Btrfs devel - Nov 2009 - "No filesystem could mount root" after adding a second device to fs

Hi,

after adding a second device to a btrfs filesystem (kernel 2.6.31)
used for root fs, I am getting a "No filesystem could mount root"
message upon reboot. Is the caveat

"btrfsctl -a is used to scan all of the block devices under /dev and
probe for Btrfs volumes. This is required after loading the btrfs
module if you''re running with more than one device in a filesystem.
"

still true? If yes, is there a technical reason for it - am I wrong to
assume that btrfs could execute the scan upon mount request when the
filesystem requires it for assembling?

Regards,

Leszek ''skolima'' Ciesielski
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On Mon, Nov 30, 2009 at 12:41:29PM +0100, Leszek Ciesielski
wrote:
> Hi,
> 
> after adding a second device to a btrfs filesystem (kernel 2.6.31)
> used for root fs, I am getting a "No filesystem could mount root"
> message upon reboot. Is the caveat
> 
> "btrfsctl -a is used to scan all of the block devices under /dev and
> probe for Btrfs volumes. This is required after loading the btrfs
> module if you''re running with more than one device in a
filesystem. "
> 
> still true? If yes, is there a technical reason for it - am I wrong to
> assume that btrfs could execute the scan upon mount request when the
> filesystem requires it for assembling?
> 
The btrfs module needs to have an idea of what devices are available for it to
use.  The btrfsctl -a needs to be done in userspace since it scans /dev for
block devices to probe to see if they are btrfs devices.  To do it in the kernel
would require calling out to userspace to run btrfsctl -a for the uncommon case
that you are booting from a multi-disk setup.  Hopefully as btrfs becomes more
and more common distributions will start putting btrfsctl -a in their
initrd''s
if they detect you have a mutli-disk root.  Thanks,

Josef
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