Title: how does GRUB read from /boot on software-RAID partition?









Some articles about GRUB and md-based RAID1 (mirroring) seem to imply that GRUB

can read files (including the kernel and the initrd file) from /boot on the

filesystem on a mirrored partition.



Since GRUB hasn't loaded the kernel file yet, GRUB can't be using the kernel

and its md driver, and therefore can't be reading the partition _as_a_RAID_

_volume_ (/dev/mdX), right?





So is GRUB just reading the partition directly to get to the file system?



Specifically, is GRUB taking advantage of the fact that the RAID metadata is

written at the end of a partition that is a component of a RAID volume (and

that a file system doesn't care if the block device it's in actually contains

more blocks than the filesystem knows about)?





If so, how reliable is that?



Should one put /boot on a plain, non-RAID partition on one disk and somehow

(manually or automatically) maintain a backup /boot partition on the second

disk, or is it fine to put /boot on a mirrored partition (so maintaining

redundancy is automatic) and let GRUB read the partition directly?





Thanks,

Daniel

--

(Plain text sometimes corrupted to HTML "courtesy" of Microsoft Exchange.) [F]

Barclay, Daniel wrote:
> Some articles about GRUB and md-based RAID1 (mirroring) seem to imply
> that GRUB
>
> can read files (including the kernel and the initrd file) from /boot
> on the
> filesystem on a mirrored partition.
>

I never tried, but I've seen several reports that it works.

>
> Since GRUB hasn't loaded the kernel file yet, GRUB can't be using the
> kernel
> and its md driver, and therefore can't be reading the partition
> _as_a_RAID_
> _volume_ (/dev/mdX), right?
>
>
> So is GRUB just reading the partition directly to get to the file system?
>

GRUB does not know anything about RAID, so I assume this is true.

> Specifically, is GRUB taking advantage of the fact that the RAID
> metadata is
> written at the end of a partition that is a component of a RAID volume
> (and
> that a file system doesn't care if the block device it's in actually
> contains
> more blocks than the filesystem knows about)?
>

I'd suppose so.

> If so, how reliable is that?
>
> Should one put /boot on a plain, non-RAID partition on one disk and
> somehow
> (manually or automatically) maintain a backup /boot partition on the
> second
> disk, or is it fine to put /boot on a mirrored partition (so maintaining
> redundancy is automatic) and let GRUB read the partition directly?
>

Again, while I haven't tried, I've seen several reports that this works.
(Don't forget to install GRUB on both disks, should one fail.) So why
make things more complicated and not automatic?

--
You know what they say -- the sweetest word in the English language is revenge.
-- Peter Beard

Eduardo M KALINOWSKI
eduardo@kalinowski.com.br
http://move.to/hpkb


--
To UNSUBSCRIBE, email to debian-user-REQUEST@lists.debian.org
with a subject of "unsubscribe". Trouble? Contact listmaster@lists.debian.org

Title: Re: how does GRUB read from /boot on software-RAID partition?







Eduardo M KALINOWSKI wrote:

> Barclay, Daniel wrote:

...

>> Since GRUB hasn't loaded the kernel file yet, GRUB can't be using the kernel

>> and its md driver, and therefore can't be reading the partition _as_a_RAID_

>> _volume_ (/dev/mdX), right?

>>

>>

>> So is GRUB just reading the partition directly to get to the file system?

>>

>

> GRUB does not know anything about RAID, so I assume this is true.



That's what I have been thinking, but I just found the message at

http://www.mail-archive.com/linux-raid@vger.kernel.org/msg09712.html

that says::



*** ... once grub2 has determined that the intended boot partition is

*** a raid partition, the raid code takes over ... and it scans for all

*** the other members of the raid array and utilizes whichever drives it

*** needs to in order to complete the boot process.* ... [i]t doesn't

*** need any member of a raid1 array to be perfect[;] it will attempt a

*** round robin read on all the sectors and only fail if all drives return

*** an error for a given read.



Is that _just_ for GRUB2 and or does the current GRUB (0.97) in Lenny

also do that?







>> ... is GRUB taking advantage of the fact that the RAID metadata is

>> written at the end of a partition ...

...

>> If so, how reliable is that?

>>

>> Should one put /boot on a plain, non-RAID partition on one disk and

>> somehow (manually or automatically) maintain a backup /boot partition on

*>> the second disk, or is it fine to put /boot on a mirrored partition (so

*>> maintaining redundancy is automatic) and let GRUB read the partition directly?

>>

>

> Again, while I haven't tried, I've seen several reports that this works.

> ... So why make things more complicated and not automatic?



I don't get why you're asking that.* I _am_ trying to avoid the complicated

and non-automatic solution (trying to check whether the simpler solution is

reliable).







Daniel

--

(Plain text sometimes corrupted to HTML "courtesy" of Microsoft Exchange.) [F]