diff --git a/btrfs.go b/btrfs.go
index a99204b..4477f86 100644
--- a/btrfs.go
+++ b/btrfs.go
@@ -1,7 +1,6 @@
 package btrfs
 
 import (
-	"bytes"
 	"fmt"
 	"github.com/dennwc/btrfs/ioctl"
 	"io"
@@ -12,8 +11,6 @@ import (
 
 const SuperMagic = 0x9123683E
 
-const xattrPrefix = "btrfs."
-
 func Open(path string, ro bool) (*FS, error) {
 	if ok, err := IsSubVolume(path); err != nil {
 		return nil, err
@@ -60,16 +57,16 @@ type Info struct {
 
 func (f *FS) Info() (out Info, err error) {
 	var arg btrfs_ioctl_fs_info_args
-	if err = ioctl.Do(f.f, _BTRFS_IOC_FS_INFO, &arg); err != nil {
-		return
-	}
-	out = Info{
-		MaxID:          arg.max_id,
-		NumDevices:     arg.num_devices,
-		FSID:           arg.fsid,
-		NodeSize:       arg.nodesize,
-		SectorSize:     arg.sectorsize,
-		CloneAlignment: arg.clone_alignment,
+	arg, err = iocFsInfo(f.f)
+	if err == nil {
+		out = Info{
+			MaxID:          arg.max_id,
+			NumDevices:     arg.num_devices,
+			FSID:           arg.fsid,
+			NodeSize:       arg.nodesize,
+			SectorSize:     arg.sectorsize,
+			CloneAlignment: arg.clone_alignment,
+		}
 	}
 	return
 }
@@ -160,10 +157,10 @@ func (f *FS) SetFlags(flags SubvolFlags) error {
 }
 
 func (f *FS) Sync() (err error) {
-	if err = ioctl.Do(f.f, _BTRFS_IOC_START_SYNC, nil); err != nil {
+	if err = ioctl.Ioctl(f.f, _BTRFS_IOC_START_SYNC, 0); err != nil {
 		return
 	}
-	return ioctl.Do(f.f, _BTRFS_IOC_WAIT_SYNC, nil)
+	return ioctl.Ioctl(f.f, _BTRFS_IOC_WAIT_SYNC, 0)
 }
 
 func (f *FS) CreateSubVolume(name string) error {
@@ -221,58 +218,4 @@ func (f *FS) ListSubvolumes(filter func(Subvolume) bool) ([]Subvolume, error) {
 	return out, nil
 }
 
-type Compression string
-
-const (
-	CompressionNone = Compression("")
-	LZO             = Compression("lzo")
-	ZLIB            = Compression("zlib")
-)
-
-const xattrCompression = xattrPrefix + "compression"
-
-func SetCompression(path string, v Compression) error {
-	var value []byte
-	if v != CompressionNone {
-		var err error
-		value, err = syscall.ByteSliceFromString(string(v))
-		if err != nil {
-			return err
-		}
-	}
-	err := syscall.Setxattr(path, xattrCompression, value, 0)
-	if err != nil {
-		return &os.PathError{Op: "setxattr", Path: path, Err: err}
-	}
-	return nil
-}
-
-func GetCompression(path string) (Compression, error) {
-	var buf []byte
-	for {
-		sz, err := syscall.Getxattr(path, xattrCompression, nil)
-		if err == syscall.ENODATA || sz == 0 {
-			return CompressionNone, nil
-		} else if err != nil {
-			return CompressionNone, &os.PathError{Op: "getxattr", Path: path, Err: err}
-		}
-		if cap(buf) < sz {
-			buf = make([]byte, sz)
-		} else {
-			buf = buf[:sz]
-		}
-		sz, err = syscall.Getxattr(path, xattrCompression, buf)
-		if err == syscall.ENODATA {
-			return CompressionNone, nil
-		} else if err == syscall.ERANGE {
-			// xattr changed by someone else, and is larger than our current buffer
-			continue
-		} else if err != nil {
-			return CompressionNone, &os.PathError{Op: "getxattr", Path: path, Err: err}
-		}
-		buf = buf[:sz]
-		break
-	}
-	buf = bytes.TrimSuffix(buf, []byte{0})
-	return Compression(buf), nil
-}
+func (f *FS) Usage() (UsageInfo, error) { return spaceUsage(f.f) }
diff --git a/btrfs_tree.go b/btrfs_tree.go
index a7e6f6b..5d0ebca 100644
--- a/btrfs_tree.go
+++ b/btrfs_tree.go
@@ -6,6 +6,9 @@ import (
 )
 
