$ btrfs subvolume list -p /
ID 261 gen 203517 parent 5 top level 5 path __active
ID 262 gen 203540 parent 261 top level 261 path home
ID 263 gen 203540 parent 261 top level 261 path var
ID 265 gen 203517 parent 261 top level 261 path usr
ID 424 gen 201050 parent 5 top level 5 path __snapshot/my_savepoint-2013-10-28-1382970930
ID 427 gen 203508 parent 5 top level 5 path __snapshot/my_savepoint-2013-11-03-1383505769
ID 428 gen 203508 parent 5 top level 5 path __rollback

$ btrfs subvolume get-default /
ID 261 gen 203517 top level 5 path __active

$ cat /etc/fstab
tmpfs       /tmp    tmpfs   nodev,nosuid,size=2G    0   0
UUID=81410cb0-62ba-408b-a5b2-a8a6354fb090 / btrfs defaults,noatime,compress=lzo 0   0 #,subvol=__active
shm   /dev/shm   tmpfs   nodev,nosuid   0   0

$ grub-mkrelpath /boot/initramfs-linux.img
/boot/initramfs-linux.img

$ cat /boot/grub/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
insmod part_gpt
insmod part_msdos
if [ -s $prefix/grubenv ]; then
  load_env
fi
if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   set boot_once=true
else
   set default="0"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
else
  menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
  set saved_entry="${prev_saved_entry}"
  save_env saved_entry
  set prev_saved_entry=
  save_env prev_saved_entry
  set boot_once=true
fi

function savedefault {
  if [ -z "${boot_once}" ]; then
    saved_entry="${chosen}"
    save_env saved_entry
  fi
}

function load_video {
  if [ x$feature_all_video_module = xy ]; then
    insmod all_video
  else
    insmod efi_gop
    insmod efi_uga
    insmod ieee1275_fb
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
  fi
}

if [ x$feature_default_font_path = xy ] ; then
   font=unicode
else
insmod part_gpt
insmod btrfs
set root='hd0,gpt2'
if [ x$feature_platform_search_hint = xy ]; then
  search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  81410cb0-62ba-408b-a5b2-a8a6354fb090
else
  search --no-floppy --fs-uuid --set=root 81410cb0-62ba-408b-a5b2-a8a6354fb090
fi
    font="/usr/share/grub/unicode.pf2"
fi

if loadfont $font ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  set locale_dir=$prefix/locale
  set lang=en_US
  insmod gettext
fi
terminal_input console
terminal_output gfxterm
set timeout=5
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/10_linux ###
menuentry 'Arch Linux, with Linux core repo kernel' --class arch --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-core repo kernel-true-81410cb0-62ba-408b-a5b2-a8a6354fb090' {
    load_video
    set gfxpayload=keep
    insmod gzio
    insmod part_gpt
    insmod btrfs
    set root='hd0,gpt2'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  81410cb0-62ba-408b-a5b2-a8a6354fb090
    else
      search --no-floppy --fs-uuid --set=root 81410cb0-62ba-408b-a5b2-a8a6354fb090
    fi
    echo    'Loading Linux core repo kernel ...'
    linux   /boot/vmlinuz-linux root=UUID=81410cb0-62ba-408b-a5b2-a8a6354fb090 rw
    echo    'Loading initial ramdisk ...'
    initrd  /boot/initramfs-linux.img
}
menuentry 'Arch Linux, with Linux core repo kernel (Fallback initramfs)' --class arch --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-core repo kernel-fallback-81410cb0-62ba-408b-a5b2-a8a6354fb090' {
    load_video
    set gfxpayload=keep
    insmod gzio
    insmod part_gpt
    insmod btrfs
    set root='hd0,gpt2'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  81410cb0-62ba-408b-a5b2-a8a6354fb090
    else
      search --no-floppy --fs-uuid --set=root 81410cb0-62ba-408b-a5b2-a8a6354fb090
    fi
    echo    'Loading Linux core repo kernel ...'
    linux   /boot/vmlinuz-linux root=UUID=81410cb0-62ba-408b-a5b2-a8a6354fb090 rw
    echo    'Loading initial ramdisk ...'
    initrd  /boot/initramfs-linux-fallback.img
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f  ${config_directory}/custom.cfg ]; then
  source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###

### BEGIN /etc/grub.d/60_memtest86+ ###
### END /etc/grub.d/60_memtest86+ ###