Re: [RFC] grub-install C rewrite

[Chris Murphy]

On Sep 26, 2013, at 2:22 PM, Lennart Sorensen <address@hidden> wrote:

> On Thu, Sep 26, 2013 at 08:49:52PM +0200, Vladimir 'φ-coder/phcoder' 
> Serbinenko wrote:
>> This is interesting testcase which wasn't brought before. This would
>> potentially involve creating several core.img or forcing UUID when using
>> multiple devices. Again, pretty easy in C and hairy in bash due to list
>> handling.
> 
> No, one core.img is fine.  The boot disk is the boot disk in each case
> (so on x86 device 0x80).  The same core.img gets inserted too all the
> devices, so that if it happens to be the boot disk, it works.  Since the
> partition is raid1, the same UUID is everywhere.  I do not expect
> booting from a raid5 device to be possible as the boot partition (althoug
> hperhaps grub is smart enough to read the kernel from an md raid5 device
> these days).

It is. Even raid6 is possible. The limitation is the BIOS being able to present 
all member devices to grub for it to assemble the raid in order to find /boot. 
But I don't know if it can work on a degraded array by rebuilding parity. If 
not, I see no point in /boot on raid5/6.

The case of btrfs raid0/1/10 is interesting. It's possible to boot all of these 
with GRUB2, I've tried up to 4 disks for those raid levels, and includes 
support for subvolumes/snapshots. I haven't extensively tried to break it with 
many snapshots, deletions, etc. So I don't know how resilient it is. The 
ability to boot raid1/10 with a missing member is useful. And it's actually a 
more complex setup to incorporate md raid and ext4 on those same disks for a 
separate /boot; or possibly losing ability to boot if /boot is on a single 
disk, which also complicates the layout. For such a layout, core.img is needed 
for each disk.

Open question is if on BIOS hardware, if a 1MB BIOSBoot is preferred over the 
64KB Btrfs bootloader pad? I don't know off hand if each member disk, or 
subsequently added disks, each have this 64KB pad or just the first member.


Chris Murphy