diff --git a/src/dirty.rs b/src/dirty.rs
new file mode 100644
index 0000000..667c9b7
--- /dev/null
+++ b/src/dirty.rs
@@ -0,0 +1,47 @@
+use core::ops::{Deref, DerefMut};
+
+/// Dirty wraps a value of type T with functions similiar to that of a Read/Write
+/// lock but simply sets a dirty flag on write(), reset on read()
+pub struct Dirty<T> {
+    value: T,
+    dirty: bool,
+}
+
+impl<T> Dirty<T> {
+    /// Create a new Dirty
+    pub fn new(val: T) -> Dirty<T> {
+        Dirty {
+            value: val,
+            dirty: false,
+        }
+    }
+
+    /// Returns true if dirty, false otherwise
+    #[allow(dead_code)]
+    pub fn dirty(&self) -> bool {
+        self.dirty
+    }
+
+    /// Reset dirty
+    pub fn sync(&mut self) {
+        self.dirty = false;
+    }
+}
+
+impl<T> Deref for Dirty<T> {
+    type Target = T;
+
+    /// Read the value
+    fn deref(&self) -> &T {
+        &self.value
+    }
+}
+
+impl<T> DerefMut for Dirty<T> {
+    /// Writable value return, sets the dirty flag
+    fn deref_mut(&mut self) -> &mut T {
+        self.dirty = true;
+        &mut self.value
+    }
+}
+
diff --git a/src/lib.rs b/src/lib.rs
index 34b1939..c0735cf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -9,6 +9,8 @@ extern crate spin;
 extern crate alloc;
 extern crate bit_set;
 
+mod dirty;
+mod vfs;
 mod sfs;
 mod structs;
 #[cfg(test)]
diff --git a/src/sfs.rs b/src/sfs.rs
index a8497da..af49cf1 100644
--- a/src/sfs.rs
+++ b/src/sfs.rs
@@ -1,6 +1,7 @@
 use spin::Mutex;
 use bit_set::BitSet;
-use alloc::{boxed::Box, Vec};
+use alloc::{boxed::Box, Vec, BTreeMap, rc::Rc};
+use dirty::Dirty;
 use super::structs::*;
 use core::mem::{uninitialized, size_of};
 use core::slice;
@@ -13,46 +14,32 @@ pub trait Device {
 }
 
 /// inode for sfs
-pub struct Inode {
+pub struct INode {
     /// on-disk inode
-    disk_inode: *mut DiskInode,
+    disk_inode: Dirty<DiskINode>,
     /// inode number
-    id: u32,
-    /// true if inode modified
-    dirty: bool,
-    /// kill inode if it hits zero
-    reclaim_count: u32,
-    /// semaphore for din
-    mutex: Mutex<()>,
+    id: INodeId,
 }
 
+type INodeId = usize;
+
 /// filesystem for sfs
 pub struct SimpleFileSystem {
     /// on-disk superblock
-    super_block: SuperBlock,
+    super_block: Dirty<SuperBlock>,
     /// blocks in use are mared 0
     free_map: BitSet,
-    /// true if super/freemap modified
-    super_dirty: bool,
-    /// buffer for non-block aligned io
-//    buffer: u8,
-    /// semaphore for fs
-//    fs_mutex: Mutex<()>,
-    /// semaphore for link/unlink and rename
-//    link_mutex: Mutex<()>,
     /// inode list
-//    inodes: Vec<Inode>,
+    inodes: BTreeMap<INodeId, Rc<INode>>,
     /// device
-    device: Mutex<Box<Device>>,
+    device: Box<Device>,
 }
 
 impl SimpleFileSystem {
     /// Create a new SFS with device
     pub fn new(mut device: Box<Device>) -> Option<Self> {
         let mut super_block: SuperBlock = unsafe{ uninitialized() };
-        let slice = unsafe{ slice::from_raw_parts_mut(
-            &mut super_block as *mut SuperBlock as *mut u8, size_of::<SuperBlock>()) };
-        if device.read_at(0, slice).is_none() {
+        if device.read_at(0, super_block.as_buf_mut()).is_none() {
             return None;
         }
         if super_block.check() == false {
@@ -60,10 +47,61 @@ impl SimpleFileSystem {
         }
 
