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List:       debian-user
Subject:    Re: grub-probe: error: cannot stat `/dev/root'
From:       "Boyd Stephen Smith Jr." <bss () iguanasuicide ! net>
Date:       2011-05-07 14:04:16
Message-ID: 201105070904.26205.bss () iguanasuicide ! net
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In <pan.2011.05.07.10.57.16@gmail.com>, Camaleón wrote:
>On Fri, 06 May 2011 10:44:21 -0700, CACook wrote:
>> This just started after my most recent (very painful) dist-upgrade.  It
>> seems that BTRFS is not compatible with grub and Debian.  I very nearly
>> lost my whole system because of this catastrophe.
>> 
>> Does anyone know the nature of the error and how to fix?
>
>You're asking for so much :-)
>
>First, the btrfs filesystem is still very experimental so only use it if
>you really (I mean *really*) know what you are doing and also know how to
>deal when things go wrong. I would even expect data loss (directly or
>indirectly related to the usage of this filesystem) so make daily backups
>if needed.

As a current btrfs user, I don't recommend you run it on the Squeeze kernel 
(2.6.32-5) because there are certain circumstances where it doesn't gracefully 
handle out-of-space issues and that "df" reports for it lie.  I recently filed 
a bug against the Debian kernel due to this; two BUG() calls within a second 
leading to a broken btrfs module and needing a reboot and some manual file 
system recovery.

I don't recommend you use the Wheezy kernel (2.6.38-2 in the package name; 
2.6.38-3 in the package version) either.  There's a least one outstanding bug 
that I filed involving the in-kernel NULL pointer dereference while mounting a 
btrfs file system.  Again, this leads to a broken btrfs module and needs a 
reboot to fix.  However, it happens easy enough and consistently enough during 
the Debian boot process that it effectively makes that kernel unusable.

I've not tested the Sid kernel; I may do that today.

I thought 2.6.39-rc4 from experimental would work, but they haven't put the 
2.6.39 kbuild infrastructure in experimental and I have an out-of-tree kernel 
module that I need for daily operations.  This is a close second just because 
I don't have any experience with it, yet.

Right now, my data losses due to btrfs, if any, are still unnoticed.  However, 
I am more worried than ever before that I won't find a file system that meets 
my unique needs before I start losing data.  My unique needs are: on-line 
growing, on- or off-line shrinking, good performance on 4+ TiB file systems 
with less than 4 GiB free space.  For the few file systems I haven't converted 
to btrfs, I am still using reiserfs.

FWIW, I don't think GRUB2 has BtrFS support, yet.  So, you'd have to store 
your kernel and initrd on a different file system.
-- 
Boyd Stephen Smith Jr.                   ,= ,-_-. =.
bss@iguanasuicide.net                   ((_/)o o(\_))
ICQ: 514984 YM/AIM: DaTwinkDaddy         `-'(. .)`-'
http://iguanasuicide.net/                    \_/

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