I have spent a couple of hours in trying to install Ubuntu 12.04 Beta 2 Server on a RAID-1 btrfs system. The answer of balchmar has helped me a lot, thanks! However, I ended up using a slightly different approach and wanna mention it here for completeness:
Prepare your disk partitioning.
You can use the LiveCD or -as in my case- a previously installed 12.04 Ubuntu server installation. Important: I had to learn that, contrary to what is mentioned in the btrfs wiki, I had to use partitions instead of entire devices like /dev/sda, so that grub can install into the MBR by the Ubuntu Installer. So, here's the partition I used. Note that I use two identical 500GB disks and two partitions each, one for btrfs and one for swap:
Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0004f49a
Device Boot Start End Blocks Id System
/dev/sda1 * 2048 945508351 472753152 83 Linux
/dev/sda2 945508352 976771071 15631360 82 Linux swap / Solaris
Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0002505f
Device Boot Start End Blocks Id System
/dev/sdb1 2048 945508351 472753152 83 Linux
/dev/sdb2 945508352 976771071 15631360 82 Linux swap / Solaris
Start a default Ubuntu 12.04 Server installation until the disk partition part. Don't make any selection there yet.
Press CTRL-ALT-F2 and enter the console. Issue the following commands, which will create a RAID-1 btrfs filesystem over /dev/sda1 and /dev/sdb1, labeling it RAID1:
make.btrfs -L RAID1 -m raid1 -d raid1 /dev/sda1 /dev/sdb1 && btrfs device scan
Press CTRL-ALT-F1 to go back to the installer and choose to manually partition your disk.
You can now choose /dev/sda1 and define the mount point '/'. Important: Do not choose to format the file system as this would delete your previously created RAID-1 one! (you may now also choose to make use of the previously defined swap devices, of course.)
Continue the installation as normal. All data will be now installed on your RAID-1 btrfs!
Not sure this is needed, but some people mention one has to run
btrfs filesystem balance
after the initial boot. You can check the result by issuing the following command:
$ sudo btrfs filesystem show
Label: 'RAID1' uuid: 49d2eaed-d598-462f-a768-e6e0d43dc7ce
Total devices 2 FS bytes used 635.45MB
devid 1 size 450.85GB used 1.26GB path /dev/sda1
devid 2 size 450.85GB used 1.26GB path /dev/sdb1
Btrfs Btrfs v0.19
To see that RAID-1 is actually used for data and metadata, issue
$ sudo btrfs filesystem df /
Data, RAID1: total=1.00GB, used=568.07MB
System, RAID1: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=256.00MB, used=67.37MB