Re: Howto mount intact left-over device of a degraded btrfs-raid?

[Josef Bacik <josef@redhat.com>]

On Mon, Jan 25, 2010 at 10:46:47PM +0100, Mr. Tux wrote:
> On Monday 25 January 2010 19.43:27 Josef Bacik wrote:
> >
> > How did you setup this array to begin with?  I'm trying to reproduce this
> > bug but I haven't been able to.  Thanks,
> >
> 
> Hi Josef,
> 
> Thanks for your help!
> 
> Linux Kernel 2.6.32.2 from kernel.org (was marked as stable at that time,  I 
> compiled it using make-kpkg with btrfs support inside the kernel -> not 
> modular)
> 
> OS: Debian Lenny 5.03
> 
> I created the btrfs raid on a two sata-II drives like this:
> 
> mkfs.btrfs -d raid1 -m raid 1 /dev/sda4 /dev/sdb4
> 
> Then I mounted the btrfs with:
> 
> mount /dev/sda4 /mnt/
> 
> cat /proc/mounts and dmesg showed the raid was mounted successfully
> 
> Then I started to copy large amount of data into the raid1 from a file server. 
> With the copying process ongoing I detached the data cable from one device to 
> simulate hardware failure. Copying process went on as planed to the drive 
> still attached.
> 
> Then I unmounted the btrfs raid like this:
> 
> umount /mnt/
> 
> and I shut down the system.
> 
> Now the goal was to boot and mount only the intact btrfs partition again as a 
> degraded raid.
> 
> I followed the multiple-device-wiki of btrfs [1]
> 
> I didn't try to remove the faulty device - I shut down the system already and 
> booted again without the faulty drive. So I tried this:
> 
> mount -o degraded /dev/sda /mnt/
> 
> This caused a memory segfault - the system still ran, but I couldn't use the 
> array any more.
> 
> So I used the next command
> 
> btrfs-vol -r missing /mnt
> 
> Complained it couldn't mount with only one member present.
> 
> My assumption: Bad luck with the kernel - or did this memory segfault happen 
> on earlier kernel versions as well?
> 
> [1] 
> http://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Replacing_Failed_Devices
> 

Hmm well I'm trying to reproduce that here and it's working fine, course I can't
pull a cable on my disk since I only have one disk to test with.  Would you mind
sending me your dmesg after you try

mount -o degraded /dev/sda1 /mnt/

so I can see whats going on.  Thanks,

Josef