diff --git a/src/backup/chunk_store.rs b/src/backup/chunk_store.rs
index d376a5b4..cd96f71a 100644
--- a/src/backup/chunk_store.rs
+++ b/src/backup/chunk_store.rs
@@ -1,18 +1,47 @@
 use failure::*;
 use std::path::{Path, PathBuf};
+use std::io::Write;
+
+use crypto::digest::Digest;
+use crypto::sha2::Sha512Trunc256;
 
 pub struct ChunkStore {
     base: PathBuf,
     chunk_dir: PathBuf,
+    hasher: Sha512Trunc256,
+
 }
 
-// see RFC4648 for base32hex definition
-const BASE32HEX_ALPHABET: &'static [u8; 32]   = b"0123456789ABCDEFGHIJKLMNOPQRSTUV";
+const HEX_CHARS: &'static [u8; 16] = b"0123456789abcdef";
 
-fn u10_to_base32hex(num: usize) -> String {
-    let lo = num & 0b11111 as usize;
-    let hi = num >> 5 as usize;
-    format!("{}{}", BASE32HEX_ALPHABET[hi] as char, BASE32HEX_ALPHABET[lo] as char)
+fn u256_to_hex(digest: &[u8; 32]) -> String {
+
+    let mut buf = Vec::<u8>::with_capacity(64);
+
+    for i in 0..32 {
+        buf.push(HEX_CHARS[(digest[i] >> 4) as usize]);
+        buf.push(HEX_CHARS[(digest[i] & 0xf) as usize]);
+    }
+
+    unsafe { String::from_utf8_unchecked(buf) }
+}
+
+fn u256_to_prefix(digest: &[u8; 32]) -> PathBuf {
+
+    let mut buf = Vec::<u8>::with_capacity(3+1+2+1);
+
+    buf.push(HEX_CHARS[(digest[0] as usize) >> 4]);
+    buf.push(HEX_CHARS[(digest[0] as usize) &0xf]);
+    buf.push(HEX_CHARS[(digest[1] as usize) >> 4]);
+    buf.push('/' as u8);
+
+    buf.push(HEX_CHARS[(digest[1] as usize) & 0xf]);
+    buf.push(HEX_CHARS[(digest[2] as usize) >> 4]);
+    buf.push('/' as u8);
+
+    let path = unsafe { String::from_utf8_unchecked(buf)};
+
+    path.into()
 }
 
 impl ChunkStore {
@@ -22,7 +51,9 @@ impl ChunkStore {
         let mut chunk_dir = base.clone();
         chunk_dir.push(".chunks");
 
-        ChunkStore { base, chunk_dir }
+        let hasher = Sha512Trunc256::new();
+
+        ChunkStore { base, chunk_dir, hasher }
     }
 
     pub fn create<P: Into<PathBuf>>(path: P) -> Result<Self, Error> {
@@ -32,10 +63,10 @@ impl ChunkStore {
         std::fs::create_dir(&me.base)?;
         std::fs::create_dir(&me.chunk_dir)?;
 
-        // create 1024 subdir
-        for i in 0..1024 {
+        // create 4096 subdir
+        for i in 0..4096 {
             let mut l1path = me.base.clone();
-            l1path.push(u10_to_base32hex(i));
+            l1path.push(format!("{:03x}",i));
             std::fs::create_dir(&l1path)?;
         }
 
@@ -53,6 +84,35 @@ impl ChunkStore {
         Ok(me)
     }
 
+    pub fn insert_chunk(&mut self, chunk: &[u8]) -> Result<([u8; 32]), Error> {
+
+        self.hasher.reset();
+        self.hasher.input(chunk);
+
+        let mut digest = [0u8; 32];
+        self.hasher.result(&mut digest);
+        println!("DIGEST {}", u256_to_hex(&digest));
+
+        let mut chunk_path = self.base.clone();
+
+        let prefix = u256_to_prefix(&digest);
+        chunk_path.push(prefix);
+
+        if let Err(_e) = std::fs::create_dir(&chunk_path) { /* ignore */ }
+
+        println!("PATH {:?}", chunk_path);
+
+        chunk_path.push(u256_to_hex(&digest));
+        //chunk_path.set_extension("tmp");
+        //chunk_path.set_extension("");
+        let mut f = std::fs::File::create(&chunk_path)?;
+        f.write_all(chunk)?;
+
+        println!("PATH {:?}", chunk_path);
+
+        Ok(digest)
+    }
+
 }
 
 
@@ -64,8 +124,9 @@ fn test_chunk_store1() {
     let chunk_store = ChunkStore::open(".testdir");
     assert!(chunk_store.is_err());
 
-    let chunk_store = ChunkStore::create(".testdir");
-    assert!(chunk_store.is_ok());
+    let mut chunk_store = ChunkStore::create(".testdir").unwrap();
+    chunk_store.insert_chunk(&[0u8, 1u8]);
+
 
     let chunk_store = ChunkStore::create(".testdir");
     assert!(chunk_store.is_err());