Re: grub2/grub-pc install not possible on mdadm

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On 02/03/2012 13:15, Stefan Priebe - Profihost AG wrote:
No. When booting with MBR, all that goes in the boot sector is enough to
load the next stage. That next stage will include the raid1x module. It
lives in the gap between the MBR and the first partition - with old
fdisk, that was 31K, with more recent fdisk it's 1MB. With no partition
table on the drives and 1.2 metadata, there's only 3.5K for it, which
isn't enough.
But that's why i've a BIOS_GRUB partition on the raid and so also on the

No, you don't have a BIOS_GRUB partition also on the disk. When you partition an array, the partition table goes inside the data area of the array.

Shouldn't it be the same size for 0.9 which starts at the
beginning, too?

With 0.9 or 1.0, because the data area is at the start of the array, and therefore at the start of the disc when you create an array over the whole disc, all the boot stuff is visible to the BIOS because it's in the same place as it would be on a bare drive.

What's actually happening with metadata 0.90 or 1.0, because they have
their data from the beginning, is that the partition table, GRUB etc
that you have installed into md127 appears to the BIOS as two drives
with boot sectors, partition tables, etc, and it then proceeds to boot
off the first drive.
So the bios needs to read the partition table? I thought the MBR would
be enough.

If you're booting with MBR (traditional BIOS), the MBR plus the gap before the first partition is enough.

If you're booting with UEFI (modern BIOS), the MBR is totally irrelevant and the BIOS has to read the GPT partition table to find the BIOS_GRUB partition.

If we're going to go any further than this, please can you indicate which you are using?

Until grub has loaded, you rely on the fact that
what you installed inside the RAID-1 is laid out in exactly the way the
BIOS expects to find it on a single drive.

What you've done with metadata 1.2 is put GRUB in a place that needs
GRUB to find it.
With metadata 0.9 i can install grub on sda and sdb even when there's no
part. table.

No, you can't; installing on sda and sdb overwrites the beginning of the array, and vice versa; you aren't installing it twice. grub-probe or grub-install understands this, which is why it works.

But with 1.2 grub says it cannot find a partition table so it cannot
install . So it seems to be a grub bug? grub-probe correctly dectecs a
1.2 metadata md raid on the disks but then still doesn't want to install
with the no partition table message.

It's not a grub bug. It's not a bug at all. When you create a 1.2 array, the data area is 1MB into the component devices, and when you partition the array, that's where its partition table goes. grub-probe looks for what the BIOS will see, and the BIOS will not see the partition table because it's not at the start of the component devices, it's 1MB in.

One way to make this work is: partition your discs (either MBR with a data partition starting 1M into the disc and spanning the rest of the disc, or UEFI with a BIOS_GRUB partition from as early as it'll let you up to 1M and a data partition from 1M and spanning the rest of the disc). You will need to do this for all your discs separately. Then create an array with any metadata and of any type supported by grub on the data partitions. The array need not be partitioned, but you want separate /boot, swap and root, so partition it. Then install grub and it will work it out.

Alternatively, partition all your discs as you previously partitioned your array. Create 3 arrays of any metadata and type, the first being for /boot, which must be of a metadata and type supported by grub. Install grub and it will work it out.


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