diff --git a/src/lib.rs b/src/lib.rs
index b7b2082..6d52650 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -16,9 +16,10 @@ extern crate quickcheck;
 extern crate quickcheck_macros;
 
 pub mod block_manager;
+pub mod checksum;
 pub mod file_utils;
 pub mod pack;
+pub mod pdata;
 pub mod shrink;
 pub mod thin;
 pub mod version;
-pub mod checksum;
diff --git a/src/pdata/btree.rs b/src/pdata/btree.rs
new file mode 100644
index 0000000..c19d89b
--- /dev/null
+++ b/src/pdata/btree.rs
@@ -0,0 +1,187 @@
+use anyhow::{anyhow, Result};
+use fixedbitset::FixedBitSet;
+use nom::{number::complete::*, IResult};
+use std::sync::{Arc, Mutex};
+
+use crate::block_manager::*;
+use crate::checksum;
+
+//------------------------------------------
+
+pub trait ValueType {
+    type Value;
+    fn unpack(data: &[u8]) -> IResult<&[u8], Self::Value>;
+}
+
+pub struct NodeHeader {
+    is_leaf: bool,
+    block: u64,
+    nr_entries: u32,
+    max_entries: u32,
+    value_size: u32,
+}
+
+#[allow(dead_code)]
+const INTERNAL_NODE: u32 = 1;
+const LEAF_NODE: u32 = 2;
+
+pub fn unpack_node_header(data: &[u8]) -> IResult<&[u8], NodeHeader> {
+    let (i, _csum) = le_u32(data)?;
+    let (i, flags) = le_u32(i)?;
+    let (i, block) = le_u64(i)?;
+    let (i, nr_entries) = le_u32(i)?;
+    let (i, max_entries) = le_u32(i)?;
+    let (i, value_size) = le_u32(i)?;
+    let (i, _padding) = le_u32(i)?;
+
+    Ok((
+        i,
+        NodeHeader {
+            is_leaf: flags == LEAF_NODE,
+            block,
+            nr_entries,
+            max_entries,
+            value_size,
+        },
+    ))
+}
+
+pub enum Node<V: ValueType> {
+    Internal {
+        header: NodeHeader,
+        keys: Vec<u64>,
+        values: Vec<u64>,
+    },
+    Leaf {
+        header: NodeHeader,
+        keys: Vec<u64>,
+        values: Vec<V::Value>,
+    },
+}
+
+pub fn unpack_node_<V: ValueType>(data: &[u8]) -> IResult<&[u8], Node<V>> {
+    use nom::multi::count;
+
+    let (i, header) = unpack_node_header(data)?;
+    let (i, keys) = count(le_u64, header.nr_entries as usize)(i)?;
+    let nr_free = header.max_entries - header.nr_entries;
+    let (i, _padding) = count(le_u64, nr_free as usize)(i)?;
+
+    if header.is_leaf {
+        let (i, values) = count(V::unpack, header.nr_entries as usize)(i)?;
+        Ok((
+            i,
+            Node::Leaf {
+                header,
+                keys,
+                values,
+            },
+        ))
+    } else {
+        let (i, values) = count(le_u64, header.nr_entries as usize)(i)?;
+        Ok((
+            i,
+            Node::Internal {
+                header,
+                keys,
+                values,
+            },
+        ))
+    }
+}
+
+pub fn unpack_node<V: ValueType>(data: &[u8]) -> Result<Node<V>> {
+    if let Ok((_i, node)) = unpack_node_(data) {
