If you decide to use the btrfs data (or/and metadata) replication, you will see the df command reports you your storage get used twice as fast as you expect. Btrfs doesn't act as for example mdadm. Each file will get striped at the block level across all devices available depending on the replication policy selected.
Create a new test file system
root@blackbox:~# mkfs.btrfs -m raid1 -d raid1 -L btrfs01 /dev/sdb /dev/sdc
WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using
adding device /dev/sdc id 2
fs created label btrfs01 on /dev/sdb
nodesize 4096 leafsize 4096 sectorsize 4096 size 298.02GB
Btrfs Btrfs v0.19
root@blackbox:~#
Mount it
root@blackbox:~# mkdir /mnt/btrfs01 root@blackbox:~# mkdir /mnt/btrfs01 root@blackbox:~# mount /dev/sdb /mnt/btrfs01/
Check the capacity available
root@blackbox:~# df -h -F btrfs Filesystem Size Used Avail Use% Mounted on /dev/sdb 299G 56K 296G 1% /mnt/btrfs01 root@blackbox:~#
Create a new file (dd)
root@blackbox:~# dd if=/dev/zero of=/mnt/btrfs01/file.10G bs=1M count=$((10*1024)) 10240+0 records in 10240+0 records out 10737418240 bytes (11 GB) copied, 176.188 s, 60.9 MB/s root@blackbox:~#
Check how much space this file uses on the file system
root@blackbox:~# df -h -F btrfs
Filesystem Size Used Avail Use% Mounted on
/dev/sdb 299G 21G 276G 7% /mnt/btrfs01
root@blackbox:~#
root@blackbox:~# btrfs-show
Label: btrfs01 uuid: d2b77569-a8be-4ec7-862a-d0fc1e6c8511
Total devices 2 FS bytes used 10.01GB
devid 1 size 149.01GB used 12.03GB path /dev/sdb
devid 2 size 149.01GB used 12.01GB path /dev/sdc
Btrfs Btrfs v0.19
root@blackbox:~#
Get the usage explanation from btrfs tool
root@blackbox:~# btrfs filesystem df /mnt/btrfs01/ Data, RAID1: total=11.00GB, used=10.00GB Data: total=8.00MB, used=0.00 System, RAID1: total=8.00MB, used=4.00KB System: total=4.00MB, used=0.00 Metadata, RAID1: total=1.00GB, used=14.11MB Metadata: total=8.00MB, used=0.00 root@blackbox:~#