diff --git a/ChangeLog b/ChangeLog
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+v0.1.1.1
+
+	* Support getting/setting the id of the default subvolume.
+
 v0.1.1.0
 
 	* Support defraging file ranges.
diff --git a/System/Linux/Btrfs.hsc b/System/Linux/Btrfs.hsc
--- a/System/Linux/Btrfs.hsc
+++ b/System/Linux/Btrfs.hsc
@@ -46,6 +46,8 @@
     , getSubvolInfoFd, getSubvolInfo
     , getSubvolByUuidFd, getSubvolByUuid
     , getSubvolByReceivedUuidFd, getSubvolByReceivedUuid
+    , getDefaultSubvolFd, getDefaultSubvol
+    , setDefaultSubvolFd, setDefaultSubvol
     -- * Defragging
     , defragFd, defrag
     , DefragRangeArgs(..), defaultDefragRangeArgs
@@ -614,7 +616,7 @@
             , siRTime = nothingIf (nv2 || rTransId == 0) rTime
             }
 
--- | Retrieve information about a subvolume.
+-- | Retrieve information about a subvolume. This is a wrapper around 'treeSearch'.
 getSubvolInfo
     :: FILEPATH -- ^ The mount point of the volume (or any file in that volume).
     -> SubvolId -- ^ The id of the subvolume.
@@ -646,7 +648,8 @@
 getSubvolByUuidFd =
     searchByUuidFd (#const BTRFS_UUID_KEY_SUBVOL)
 
--- | Find the id of a subvolume, given its UUID.
+-- | Find the id of a subvolume, given its UUID. This is a wrapper around
+-- 'treeSearch'.
 --
 -- /Requires Linux 3.12 or later./
 getSubvolByUuid
@@ -661,7 +664,8 @@
 getSubvolByReceivedUuidFd =
     searchByUuidFd (#const BTRFS_UUID_KEY_RECEIVED_SUBVOL)
 
--- | Find the id of a subvolume, given its 'siReceivedUuid'.
+-- | Find the id of a subvolume, given its 'siReceivedUuid'. This is a
+-- wrapper around 'treeSearch'.
 --
 -- /Requires Linux 3.12 or later./
 getSubvolByReceivedUuid
@@ -672,6 +676,53 @@
     withFd path ReadOnly $ \fd ->
         getSubvolByReceivedUuidFd fd uuid
 
+getDefaultSubvolFd :: Fd -> IO SubvolId
+getDefaultSubvolFd fd = do
+    let sk = defaultSearchKey
+            { skTreeId      = (#const BTRFS_ROOT_TREE_OBJECTID)
+            , skMinObjectId = (#const BTRFS_ROOT_TREE_DIR_OBJECTID)
+            , skMaxObjectId = (#const BTRFS_ROOT_TREE_DIR_OBJECTID)
+            , skMinType     = (#const BTRFS_DIR_ITEM_KEY)
+            , skMaxType     = (#const BTRFS_DIR_ITEM_KEY)
+            }
+    l <- treeSearchListFd fd sk $ \_ ptr -> do
+        LE16 nameLen <- (#peek struct btrfs_dir_item, name_len) ptr
+        let cName = ptr `plusPtr` (#size struct btrfs_dir_item)
+        name <- peekCStringLen (cName, fromIntegral nameLen)
+        if name /= "default" then
+            return Nothing
+        else do
+            let location = ptr `plusPtr` (#offset struct btrfs_dir_item, location)
+            LE64 objectId <- (#peek struct btrfs_disk_key, objectid) location
+            return (Just objectId)
+    case l of
+        [] -> ioError $ mkIOError doesNotExistErrorType "getDefaultSubvolFd" Nothing Nothing
+        (objectId : _) -> return objectId
+
+-- | Find the id of the default subvolume. This is a wrapper around
+-- 'treeSearch'.
+getDefaultSubvol
+    :: FILEPATH -- ^ The mount point of the volume (or any file in that volume).
+    -> IO SubvolId
+getDefaultSubvol path = withFd path ReadOnly getDefaultSubvolFd
+
+setDefaultSubvolFd :: Fd -> ObjectId -> IO ()
+setDefaultSubvolFd fd objectId = do
+    alloca $ \ptr -> do
+        poke ptr objectId
+        throwErrnoIfMinus1_ "setDefaultSubvolFd" $
+            ioctl fd (#const BTRFS_IOC_DEFAULT_SUBVOL) ptr
+
+-- | Set the default subvolume.
+--
+-- Note: calls the @BTRFS_IOC_DEFAULT_SUBVOL@ @ioctl@.
+setDefaultSubvol
+    :: FILEPATH -- ^ The mount point of the volume (or any file in that volume).
+    -> SubvolId -- ^ The id of the new default subvolume.
+    -> IO ()
+setDefaultSubvol path subvolId =
+    withFd path ReadOnly $ \fd -> setDefaultSubvolFd fd subvolId
+
 --------------------------------------------------------------------------------
 
