sneaky dual-boot [LWN.net]

sneaky dual-boot

Posted Apr 24, 2022 20:15 UTC (Sun) by ballombe (subscriber, #9523)
Parent article: Fedora considers deprecating legacy BIOS

Sometime it is useful to setup the computer so that a different OS boots depending whether UEFI or BIOS is used.
It is also possible to have two different partitions tables (GPT and legacy) on the same disk.

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sneaky dual-boot

Posted Apr 25, 2022 0:46 UTC (Mon) by pabs (subscriber, #43278) [Link] (3 responses)

What about the opposite; is it possible to set up a system so that it can boot in either UEFI or BIOS mode? IIRC current install systems don't do that, but can themselves be booted in either UEFI or BIOS mode.

sneaky dual-boot

Posted Apr 25, 2022 11:44 UTC (Mon) by lsl (subscriber, #86508) [Link]

Fedora's cloud image does that, I think.

sneaky dual-boot

Posted Apr 27, 2022 11:57 UTC (Wed) by plugwash (subscriber, #29694) [Link] (1 responses)

Many distro install images nowadays support four different boot methods.

Legacy HD-media
Legacy optical media
UEFI HD-media
UEFI CD

In principle it would be possible for an installed system to support both UEFI and Legacy. I do see a few issues though.

1. Operating systems using UEFI installed on fixed drives are supposed to register themselves with the firmware(which requires the installer to berunning in UEFImode) rather than relying on a fixed entry point on the drive. In practice it's possible to boot using the "removable media path" even on a fixed drive but it's not the official way to do things.
2. While the combination of BIOS boot and GPT is technically feasible it's notexactly standard, I'm not sure if grub supports it or if a hybrid partion table (which is it's own can of worms) is needed.
3. There is no gaurantee that what works in one mode will work in the other.
4. Multiboot becomes even more of a can of worms than usual.

sneaky dual-boot

Posted Apr 27, 2022 14:55 UTC (Wed) by jem (subscriber, #24231) [Link]

While the combination of BIOS boot and GPT is technically feasible it's notexactly standard, I'm not sure if grub supports it or if a hybrid partion table (which is it's own can of worms) is needed.

GRUB supports booting from a GPT disk in BIOS mode, but you will need an extra BIOS boot partition. A disk with an MBR partition table usually contains a gap between the boot record and the first partition, which GRUB takes advantage of. GPT-partitioned disks do not (typically) have this "no man's land", so a separate partition is used instead. Using a partition is cleaner anyway, and GPT also does not have a practical limit on the number of partitions a disk can contain, so one extra partition doesn't make a big difference.

sneaky dual-boot

Posted Apr 26, 2022 12:43 UTC (Tue) by nix (subscriber, #2304) [Link]

Done that mistakenly, on several occasions. It is *incredibly* confusing! The machine hardly ever gives any sort of on-screen indication of how it's booting, so the symptom is often that you install one OS, reboot, and the OS the machine used to run leaps into action as if it was not installed over, with the new one invisible (because it's using a completely different partitioning scheme) and with the old one often misbehaving (because parts of it have just been overwritten more or less completely at random).

This fubar is becoming rarer now because few machines with existing DOS partition tables are getting GPT-reformatted, and the installers that do that are using wipefs or equivalent and thus are *actually* wiping out the old one properly. But oh good grief was it confusing for a few years.