, which would eliminate any bios errors or indeterministic
behavior (such as when the HPE bios randomly decides to reset the boot
order). I've also had boot orders getting surprise re-arranged in some
SuperMicro systems.

It seems like a simple task and I would love to contribute by writing and
submit a pull request for this -- except I am not a developer and anything
I write would look like something MacGyver would create rather than
professional code.



Thanks,


Daniel


On Wed, Dec 6, 2017 at 7:30 AM, Mark Shuttleworth <mark at ubuntu.com> wrote:

>
> 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
>
>
>
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