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Does Uberstudent 3.0 support GPT?
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Author:  Guest [ Fri Jun 14, 2013 4:00 am ]
Post subject:  Does Uberstudent 3.0 support GPT?


I've just downloaded Uberstudent 3.0 and intend to dual boot it with FreeBSD 9.1 (already installed but a bit of an adventure for me) and FreeBSD now uses gpt partitioning by default. Before I set to, does anyone know if the version of grub in Uberstudent 3.0 support gpt partitions? If this is not the case then I have the workaround from http://www.rodsbooks.com/gdisk/booting.html so it is not the end of the world.

Thanks in advance.

Author:  Stephen Ewen [ Fri Jun 14, 2013 6:28 am ]
Post subject:  Re: Does Uberstudent 3.0 support GPT?

I've never tried installing FreeBSD and a Debian-based OS on the same machine, but grub 2.00:13 is in both architectures of UberStudent and does contain /usr/lib/grub/i386-pc/part_gpt.mod. Still, even when two distros use grub as default, it does not mean they play well together without some work.

Let me know how it goes. You should probably read this and follow the links before any attempt. :)

I have tried installing Fedora and a Debian-based OS on the same machine. You really don't want to do that. :!:

As an alternative to unusual boot configurations, you might consider just using VirtualBox.

Author:  Guest [ Sat Jun 15, 2013 12:40 am ]
Post subject:  Re: Does Uberstudent 3.0 support GPT?

Ah thanks for the link.

I gave it a whirl anyhow and it was educational for me! So I thought I'd give some feedback.

The Uberstudent installer detected the FreeBSD partitions as "Unknown Linux". This is supposed to happen to all installers by the standard for GPT. So that was good. I had deliberately left half the hard disk (~60 GB) free disk space on the hard drive. And the system installed flawlessly. Then it came to installing grub ... from memory:

"Installing grub to /dev/mmcblk0 ... "

Oh dear! I knew immediately that it would not boot to Uberstudent. I'd left an SD card in the slot and, fortunately, my card slot is not connected via the USB bus. Therefore the machine can _never boot from this slot. So at this point I have an installed Uberstudent but cannot boot it unless I learn how to chain it from FreeBSD's Loader or create a separate boot stick.

So I looked around the system to see what I'd got. Uberstudent has grub-pc but not grub-efi. Therefore it has to install via the mbr and grub2 code, just like any normal bios computer. I stand to be corrected on what I say now but, to my understanding, GPT creates 5 data blocks - a protective mbr, the beginning of disk boot code and partition table and backups of the latter two on the end of the disk.

The pmbr contains no boot code but point to sector 1 (the second sector) and has a partition table saying 0, 0, 0, EE. So now any old-fashioned system (meaning most all PC's built before mid-2011 and a lot afterwards) will see an unknown operating system using the whole of the hard disk. This is deliberate. The person installing the another system with BIOS-mbr understanding only now has the choice to abort or install to a separate disk - hence the term 'protective' mbr.

So the processor now reads sector 1 to grab some boot code. But sector 1 is where grub and grub2 both put their own code, traditionally. Grub-pc has to find a new home for its boot code.

(Grub-legacy does not understand GPT but it has been hacked to work around it. This is stop-gap as once disks get much bigger grub-legacy won't be able to see the end of them because it must work with an mbr or image of one.)

Theoretically an mbr can only handle disks up to 2.2 Tb though there are work-arounds to 4 and 8 Tb. The gpt system can have up to 128 partitions, with no distinction between primary and logical, and can handle disk sizes up to 9.2 Zetta-bytes. That is 9.2 billion Tb (I think!) In 2010 it was estimated that the total world data held on all computers was about 1 Zb. One sextillion bytes or 10^21 bytes. (Avogadro eat your heart out - we'll reach your number sooner or later. Avogadro's constant is 602.3 zetta per mole.)

Anyhow, to come back on point, a 128-partition table takes up more disk space that a 4 partition table. GPT must use quite a few of the first sectors on the disk. After the fact of installing, I can see that there is a 64Kb partition at the beginning of the Uberstudent disk area. I presume this is a sort of grub-pc partition to store grub2 boot code. (I need to investigate further what has happened.) I don't think installing grub-efi will have any effect as the machine is a traditional BIOS-mbr machine by construction and the BIOS would not understand UEFI.

So, in summary, because of the module Steve pointed out (part_gpt.mod) grub has respected the "Unknown Linux" operating system (the standard actually says "Unknown Operating System") and chosen _not to overwrite the early sectors of /devsda. Therefore it looked for another disk on which to write its initial code and happened to see my SD card first.

Now if I install uberstudent again, this time to a usb stick, then it should install as normal and grub-mkconfig will see both installations. Then I will have a combined boot/recovery disk if anything goes wrong. That will be the next task then.

Thanks Steve.,

Author:  Guest [ Sat Jun 15, 2013 11:46 pm ]
Post subject:  Re: Does Uberstudent 3.0 support GPT?

Well it worked as expected. I installed Uberstudent to a USB stick and rebooted to the USB. Both installations had been picked up so I just had to choose the second to boot to the hard drive.

I learnt further that when the 1st installation went ahead on the hard drive the grub boot partition was a pgt type (guess it had to be) and it was labelled grub-bios. It was 1M in size and not 64k as I said to Steve in the last post. The 64k partition belonged to FreeBSD's 4 stage boot loading sequence. boot0, boot1, boot2 and loader.

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