 defragFd :: Fd -> IO ()
@@ -693,14 +744,14 @@
     , draLength :: FileSize
         -- ^ Number of bytes to defrag, use 'maxBound' to say all.
     , draExtentThreshold :: Word32
-        -- ^ Any extent bigger than this size will be considered already
-        -- defragged. Use 0 to take the kernel default, use 1 to say every
-        -- single extent must be rewritten.
+        -- ^ Any extent of size bigger or equal to this number will be
+        -- considered already defragged. Use 0 for the kernel default.
     , draCompress :: Maybe CompressionType
         -- ^ Compress the file while defragmenting.
     , draFlush :: Bool
         -- ^ Flush data to disk immediately after defragmenting.
     }
+  deriving (Show, Eq)
 
 -- | Defaults for 'defragRange'. Selects the entire file, no compression,
 -- and no flushing.
diff --git a/System/Linux/Btrfs/ByteString.hsc b/System/Linux/Btrfs/ByteString.hsc
--- a/System/Linux/Btrfs/ByteString.hsc
+++ b/System/Linux/Btrfs/ByteString.hsc
@@ -46,6 +46,8 @@
     , getSubvolInfoFd, getSubvolInfo
     , getSubvolByUuidFd, getSubvolByUuid
     , getSubvolByReceivedUuidFd, getSubvolByReceivedUuid
+    , getDefaultSubvolFd, getDefaultSubvol
+    , setDefaultSubvolFd, setDefaultSubvol
     -- * Defragging
     , defragFd, defrag
     , DefragRangeArgs(..), defaultDefragRangeArgs
@@ -614,7 +616,7 @@
             , siRTime = nothingIf (nv2 || rTransId == 0) rTime
             }
 
--- | Retrieve information about a subvolume.
+-- | Retrieve information about a subvolume. This is a wrapper around 'treeSearch'.
 getSubvolInfo
     :: FILEPATH -- ^ The mount point of the volume (or any file in that volume).
     -> SubvolId -- ^ The id of the subvolume.
@@ -646,7 +648,8 @@
 getSubvolByUuidFd =
     searchByUuidFd (#const BTRFS_UUID_KEY_SUBVOL)
 
--- | Find the id of a subvolume, given its UUID.
+-- | Find the id of a subvolume, given its UUID. This is a wrapper around
+-- 'treeSearch'.
 --
 -- /Requires Linux 3.12 or later./
 getSubvolByUuid
@@ -661,7 +664,8 @@
 getSubvolByReceivedUuidFd =
     searchByUuidFd (#const BTRFS_UUID_KEY_RECEIVED_SUBVOL)
 
--- | Find the id of a subvolume, given its 'siReceivedUuid'.
+-- | Find the id of a subvolume, given its 'siReceivedUuid'. This is a
+-- wrapper around 'treeSearch'.
 --
 -- /Requires Linux 3.12 or later./
 getSubvolByReceivedUuid
@@ -672,6 +676,53 @@
     withFd path ReadOnly $ \fd ->
         getSubvolByReceivedUuidFd fd uuid
 
