I set up a btrfs partition using two 3 TB drives in RAID 1:
$ mkfs.btrfs -m raid1 -d raid1 /dev/sdd /dev/sde
$ mount /dev/sdd /media/media
Copied a bunch of data onto the partition, with the result:
$ du -hs /media/media
2.5T /media/media
$ btrfs filesystem show
Label: none uuid: 9c2dd511-33bf-454e-9169-594cd84bee05
Total devices 2 FS bytes used 2.41TB
devid 2 size 2.73TB used 2.41TB path /dev/sde
devid 1 size 2.73TB used 2.41TB path /dev/sdd
## I'm spoofing this a bit as I don't have a log of the response at this point
Next, I added another 3 TB drive and a 4 TB drive:
$ btrfs device add /dev/sdb /dev/sdc /media/media
$ btrfs filesystem balance
Done, had to relocate 2538 out of 2538 chunks
$ btrfs filesystem show
Label: none uuid: 9c2dd511-33bf-454e-9169-594cd84bee05
Total devices 4 FS bytes used 2.41TB
devid 2 size 2.73TB used 945.03GB path /dev/sde
devid 1 size 2.73TB used 944.04GB path /dev/sdd
devid 4 size 2.73TB used 1.24TB path /dev/sdc
devid 3 size 3.64TB used 1.84TB path /dev/sdb
## actual output this time
$ root@leviathan:/media# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 95G 82G 8.7G 91% /
## SNIP
/dev/sdd 12T 4.9T 6.8T 42% /media/media
Now, the output from btrfs fi sh sums to 4.92 TB, double the amount of disk space reported by du -hs, not 4x as I'd expect from a RAID 1 array using four disks. I had planned to convert the RAID1 to RAID10 using btrfs balance start -dconvert=raid10 -mconvert=raid10 /media/media, but the output seems to indicate that it's already behaving as RAID 10. Am I off base here? Do I not need to convert? Or have I screwed something else up?
Additional information, if it matters:
# btrfs version
Btrfs v0.20-rc1
# uname -a
Linux leviathan 3.11.0-18-generic #32-Ubuntu SMP Tue Feb 18 21:11:14 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 13.10
Release: 13.10
Codename: saucy
