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

From: Eric Sandeen <sandeen@sandeen.net>
To: Dave Chinner <david@fromorbit.com>
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: Sun, 05 Feb 2012 18:13:28 -0600	[thread overview]
Message-ID: <4F2F1B28.3060308@sandeen.net> (raw)
In-Reply-To: <20120206001123.GJ6922@dastard>

On 2/5/12 6:11 PM, Dave Chinner wrote:
> 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...

well, it was worse than that.  I can work around the grub-on-a-partition
problem, but what I ran into was grub reading & writing to/from the block
device under a mounted filesystem - corruption and hilarity ensued.

-Eric

> 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.

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