Re: Placing the root partition on an XFS filesystem is not supported

From: Dave Chinner <david@fromorbit.com>
To: Eric Sandeen <sandeen@sandeen.net>
Cc: linux-xfs@oss.sgi.com, Marcos Mello <marcosfrm@gmail.com>
Subject: Re: Placing the root partition on an XFS filesystem is not supported
Date: Mon, 6 Feb 2012 11:11:23 +1100	[thread overview]
Message-ID: <20120206001123.GJ6922@dastard> (raw)
In-Reply-To: <4F2C2C69.5020103@sandeen.net>

On Fri, Feb 03, 2012 at 12:50:17PM -0600, Eric Sandeen wrote:
> On 2/3/12 8:39 AM, Marcos Mello wrote:
> > Eric Sandeen <sandeen <at> sandeen.net> writes:
> > 
> >>
> >> In general there is no problem with xfs on a root partition.  However, the
> > installer
> >> may not make it easy or available for you.
> >>
> >> (I never use xfs for /boot though, I don't trust grub enough for that
> > honestly).
> >>
> >> -Eric
> > 
> > Same thing on Fedora 16. Let's hope some day Anaconda will change that.
> 
> F16 prevents it?  I didn't see it in the upstream tree.  That should
> not be so.  :/
> 
> > About GRUB with a XFS /boot the problem was with GRUB Lagacy, wasn't it?
> > Or GRUB2 is still buggy?
> 
> I have no idea, actually.  I delved into grub a bit, it was disturbing
> enough that I have not tried to look at grub2.  :)

Certainly the problem exists with legacy grub - it assumes that it
can write to the first sector or any disk or partition which
overwrites the XFS superblock...

The grub2 manual:

http://www.gnu.org/software/grub/manual/grub.html#BIOS-installation

indicates that if you are using BIOS/MBR based booting, then grub2
still writes to the first sector of the partition that contains the
grub directory to install the stage 1.5 loader.  Indeed:

"boot.img

On PC BIOS systems, this image is the first part of GRUB to start.
It is written to a master boot record (MBR) or to the boot sector of
a partition. Because a PC boot sector is 512 bytes, the size of this
image is exactly 512 bytes. 

The sole function of boot.img is to read the first sector of the
core image from a local disk and jump to it. Because of the size
restriction, boot.img cannot understand any file system structure,
so grub-setup hardcodes the location of the first sector of the core
image into boot.img when installing GRUB. "

IOWs, you have to treat grub2 identically to legacy grub in that it
thinks it owns the first sector of any partition on the disk.
Therefore, you need a separate /boot partition that is not formated
with XFS to be safe.

There's a reason I went back to using LILO....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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