I asked a similar question 2 years ago.
However in my case, I was only planning to copy a single device onto raid0.
I eventually found a solution. At the time you couldn't convert from raid0 to raid10, but it looks like that since kernel 3.3, you can now. So that solution may work for you in the end.
A problem with that approach is that it copies the fsuid. Which means you can't mount both the FS and its copy on the same machine. At the time, there was no tool to change the
fsuid of a FS, but it might have changed now.
The idea is to add a copy-on-write layer on top of the original device so that it can be written to, but any modification is done somewhere else which you can discard later on. That means you need additional storage space (for instance on an external drive).
Then mount that COW'd FS instead of the original, add the devices for the FS copy and remove the COW's device.
For copy-on-write, you can use the device mapper.
For the disposable copy on write area, here I use a loop device.
Let's say you want to clone
Create the COW back store:
truncate -s 100G /media/STORE/snap-store
losetup /dev/loop0 /media/STORE/snap-store
Now unmount the origin FS if mounted and
modprobe -r btrfs to make sure it's not going to interfere and make it forget its device scan.
Then make the COW'd device:
echo "echo 0 $(blockdev --getsize /dev/sda) snapshot /dev/sda /dev/loop0 N 8 | dmsetup create cowed
/dev/mapper/cowed is like
/dev/sda except that anything written to it will end up in
/dev/sda will be untouched.
Now, you can mount it:
mount /dev/mapper/cowed /mnt
Add the other devices:
btrfs dev add /dev/sd[bcde] /mnt
And remove the old one:
btrfs dev del /dev/mapper/cowed /mnt
When that's over, you may want to shutdown and unplug or make
/dev/sda readonly as because it's got the same fsuid as the other ones,
btrfs might still mess up with it.
Now, if I understand correctly, assuming you've got recent btrfs-prog, you should be able to do a:
btrfs balance start -d convert=raid10 /mnt
To convert to raid10. In theory, that should make sure that every data chunk is copied on a least 2 disks.
I would strongly recommend that you do tests on a dummy btrfs on loop devices first as all that is from memory and I might have gotten it wrong (see for instance my initial answer before my edit).
Note that since kernel 3.6, btrfs implements send/receive a bit like in zfs. That might be an option for you.