btrfs raid1 and btrfs raid10 arrays NOT REDUNDANT

Jim Salter
2014-01-03T22:25:53Z

I'm using Ubuntu 12.04.3 with an up-to-date 3.11 kernel, and the
btrfs-progs from Debian Sid (since the ones from Ubuntu are ancient).

I discovered to my horror during testing today that neither raid1 nor
raid10 arrays are fault tolerant of losing an actual disk.

mkfs.btrfs -d raid10 -m raid10 /dev/vdc /dev/vdd /dev/vdd /dev/vde
mkdir /test
mount /dev/vdb /test
echo "test" > /test/test
btrfs filesystem sync /test
shutdown -hP now

After shutting down the VM, I can remove ANY of the drives from the
btrfs raid10 array, and be unable to mount the array. In this case, I
removed the drive that was at /dev/vde, then restarted the VM.

btrfs fi show
Label: none uuid: 94af1f5d-6ad2-4582-ab4a-5410c410c455
Total devices 4 FS bytes used 156.00KB
devid 3 size 1.00GB used 212.75MB path /dev/vdd
devid 3 size 1.00GB used 212.75MB path /dev/vdc
devid 3 size 1.00GB used 232.75MB path /dev/vdb
*** Some devices missing

OK, we have three of four raid10 devices present. Should be fine. Let's
mount it:

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

What's the kernel log got to say about it?

dmesg | tail -n 4
[ 536.694363] device fsid 94af1f5d-6ad2-4582-ab4a-5410c410c455 devid 1
transid 7 /dev/vdb
[ 536.700515] btrfs: disk space caching is enabled
[ 536.703491] btrfs: failed to read the system array on vdd
[ 536.708337] btrfs: open_ctree failed

Same behavior persists whether I create a raid1 or raid10 array, and
whether I create it as that raid level using mkfs.btrfs or convert it
afterwards using btrfs balance start -dconvert=raidn -mconvert=raidn.
Also persists even if I both scrub AND sync the array before shutting
the machine down and removing one of the disks.

What's up with this? This is a MASSIVE bug, and I haven't seen anybody
else talking about it... has nobody tried actually failing out a disk
yet, or what?