+        Ok(node)
+    } else {
+        Err(anyhow!("couldn't unpack btree node"))
+    }
+}
+
+//------------------------------------------
+
+pub struct ValueU64;
+
+impl ValueType for ValueU64 {
+    type Value = u64;
+    fn unpack(i: &[u8]) -> IResult<&[u8], u64> {
+        le_u64(i)
+    }
+}
+
+//------------------------------------------
+
+pub trait NodeVisitor<V: ValueType> {
+    fn visit<'a>(&mut self, w: &BTreeWalker, b: &Block, node: &Node<V>) -> Result<()>;
+}
+
+#[derive(Clone)]
+pub struct BTreeWalker {
+    pub engine: Arc<AsyncIoEngine>,
+    pub seen: Arc<Mutex<FixedBitSet>>,
+}
+
+impl BTreeWalker {
+    pub fn new(engine: AsyncIoEngine) -> BTreeWalker {
+        let nr_blocks = engine.get_nr_blocks() as usize;
+        let r: BTreeWalker = BTreeWalker {
+            engine: Arc::new(engine),
+            seen: Arc::new(Mutex::new(FixedBitSet::with_capacity(nr_blocks))),
+        };
+        r
+    }
+
+    pub fn walk_nodes<NV, V>(&mut self, visitor: &mut NV, bs: &Vec<u64>) -> Result<()>
+    where
+        NV: NodeVisitor<V>,
+        V: ValueType,
+    {
+        let mut blocks = Vec::new();
+        let seen = self.seen.lock().unwrap();
+        for b in bs {
+            if !seen[*b as usize] {
+                blocks.push(Block::new(*b));
+            }
+        }
+        drop(seen);
+
+        self.engine.read_many(&mut blocks)?;
+
+        for b in blocks {
+            self.walk_node(visitor, &b)?;
+        }
+
+        Ok(())
+    }
+
+    pub fn walk_node<NV, V>(&mut self, visitor: &mut NV, b: &Block) -> Result<()>
+    where
+        NV: NodeVisitor<V>,
+        V: ValueType,
+    {
+        let mut seen = self.seen.lock().unwrap();
+        seen.insert(b.loc as usize);
+        drop(seen);
+
+        let bt = checksum::metadata_block_type(b.get_data());
+        if bt != checksum::BT::NODE {
+            return Err(anyhow!("checksum failed for node {}, {:?}", b.loc, bt));
+        }
+
+        let node = unpack_node::<V>(&b.get_data())?;
+        visitor.visit(self, &b, &node)?;
+
+        if let Node::Internal {
+            header: _h,
+            keys: _k,
+            values,
+        } = node
+        {
+            self.walk_nodes(visitor, &values)?;
+        }
+
+        Ok(())
+    }
+}
+
+//------------------------------------------
diff --git a/src/pdata/mod.rs b/src/pdata/mod.rs
new file mode 100644
index 0000000..83204ba
--- /dev/null
+++ b/src/pdata/mod.rs
@@ -0,0 +1,2 @@
+pub mod btree;
+
diff --git a/src/thin/check.rs b/src/thin/check.rs
index 90d9703..46bcfcc 100644
--- a/src/thin/check.rs
+++ b/src/thin/check.rs
@@ -1,5 +1,4 @@
 use anyhow::{anyhow, Result};
-use fixedbitset::FixedBitSet;
 use nom::{number::complete::*, IResult};
 use std::path::Path;
 use std::sync::{Arc, Mutex};
@@ -7,188 +6,8 @@ use std::time::Instant;
 use threadpool::ThreadPool;
 