 const (
+	_BTRFS_BLOCK_GROUP_TYPE_MASK = (blockGroupData |
+		blockGroupSystem |
+		blockGroupMetadata)
 	_BTRFS_BLOCK_GROUP_PROFILE_MASK = (blockGroupRaid0 |
 		blockGroupRaid1 |
 		blockGroupRaid5 |
diff --git a/ioctl_h.go b/ioctl_h.go
index 8a8ef98..0691e89 100644
--- a/ioctl_h.go
+++ b/ioctl_h.go
@@ -678,8 +678,49 @@ func iocDefaultSubvol(f *os.File, out *uint64) error {
 	return ioctl.Do(f, _BTRFS_IOC_DEFAULT_SUBVOL, out)
 }
 
-func iocSpaceInfo(f *os.File, out *btrfs_ioctl_space_args) error {
-	return ioctl.Do(f, _BTRFS_IOC_SPACE_INFO, out)
+type spaceInfo struct {
+	Flags      uint64
+	TotalBytes uint64
+	UsedBytes  uint64
+}
+
+func iocSpaceInfo(f *os.File) ([]spaceInfo, error) {
+	arg := &btrfs_ioctl_space_args{}
+	if err := ioctl.Do(f, _BTRFS_IOC_SPACE_INFO, arg); err != nil {
+		return nil, err
+	}
+	n := arg.total_spaces
+	if n == 0 {
+		return nil, nil
+	}
+	const (
+		argSize  = unsafe.Sizeof(btrfs_ioctl_space_args{})
+		infoSize = unsafe.Sizeof(btrfs_ioctl_space_info{})
+	)
+	buf := make([]byte, argSize+uintptr(n)*infoSize)
+	basePtr := unsafe.Pointer(&buf[0])
+	arg = (*btrfs_ioctl_space_args)(basePtr)
+	arg.space_slots = n
+	if err := ioctl.Do(f, _BTRFS_IOC_SPACE_INFO, arg); err != nil {
+		return nil, err
+	} else if arg.total_spaces == 0 {
+		return nil, nil
+	}
+	if n > arg.total_spaces {
+		n = arg.total_spaces
+	}
+	out := make([]spaceInfo, n)
+	ptr := uintptr(basePtr) + argSize
+	for i := 0; i < int(n); i++ {
+		info := (*btrfs_ioctl_space_info)(unsafe.Pointer(ptr))
+		out[i] = spaceInfo{
+			Flags:      info.flags,
+			TotalBytes: info.total_bytes,
+			UsedBytes:  info.used_bytes,
+		}
+		ptr += infoSize
+	}
+	return out, nil
 }
 
 func iocStartSync(f *os.File, out *uint64) error {
@@ -712,8 +753,16 @@ func iocScrubProgress(f *os.File, out *btrfs_ioctl_scrub_args) error {
 	return ioctl.Do(f, _BTRFS_IOC_SCRUB_PROGRESS, out)
 }
 
-func iocDevInfo(f *os.File, out *btrfs_ioctl_dev_info_args) error {
-	return ioctl.Do(f, _BTRFS_IOC_DEV_INFO, out)
+func iocFsInfo(f *os.File) (out btrfs_ioctl_fs_info_args, err error) {
+	err = ioctl.Do(f, _BTRFS_IOC_FS_INFO, &out)
+	return
+}
+
+func iocDevInfo(f *os.File, devid uint64, uuid UUID) (out btrfs_ioctl_dev_info_args, err error) {
+	out.devid = devid
+	out.uuid = uuid
+	err = ioctl.Do(f, _BTRFS_IOC_DEV_INFO, &out)
+	return
 }
 
