notes on switching from Linux md RAID5 to btrfs RAID10

btrfs-raid10-notes.txt

assumptions:
/dev/sd[bcde]1 in Linux md (/dev/md0) RAID5/RAID6 mounted as /storage.
Less space used then capacity of one drive.

1.
stop all services accessing the array first.
Remounting the array ro might be a good idea (mount /dev/md0 -o remount,ro).

2.
remove one disk from array
# mdadm /dev/md0 --fail /dev/sdb1
# mdadm /dev/md0 --remove /dev/sdb1
# mdadm --zero-superblock /dev/sdb1

cat /proc/mdstat should now look like this (note the UUU_):

Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] [linear] [multipath] 
md0 : active raid6 sde1[3] sdd1[1] sdb1[4](F) sdc1[0]
5859110912 blocks super 1.2 level 6, 512k chunk, algorithm 2 [4/3] [UUU_]
bitmap: 0/22 pages [0KB], 65536KB chunk

unused devices: <none>

3.
create new btrfs volume on the removed drive:

# mkfs.btrfs /dev/sdb1

4.
copy over files (make sure a single drive has enough space)

# mount /dev/sdb1 /storage.new
# cp -a /storage/* /storage.new

check consistency

5-
dismantle old array:

# umount /storage
# mdadm --stop /dev/md0
# mdadm --zero-superblock /dev/sdc1
# mdadm --zero-superblock /dev/sdd1
# mdadm --zero-superblock /dev/sde1

6.
add drives to btrfs volume:
# btrfs device add /dev/sd[cde]1 /storage.new

7.
# remount new array in old location
umount /storage.new
#mount /dev/sdb1 /storage

8.
# switch btrfs profile for data and metadata to raid10
btrfs balance start -dconvert=raid10 -mconvert=raid10 /storage

check progress with (or  btrfs balance status /storage)
# btrfs fi show
Label: 'root'  uuid: 4967ac6e-9073-41f1-b1eb-a6f6e052da7a
        Total devices 1 FS bytes used 25.06GiB
        devid    1 size 38.87GiB used 38.87GiB path /dev/sda4

Label: 'storage'  uuid: a48e0409-d677-4fbb-9a80-4c599d30b4ac
        Total devices 4 FS bytes used 2.26TiB
        devid    1 size 2.73TiB used 2.27TiB path /dev/sdb1
        devid    2 size 2.73TiB used 44.50GiB path /dev/sdc1
        devid    3 size 2.73TiB used 44.50GiB path /dev/sdd1
        devid    4 size 2.73TiB used 44.50GiB path /dev/sde1

9.
set label
# btrfs fi label /storage storage

10.
fix /etc/fstab, use UUID= to mount (you can check UUIDs with # blkid)
remove mdadm from autostart