         Some(SimpleFileSystem {
-            super_block,
+            super_block: Dirty::new(super_block),
             free_map: BitSet::new(),
-            super_dirty: false,
-            device: Mutex::new(device),
+            inodes: BTreeMap::<INodeId, Rc<INode>>::new(),
+            device,
         })
     }
+    /// Allocate a block, return block id
+    fn alloc_block(&mut self) -> Option<usize> {
+        let id = self.free_map.alloc();
+        if id.is_some() {
+            self.super_block.unused_blocks -= 1;    // will panic if underflow
+        }
+        id
+    }
+    /// Free a block
+    fn free_block(&mut self, block_id: usize) {
+        assert!(!self.free_map.contains(block_id));
+        self.free_map.insert(block_id);
+        self.super_block.unused_blocks += 1;
+    }
+    /// Get inode by id
+    fn get_inode(&self, id: INodeId) -> Option<Rc<INode>> {
+        self.inodes.get(&id).map(|rc| rc.clone())
+    }
+    /// Write back super block if dirty
+    fn sync(&mut self) {
+        let SimpleFileSystem {
+            ref mut super_block,
+            ref mut device,
+            ..
+        } = self;
+
+        if super_block.dirty() {
+            device.write_at(0, super_block.as_buf());
+            super_block.sync();
+        }
+    }
+}
+
+trait BitsetAlloc {
+    fn alloc(&mut self) -> Option<usize>;
+}
+
+impl BitsetAlloc for BitSet {
+    fn alloc(&mut self) -> Option<usize> {
+        // TODO: more efficient
+        let id = (0 .. self.len()).find(|&i| self.contains(i));
+        if let Some(id) = id {
+            self.remove(id);
+        }
+        id
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
 }
\ No newline at end of file
diff --git a/src/structs.rs b/src/structs.rs
index 8f23d4c..7b4dca3 100644
--- a/src/structs.rs
+++ b/src/structs.rs
@@ -1,3 +1,6 @@
+use core::slice;
+use core::mem::size_of_val;
+
 /// On-disk superblock
 #[repr(C, packed)]
 pub struct SuperBlock {
@@ -14,7 +17,7 @@ pub struct SuperBlock {
 /// inode (on disk)
 #[repr(C, packed)]
 #[derive(Debug)]
-pub struct DiskInode {
+pub struct DiskINode {
     /// size of the file (in bytes)
     pub size: u32,
     /// one of SYS_TYPE_* above
@@ -35,7 +38,7 @@ pub struct DiskInode {
 #[repr(C, packed)]
 pub struct DiskEntry {
     /// inode number
-    pub inode_number: u32,
+    pub id: u32,
     /// file name
     pub name: [u8; MAX_FNAME_LEN + 1],
 }
@@ -46,6 +49,19 @@ impl SuperBlock {
     }
 }
 
+/// Convert structs to [u8] slice
+pub trait AsBuf {
+    fn as_buf(&self) -> &[u8] {
+        unsafe{ slice::from_raw_parts(self as *const _ as *const u8, size_of_val(self)) }
+    }
+    fn as_buf_mut(&mut self) -> &mut [u8] {
+        unsafe{ slice::from_raw_parts_mut(self as *mut _ as *mut u8, size_of_val(self)) }
+    }
+}
+
+impl AsBuf for SuperBlock {}
+impl AsBuf for DiskINode {}
+
 /*
  * Simple FS (SFS) definitions visible to ucore. This covers the on-disk format
  * and is used by tools that work on SFS volumes, such as mksfs.
@@ -76,4 +92,17 @@ pub const BLK_NENTRY: usize = BLKSIZE / 4;
 /// file types
 pub enum FileType {
     Invalid = 0, File = 1, Dir = 2, Link = 3,
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn struct_size() {
+        use core::mem::size_of;
+        assert!(size_of::<SuperBlock>() <= BLKSIZE);
+        assert!(size_of::<DiskINode>() <= BLKSIZE);
+        assert!(size_of::<DiskEntry>() <= BLKSIZE);
+    }
 }
\ No newline at end of file
diff --git a/src/vfs.rs b/src/vfs.rs
new file mode 100644
index 0000000..fd5c96b
--- /dev/null
+++ b/src/vfs.rs
@@ -0,0 +1,17 @@
+/// Abstract operations on a inode.
+pub trait INodeOps {
+    fn open(&mut self, flags: u32) -> Result<(), ()>;
+    fn close(&mut self) -> Result<(), ()>;
+    fn read(&mut self, buf: &mut [u8]) -> Result<(), ()>;
+    fn write(&mut self, buf: &[u8]) -> Result<(), ()>;
+//    fn fstat(&mut self, buf: &[u8]) -> Result<(), ()>;
+//    fn fsync(&mut self) -> Result<(), ()>;
+//    fn name_file(&mut self) -> Result<(), ()>;
+//    fn reclaim(&mut self) -> Result<(), ()>;
+//    fn get_type(&mut self) -> Result<u32, ()>;
+//    fn try_seek(&mut self, offset: u64) -> Result<(), ()>;
+//    fn truncate(&mut self, len: u64) -> Result<(), ()>;
+//    fn create(&mut self, name: &'static str, excl: bool) -> Result<(), ()>;
+//    fn loopup(&mut self, path: &'static str) -> Result<(), ()>;
+//    fn io_ctrl(&mut self, op: u32, data: &[u8]) -> Result<(), ()>;
+}
\ No newline at end of file