It was a simple:

mkfs.btrfs -L backup -d single /dev/loop2

But it also happens without the options, like:

mkfs.btrfs /dev/loop2


/dev/loop2 is a loop device, which is aes encrypted. The output of "losetup /dev/loop2":

/dev/loop2: [0010]:5324 
(/dev/disk/by-id/ata-WDC_WD6400AAKS-22A7B2_WD-WCASY7780706-part3) encryption=AES128


Thanks you for looking into this!
While writing this I read your second mail. The strace output is attached.

Regards,
Felix

P.S.: The btrfs wiki is down. If someone reads this and have time to answer:

Means "-d single" that the data are not spread across the devices? The default is
raid0, right? I want some kind of security that if a device crashes only the
files on that damaged device are gone. Raid1 will be the next step, but act. I don't
have money for that :)


On 22. January 2011 - 15:11, Hugo Mills wrote:
> Date: Sat, 22 Jan 2011 15:11:24 +0000
> From: Hugo Mills <hugo-lkml@carfax.org.uk>
> To: Felix Blanke <felixblanke@gmail.com>
> Cc: linux-btrfs@vger.kernel.org
> Subject: Re: Bug in mkfs.btrfs?!
> Mail-Followup-To: Hugo Mills <hugo-lkml@carfax.org.uk>, Felix Blanke
>  <felixblanke@gmail.com>, linux-btrfs@vger.kernel.org
> 
> On Sat, Jan 22, 2011 at 03:52:22PM +0100, Felix Blanke wrote:
> > I was able to create the fs with the --force options. But that bug should be fixed,
> > too :)
> > 
> > Is that link maybe too long? It is created by udev, and there was never a problem
> > using that link.
> 
>    Yes, it certainly looks like it. There's 63 characters in the name
> in your strace report, so it looks like it's being truncated somewhere.
> 
>    What was the exact command line you used to get the error?
> 
>    Hugo.
> 
> > Regards,
> > Felix
> > 
> > On 22. January 2011 - 15:45, Felix Blanke wrote:
> > > Date: Sat, 22 Jan 2011 15:45:13 +0100
> > > From: Felix Blanke <felixblanke@gmail.com>
> > > To: linux-btrfs@vger.kernel.org
> > > Subject: Bug in mkfs.btrfs?!
> > > 
> > > Hi,
> > > 
> > > I wanted to create a new btrfs fs for my backups.
> > > When trying to mkfs.btrfs for that  device, I'm getting
> > > 
> > > "error checking /dev/loop2 mount status"
> > > 
> > > 
> > > With strace I see where the problem is:
> > > 
> > > lstat("/dev/disk/by-id/ata-INTEL_SSDSA2M160G2GC_CVPO939201JX160AGN-par",
> > > 0x7fffa30b3cf0) = -1 ENOENT (No such file or directory)
> > > 
> > > 
> > > 
> > > The problem is there is something missing at the end of the link, should be
> > > something like "-part1", "-part2" etc.
> > > 
> > > 
> > > I'll try the patch with the --foce option.
> > > 
> > > Regards,
> > > Felix
> > > 
> > ---end quoted text---
> 
> -- 
> === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
>   PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
>      --- Great oxymorons of the world, no. 9: Standard Deviation ---     


---end quoted text---