+getDefaultSubvolFd :: Fd -> IO SubvolId
+getDefaultSubvolFd fd = do
+    let sk = defaultSearchKey
+            { skTreeId      = (#const BTRFS_ROOT_TREE_OBJECTID)
+            , skMinObjectId = (#const BTRFS_ROOT_TREE_DIR_OBJECTID)
+            , skMaxObjectId = (#const BTRFS_ROOT_TREE_DIR_OBJECTID)
+            , skMinType     = (#const BTRFS_DIR_ITEM_KEY)
+            , skMaxType     = (#const BTRFS_DIR_ITEM_KEY)
+            }
+    l <- treeSearchListFd fd sk $ \_ ptr -> do
+        LE16 nameLen <- (#peek struct btrfs_dir_item, name_len) ptr
+        let cName = ptr `plusPtr` (#size struct btrfs_dir_item)
+        name <- peekCStringLen (cName, fromIntegral nameLen)
+        if name /= "default" then
+            return Nothing
+        else do
+            let location = ptr `plusPtr` (#offset struct btrfs_dir_item, location)
+            LE64 objectId <- (#peek struct btrfs_disk_key, objectid) location
+            return (Just objectId)
+    case l of
+        [] -> ioError $ mkIOError doesNotExistErrorType "getDefaultSubvolFd" Nothing Nothing
+        (objectId : _) -> return objectId
+
+-- | Find the id of the default subvolume. This is a wrapper around
+-- 'treeSearch'.
+getDefaultSubvol
+    :: FILEPATH -- ^ The mount point of the volume (or any file in that volume).
+    -> IO SubvolId
+getDefaultSubvol path = withFd path ReadOnly getDefaultSubvolFd
+
+setDefaultSubvolFd :: Fd -> ObjectId -> IO ()
+setDefaultSubvolFd fd objectId = do
+    alloca $ \ptr -> do
+        poke ptr objectId
+        throwErrnoIfMinus1_ "setDefaultSubvolFd" $
+            ioctl fd (#const BTRFS_IOC_DEFAULT_SUBVOL) ptr
+
+-- | Set the default subvolume.
+--
+-- Note: calls the @BTRFS_IOC_DEFAULT_SUBVOL@ @ioctl@.
+setDefaultSubvol
+    :: FILEPATH -- ^ The mount point of the volume (or any file in that volume).
+    -> SubvolId -- ^ The id of the new default subvolume.
+    -> IO ()
+setDefaultSubvol path subvolId =
+    withFd path ReadOnly $ \fd -> setDefaultSubvolFd fd subvolId
+
 --------------------------------------------------------------------------------
 
 defragFd :: Fd -> IO ()
@@ -693,14 +744,14 @@
     , draLength :: FileSize
         -- ^ Number of bytes to defrag, use 'maxBound' to say all.
     , draExtentThreshold :: Word32
-        -- ^ Any extent bigger than this size will be considered already
-        -- defragged. Use 0 to take the kernel default, use 1 to say every
-        -- single extent must be rewritten.
+        -- ^ Any extent of size bigger or equal to this number will be
+        -- considered already defragged. Use 0 for the kernel default.
     , draCompress :: Maybe CompressionType
         -- ^ Compress the file while defragmenting.
     , draFlush :: Bool
         -- ^ Flush data to disk immediately after defragmenting.
     }
+  deriving (Show, Eq)
 
 -- | Defaults for 'defragRange'. Selects the entire file, no compression,
 -- and no flushing.
diff --git a/btrfs.cabal b/btrfs.cabal
--- a/btrfs.cabal
+++ b/btrfs.cabal
@@ -1,5 +1,5 @@
 name:                btrfs
-version:             0.1.1.0
+version:             0.1.1.1
 synopsis:            Bindings to the btrfs API
 description:
   This package provides low-level bindings to the btrfs API (i.e. the