Re: GRUB and the risk of block list corruption in extX

[Martin Wilck <martin.wilck@ts.fujitsu.com>]

On 02/08/2013 07:42 PM, Lennart Sorensen wrote:

> Of course the block list breaks if the file in the filesystem is modified
> or moved without updating the block list, which used to break lilo all the
> time whenever one forgot to run the lilo command after making a change.

True. lilo accessed the conf file itself via block list, IIRC.  The conf
file is typically changed often. GRUB's "core.img", on the contrary,
tends to be pretty static. Moreover, it's usually created using
grub2-install, which runs grub-setup anyway after core.img is rewritten
(thus recreating the block list if necessary).

> Sure grub 0.9x was a bit less fragile than lilo, but block lists for
> files that could potentially be changed is fragile.

"files that could potentially be changed" is important here. As long as
"core.img" itself isn't touched, the risk of the block list being
corrupted is very small; at least that's what the ext4 developers said
on the parallel thread on ext4-devel. The risk can be further reduced
with "chattr +i core.img". Grub 0.9x' stage1_5_* files are 100% static,
and thus practically safe.

> Embedding enough of grub in the first track or a boot partition (as EFI
> systems support, as do a number of non x86 architectures) gives a much
> more reliable system since it can read anything else it needs using the
> filesystem and hence doesn't break if files are changed.

I understand that, but sometimes it's just not enough.

Martin

-- 
Dr. Martin Wilck
PRIMERGY System Software Engineer
x86 Server Engineering

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