krzf83@gmail.com wrote:

# uname -a
Linux dhcppc1 3.0.1-xxxx-std-ipv6-64 #1 SMP Sun Aug 14 17:06:21 CEST
2011 x86_64 x86_64 x86_64 GNU/Linux

mkdir test5
cd test5
dd if=/dev/null of=img5 bs=1 seek=2G
dd if=/dev/null of=img6 bs=1 seek=2G
losetup /dev/loop2 img5
losetup /dev/loop3 img6
mkfs.btrfs -d raid1 -m raid1 /dev/loop2 /dev/loop3
btrfs device scan
btrfs filesystem show

Label: none  uuid: d7ba6c4e-04ed-49f5-88cd-8432c948e822
        Total devices 2 FS bytes used 28.00KB
        devid    1 size 2.00GB used 437.50MB path /dev/loop4
        devid    2 size 2.00GB used 417.50MB path /dev/loop5

mkdir dir
mount -t btrfs /dev/loop2 dir
umount dir
losetup -d /dev/loop3
mount -t btrfs -o degraded /dev/loop2 dir

mount: wrong fs type, bad option, bad superblock on /dev/loop2,
       missing codepage or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

It works on latest kernel (3.1.0-rc2)

--
Li Zefan