 func iocBalanceCtl(f *os.File, out *int32) error {
diff --git a/usage.go b/usage.go
new file mode 100644
index 0000000..6f10f6a
--- /dev/null
+++ b/usage.go
@@ -0,0 +1,175 @@
+package btrfs
+
+import (
+	"os"
+	"sort"
+	"syscall"
+)
+
+func cmpChunkBlockGroup(f1, f2 uint64) int {
+	var mask uint64
+
+	if (f1 & _BTRFS_BLOCK_GROUP_TYPE_MASK) ==
+		(f2 & _BTRFS_BLOCK_GROUP_TYPE_MASK) {
+		mask = _BTRFS_BLOCK_GROUP_PROFILE_MASK
+	} else if f2&blockGroupSystem != 0 {
+		return -1
+	} else if f1&blockGroupSystem != 0 {
+		return +1
+	} else {
+		mask = _BTRFS_BLOCK_GROUP_TYPE_MASK
+	}
+
+	if (f1 & mask) > (f2 & mask) {
+		return +1
+	} else if (f1 & mask) < (f2 & mask) {
+		return -1
+	} else {
+		return 0
+	}
+}
+
+type spaceInfoByBlockGroup []spaceInfo
+
+func (a spaceInfoByBlockGroup) Len() int      { return len(a) }
+func (a spaceInfoByBlockGroup) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
+func (a spaceInfoByBlockGroup) Less(i, j int) bool {
+	return cmpChunkBlockGroup(a[i].Flags, a[j].Flags) < 0
+}
+
+type UsageInfo struct {
+	Total       uint64
+	TotalUnused uint64
+	TotalUsed   uint64
+	TotalChunks uint64
+
+	FreeEstimated uint64
+	FreeMin       uint64
+
+	LogicalDataChunks uint64
+	RawDataChunks     uint64
+	RawDataUsed       uint64
+
+	LogicalMetaChunks uint64
+	RawMetaChunks     uint64
+	RawMetaUsed       uint64
+
+	SystemUsed   uint64
+	SystemChunks uint64
+
+	DataRatio     float64
+	MetadataRatio float64
+
+	GlobalReserve     uint64
+	GlobalReserveUsed uint64
+}
+
+const minUnallocatedThreshold = 16 * 1024 * 1024
+
+func spaceUsage(f *os.File) (UsageInfo, error) {
+	info, err := iocFsInfo(f)
+	if err != nil {
+		return UsageInfo{}, err
+	}
+	var u UsageInfo
+	for i := uint64(0); i <= info.max_id; i++ {
+		dev, err := iocDevInfo(f, i, UUID{})
+		if err == syscall.ENODEV {
+			continue
+		} else if err != nil {
+			return UsageInfo{}, err
+		}
+		u.Total += dev.total_bytes
+	}
+
+	spaces, err := iocSpaceInfo(f)
+	if err != nil {
+		return UsageInfo{}, err
+	}
+	sort.Sort(spaceInfoByBlockGroup(spaces))
+	var (
+		maxDataRatio int = 1
+		mixed        bool
+	)
+	for _, s := range spaces {
+		ratio := 1
+		switch {
+		case s.Flags&blockGroupRaid0 != 0:
+			ratio = 1
+		case s.Flags&blockGroupRaid1 != 0:
+			ratio = 2
+		case s.Flags&blockGroupRaid5 != 0:
+			ratio = 0
+		case s.Flags&blockGroupRaid6 != 0:
+			ratio = 0
+		case s.Flags&blockGroupDup != 0:
+			ratio = 2
+		case s.Flags&blockGroupRaid10 != 0:
+			ratio = 2
+		}
+		if ratio > maxDataRatio {
+			maxDataRatio = ratio
+		}
+		if s.Flags&spaceInfoGlobalRsv != 0 {
+			u.GlobalReserve = s.TotalBytes
+			u.GlobalReserveUsed = s.UsedBytes
+		}
+		if s.Flags&(blockGroupData|blockGroupMetadata) == (blockGroupData | blockGroupMetadata) {
+			mixed = true
+		}
+		if s.Flags&blockGroupData != 0 {
+			u.RawDataUsed += s.UsedBytes * uint64(ratio)
+			u.RawDataChunks += s.TotalBytes * uint64(ratio)
+			u.LogicalDataChunks += s.TotalBytes
+		}
+		if s.Flags&blockGroupMetadata != 0 {
+			u.RawMetaUsed += s.UsedBytes * uint64(ratio)
+			u.RawMetaChunks += s.TotalBytes * uint64(ratio)
+			u.LogicalMetaChunks += s.TotalBytes
+		}
+		if s.Flags&blockGroupSystem != 0 {
+			u.SystemUsed += s.UsedBytes * uint64(ratio)
