Grub2/raid quickie ...

Grub2/raid quickie ...

[Wols Lists]

After my last shenanigans, I've now got my main system booting
successfully from a raid / partition. HOWEVER.

I'm somewhat puzzled by grub.cfg, and this is in part behind why my dev
system won't boot raid properly, I think. Grub uses two different UUIDs!?!?

One of them, (mduuid/...) is dead easy to find, "mdadm -D" gives the md
uuid of the array. But the other one, the "root=UUID=...", I'm damned if
I can find that id anywhere! Does anyone know a command, that will tell
me what that id is, so I can fix grub.cfg on my development system?

(As an aside, how do I retrospectively force a name onto an array so
that I can guarantee it will be called /dev/md100, or /dev/mdhome, or
whatever? I daren't use the /dev/mdxxx name at the moment, because they
change and the result is a system that fails to boot, or is unusable, or
some other malarkey. Dead easy for me to fix, but my wife will go spare!)

Cheers,
Wol

Re: Grub2/raid quickie ...

[NeilBrown]

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On Thu, 30 Oct 2014 00:08:43 +0000 Wols Lists <antlists@youngman.org.uk>
wrote:

> After my last shenanigans, I've now got my main system booting
> successfully from a raid / partition. HOWEVER.
> 
> I'm somewhat puzzled by grub.cfg, and this is in part behind why my dev
> system won't boot raid properly, I think. Grub uses two different UUIDs!?!?
> 
> One of them, (mduuid/...) is dead easy to find, "mdadm -D" gives the md
> uuid of the array. But the other one, the "root=UUID=...", I'm damned if
> I can find that id anywhere! Does anyone know a command, that will tell
> me what that id is, so I can fix grub.cfg on my development system?

blkid /dev/whatever

> 
> (As an aside, how do I retrospectively force a name onto an array so
> that I can guarantee it will be called /dev/md100, or /dev/mdhome, or
> whatever? I daren't use the /dev/mdxxx name at the moment, because they
> change and the result is a system that fails to boot, or is unusable, or
> some other malarkey. Dead easy for me to fix, but my wife will go spare!)

 mdadm --assemble /dev/mdhome --update=name  list-of-devices

You cannot do this without stopping the array, sorry.  Boot off a usb or CD
is necessary.

NeilBrown


> 
> Cheers,
> Wol
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Re: Grub2/raid quickie ...

[Chris Murphy]

On Oct 29, 2014, at 6:08 PM, Wols Lists <antlists@youngman.org.uk> wrote:

> But the other one, the "root=UUID=...", I'm damned if
> I can find that id anywhere! Does anyone know a command, that will tell
> me what that id is, so I can fix grub.cfg on my development system?

blkid

The UUID being referenced by grub-mkconfig is the first UUID reported by blkid, which is the filesystem volume UUID. It's not the same thing as the partition UUID found on GPT disks (a.k.a. unique partition GUID).


> 
> (As an aside, how do I retrospectively force a name onto an array so
> that I can guarantee it will be called /dev/md100, or /dev/mdhome, or
> whatever? I daren't use the /dev/mdxxx name at the moment, because they
> change and the result is a system that fails to boot, or is unusable, or
> some other malarkey. Dead easy for me to fix, but my wife will go spare!)

Pretty sure that goes in mdadm.conf and then you need to rebuild the initramfs since that mdadm.conf gets pulled in and referenced during early boot. Or I'm 100% wrong, and hopefully someone else says what's correct.


Chris Murphy