localhost ~ # uname -a
Linux localhost 2.6.30-gentoo-r4-btrfs-git-1 #1 SMP Mon Aug 10 21:05:04 BRT 2009 i686 QEMU Virtual CPU version 0.10.50 GenuineIntel GNU/Linux
localhost ~ # mkfs.btrfs /dev/hdb1

WARNING! - Btrfs v0.19-1-g4f89b6e IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

fs created label (null) on /dev/hdb1
        nodesize 4096 leafsize 4096 sectorsize 4096 size 972.65MB
Btrfs v0.19-1-g4f89b6e
localhost ~ # mount /dev/hdb1 /mnt/teste -o compress
mount: you must specify the filesystem type
localhost ~ # modprobe btrfs
localhost ~ # mount /dev/hdb1 /mnt/teste -o compress
localhost ~ # cd /mnt/teste
localhost teste # ls
localhost teste # df -h /dev/hdb1
Filesystem            Size  Used Avail Use% Mounted on
/dev/hdb1             973M   28K  973M   1% /mnt/teste
localhost teste # dd if=/dev/zero of=zero.dd
dd: writing to `zero.dd': No space left on device
1941722+0 records in
1941721+0 records out
994161152 bytes (994 MB) copied, 57.5689 s, 17.3 MB/s
localhost teste # sync
localhost teste # df -h /dev/hdb1
Filesystem            Size  Used Avail Use% Mounted on
/dev/hdb1             973M  788M  186M  81% /mnt/teste
localhost teste # ls -lh
total 949M
-rw-r--r-- 1 root root 949M Aug 11 14:45 zero.dd
localhost teste # btrfs-show 
failed to read /dev/hdc
Label: none  uuid: 79848a50-9c4d-4c95-8c48-c13e97c21a0c
        Total devices 1 FS bytes used 787.21MB
        devid    1 size 972.65MB used 911.25MB path /dev/hdb1

Btrfs v0.19-1-g4f89b6e
localhost teste # rm zero.dd 
localhost teste # dcfldd if=/dev/zero of=zero.dcfldd
719872 blocks (22496Mb) written.
719993+0 records in
719992+0 records out
localhost teste # ls -lh
total 22G
-rw-r--r-- 1 root root 22G Aug 11 14:53 zero.dcfldd
localhost teste # df -h /dev/hdb1
Filesystem            Size  Used Avail Use% Mounted on
/dev/hdb1             973M  737M  237M  76% /mnt/teste
localhost teste # btrfs-show 
failed to read /dev/hdc
Label: none  uuid: 79848a50-9c4d-4c95-8c48-c13e97c21a0c
        Total devices 1 FS bytes used 736.09MB
        devid    1 size 972.65MB used 911.25MB path /dev/hdb1

Btrfs v0.19-1-g4f89b6e
localhost teste # rm zero.dcfldd 
Killed
localhost teste #