 use crate::block_manager::{AsyncIoEngine, Block, IoEngine};
-use crate::checksum;
 use crate::thin::superblock::*;
-
-//------------------------------------------
-
-trait ValueType {
-    type Value;
-    fn unpack(data: &[u8]) -> IResult<&[u8], Self::Value>;
-}
-
-#[allow(dead_code)]
-struct NodeHeader {
-    is_leaf: bool,
-    block: u64,
-    nr_entries: u32,
-    max_entries: u32,
-    value_size: u32,
-}
-
-#[allow(dead_code)]
-const INTERNAL_NODE: u32 = 1;
-const LEAF_NODE: u32 = 2;
-
-fn unpack_node_header(data: &[u8]) -> IResult<&[u8], NodeHeader> {
-    let (i, _csum) = le_u32(data)?;
-    let (i, flags) = le_u32(i)?;
-    let (i, block) = le_u64(i)?;
-    let (i, nr_entries) = le_u32(i)?;
-    let (i, max_entries) = le_u32(i)?;
-    let (i, value_size) = le_u32(i)?;
-    let (i, _padding) = le_u32(i)?;
-
-    Ok((
-        i,
-        NodeHeader {
-            is_leaf: flags == LEAF_NODE,
-            block,
-            nr_entries,
-            max_entries,
-            value_size,
-        },
-    ))
-}
-
-enum Node<V: ValueType> {
-    Internal {
-        header: NodeHeader,
-        keys: Vec<u64>,
-        values: Vec<u64>,
-    },
-    Leaf {
-        header: NodeHeader,
-        keys: Vec<u64>,
-        values: Vec<V::Value>,
-    },
-}
-
-fn unpack_node_<V: ValueType>(data: &[u8]) -> IResult<&[u8], Node<V>> {
-    use nom::multi::count;
-
-    let (i, header) = unpack_node_header(data)?;
-    let (i, keys) = count(le_u64, header.nr_entries as usize)(i)?;
-
-    let nr_free = header.max_entries - header.nr_entries;
-    let (i, _padding) = count(le_u64, nr_free as usize)(i)?;
-
-    if header.is_leaf {
-        let (i, values) = count(V::unpack, header.nr_entries as usize)(i)?;
-        Ok((
-            i,
-            Node::Leaf {
-                header,
-                keys,
-                values,
-            },
-        ))
-    } else {
-        let (i, values) = count(le_u64, header.nr_entries as usize)(i)?;
-        Ok((
-            i,
-            Node::Internal {
-                header,
-                keys,
-                values,
-            },
-        ))
-    }
-}
-
-fn unpack_node<V: ValueType>(data: &[u8]) -> Result<Node<V>> {
-    if let Ok((_i, node)) = unpack_node_(data) {
-        Ok(node)
-    } else {
-        Err(anyhow!("couldn't unpack btree node"))
-    }
-}
-
-//------------------------------------------
-
-struct ValueU64;
-
-impl ValueType for ValueU64 {
-    type Value = u64;
-    fn unpack(i: &[u8]) -> IResult<&[u8], u64> {
-        le_u64(i)
-    }
-}
-
-//------------------------------------------
-
-trait NodeVisitor<V: ValueType> {
-    fn visit<'a>(&mut self, w: &BTreeWalker, b: &Block, node: &Node<V>) -> Result<()>;
-}
-
-#[derive(Clone)]
-struct BTreeWalker {
-    engine: Arc<AsyncIoEngine>,
-    seen: Arc<Mutex<FixedBitSet>>,
-}
-
-impl BTreeWalker {
-    fn new(engine: AsyncIoEngine) -> BTreeWalker {
-        let nr_blocks = engine.get_nr_blocks() as usize;
-        let r: BTreeWalker = BTreeWalker {
-            engine: Arc::new(engine),
-            seen: Arc::new(Mutex::new(FixedBitSet::with_capacity(nr_blocks))),
-        };
-        r
-    }
-
-    fn walk_nodes<NV, V>(&mut self, visitor: &mut NV, bs: &Vec<u64>) -> Result<()>
-    where
-        NV: NodeVisitor<V>,
-        V: ValueType,
-    {
-        let mut blocks = Vec::new();
-        let seen = self.seen.lock().unwrap();
-        for b in bs {
-            if !seen[*b as usize] {
-                blocks.push(Block::new(*b));
-            }
-        }
-        drop(seen);
-
-        self.engine.read_many(&mut blocks)?;
-
-        for b in blocks {
-            self.walk_node(visitor, &b)?;
-        }
-
-        Ok(())
-    }
-
-    fn walk_node<NV, V>(&mut self, visitor: &mut NV, b: &Block) -> Result<()>
-    where
-        NV: NodeVisitor<V>,
-        V: ValueType,
-    {
-        let mut seen = self.seen.lock().unwrap();
-        seen.insert(b.loc as usize);
-        drop(seen);
-
-        let bt = checksum::metadata_block_type(b.get_data());
-        if bt != checksum::BT::NODE {
-            return Err(anyhow!("checksum failed for node {}, {:?}", b.loc, bt));
-        }
-
-        let node = unpack_node::<V>(&b.get_data())?;
-        visitor.visit(self, &b, &node)?;
-
-        if let Node::Internal {
-            header: _h,
-            keys: _k,
-            values,
-        } = node
-        {
-            self.walk_nodes(visitor, &values)?;
-        }
-
-        Ok(())
-    }
-}
+use crate::pdata::btree::{ValueType, Node, BTreeWalker, NodeVisitor, ValueU64};
 
 //------------------------------------------
 
@@ -241,7 +60,7 @@ impl NodeVisitor<ValueU64> for TopLevelVisitor {
 
             w.engine.read_many(&mut blocks)?;
 
-	    // FIXME: with a thread pool we need to return errors another way.
+            // FIXME: with a thread pool we need to return errors another way.
             let nr_workers = 4;
             let pool = ThreadPool::new(nr_workers);
 
@@ -249,7 +68,7 @@ impl NodeVisitor<ValueU64> for TopLevelVisitor {
             for b in blocks {
                 let thin_id = thin_ids[n];
                 n += 1;
-                
+
                 let mut w = w.clone();
                 pool.execute(move || {
                     let mut v = BottomLevelVisitor {};