+			u.SystemChunks += s.TotalBytes * uint64(ratio)
+		}
+	}
+	u.TotalChunks = u.RawDataChunks + u.SystemChunks
+	u.TotalUsed = u.RawDataUsed + u.SystemUsed
+	if !mixed {
+		u.TotalChunks += u.RawMetaChunks
+		u.TotalUsed += u.RawMetaUsed
+	}
+	u.TotalUnused = u.Total - u.TotalChunks
+
+	u.DataRatio = float64(u.RawDataChunks) / float64(u.LogicalDataChunks)
+	if mixed {
+		u.MetadataRatio = u.DataRatio
+	} else {
+		u.MetadataRatio = float64(u.RawMetaChunks) / float64(u.LogicalMetaChunks)
+	}
+
+	// We're able to fill at least DATA for the unused space
+	//
+	// With mixed raid levels, this gives a rough estimate but more
+	// accurate than just counting the logical free space
+	// (l_data_chunks - l_data_used)
+	//
+	// In non-mixed case there's no difference.
+	u.FreeEstimated = uint64(float64(u.RawDataChunks-u.RawDataUsed) / u.DataRatio)
+
+	// For mixed-bg the metadata are left out in calculations thus global
+	// reserve would be lost. Part of it could be permanently allocated,
+	// we have to subtract the used bytes so we don't go under zero free.
+	if mixed {
+		u.FreeEstimated -= u.GlobalReserve - u.GlobalReserveUsed
+	}
+	u.FreeMin = u.FreeEstimated
+
+	// Chop unallocatable space
+	// FIXME: must be applied per device
+	if u.TotalUnused >= minUnallocatedThreshold {
+		u.FreeEstimated += uint64(float64(u.TotalUnused) / u.DataRatio)
+		// Match the calculation of 'df', use the highest raid ratio
+		u.FreeMin += u.TotalUnused / uint64(maxDataRatio)
+	}
+	return u, nil
+}
diff --git a/xattr.go b/xattr.go
new file mode 100644
index 0000000..ae4b7d2
--- /dev/null
+++ b/xattr.go
@@ -0,0 +1,66 @@
+package btrfs
+
+import (
+	"bytes"
+	"os"
+	"syscall"
+)
+
+const (
+	xattrPrefix      = "btrfs."
+	xattrCompression = xattrPrefix + "compression"
+)
+
+type Compression string
+
+const (
+	CompressionNone = Compression("")
+	LZO             = Compression("lzo")
+	ZLIB            = Compression("zlib")
+)
+
+func SetCompression(path string, v Compression) error {
+	var value []byte
+	if v != CompressionNone {
+		var err error
+		value, err = syscall.ByteSliceFromString(string(v))
+		if err != nil {
+			return err
+		}
+	}
+	err := syscall.Setxattr(path, xattrCompression, value, 0)
+	if err != nil {
+		return &os.PathError{Op: "setxattr", Path: path, Err: err}
+	}
+	return nil
+}
+
+func GetCompression(path string) (Compression, error) {
+	var buf []byte
+	for {
+		sz, err := syscall.Getxattr(path, xattrCompression, nil)
+		if err == syscall.ENODATA || sz == 0 {
+			return CompressionNone, nil
+		} else if err != nil {
+			return CompressionNone, &os.PathError{Op: "getxattr", Path: path, Err: err}
+		}
+		if cap(buf) < sz {
+			buf = make([]byte, sz)
+		} else {
+			buf = buf[:sz]
+		}
+		sz, err = syscall.Getxattr(path, xattrCompression, buf)
+		if err == syscall.ENODATA {
+			return CompressionNone, nil
+		} else if err == syscall.ERANGE {
+			// xattr changed by someone else, and is larger than our current buffer
+			continue
+		} else if err != nil {
+			return CompressionNone, &os.PathError{Op: "getxattr", Path: path, Err: err}
+		}
+		buf = buf[:sz]
+		break
+	}
+	buf = bytes.TrimSuffix(buf, []byte{0})
+	return Compression(buf), nil
+}