diff --git a/src/api3.rs b/src/api3.rs
index 2fb3f200..cb525add 100644
--- a/src/api3.rs
+++ b/src/api3.rs
@@ -34,6 +34,17 @@ fn lookup_datastore(name: &str) -> Result<Arc<DataStore>, Error> {
     bail!("store not found");
 }
 
+// this is just a test for mutability/mutex handling  - will remove later
+fn test_sync_api_handler2(param: Value, _info: &ApiMethod) -> Result<Value, Error> {
+    println!("This is a test {}", param);
+
+    let datastore = lookup_datastore("store1")?;
+
+    datastore.garbage_collection();
+
+    Ok(json!(null))
+}
+
 fn test_sync_api_handler(param: Value, _info: &ApiMethod) -> Result<Value, Error> {
     println!("This is a test {}", param);
 
diff --git a/src/backup/chunk_store.rs b/src/backup/chunk_store.rs
index 16ae7902..5a3bf320 100644
--- a/src/backup/chunk_store.rs
+++ b/src/backup/chunk_store.rs
@@ -16,7 +16,6 @@ pub struct ChunkStore {
     name: String, // used for error reporting
     base: PathBuf,
     chunk_dir: PathBuf,
-    hasher: Sha512Trunc256,
     mutex: Mutex<bool>,
     _lockfile: File,
 }
@@ -109,13 +108,12 @@ impl ChunkStore {
             name: name.to_owned(),
             base,
             chunk_dir,
-            hasher: Sha512Trunc256::new(),
-            _lockfile: lockfile,
+             _lockfile: lockfile,
             mutex: Mutex::new(false)
         })
     }
 
-    pub fn touch_chunk(&mut self, digest:&[u8]) ->  Result<(), Error> {
+    pub fn touch_chunk(&self, digest:&[u8]) ->  Result<(), Error> {
 
         // fixme:  nix::sys::stat::utimensat
         let mut chunk_path = self.chunk_dir.clone();
@@ -162,7 +160,7 @@ impl ChunkStore {
         Ok(())
     }
 
-    pub fn sweep_used_chunks(&mut self) -> Result<(), Error> {
+    pub fn sweep_used_chunks(&self) -> Result<(), Error> {
 
         use nix::fcntl::OFlag;
         use nix::sys::stat::Mode;
@@ -215,13 +213,12 @@ impl ChunkStore {
         Ok(())
     }
 
-    pub fn insert_chunk(&mut self, chunk: &[u8]) -> Result<(bool, [u8; 32]), Error> {
-
-        self.hasher.reset();
-        self.hasher.input(chunk);
+    pub fn insert_chunk(&self, chunk: &[u8]) -> Result<(bool, [u8; 32]), Error> {
+        let mut hasher = Sha512Trunc256::new();
+        hasher.input(chunk);
 
         let mut digest = [0u8; 32];
-        self.hasher.result(&mut digest);
+        hasher.result(&mut digest);
         //println!("DIGEST {}", digest_to_hex(&digest));
 
         let mut chunk_path = self.chunk_dir.clone();
@@ -284,7 +281,7 @@ fn test_chunk_store1() {
     let chunk_store = ChunkStore::open("test", ".testdir");
     assert!(chunk_store.is_err());
 
-    let mut chunk_store = ChunkStore::create("test", ".testdir").unwrap();
+    let chunk_store = ChunkStore::create("test", ".testdir").unwrap();
     let (exists, _) = chunk_store.insert_chunk(&[0u8, 1u8]).unwrap();
     assert!(!exists);
 
diff --git a/src/backup/datastore.rs b/src/backup/datastore.rs
index de903562..cdf4a435 100644
--- a/src/backup/datastore.rs
+++ b/src/backup/datastore.rs
@@ -34,9 +34,9 @@ impl DataStore {
         Ok(index)
     }
 
-    pub fn open_image_reader<P: AsRef<Path>>(&mut self, filename: P) -> Result<ImageIndexReader, Error> {
+    pub fn open_image_reader<P: AsRef<Path>>(&self, filename: P) -> Result<ImageIndexReader, Error> {
 
-        let index = ImageIndexReader::open(&mut self.chunk_store, filename.as_ref())?;
+        let index = ImageIndexReader::open(&self.chunk_store, filename.as_ref())?;
 
         Ok(index)
     }
@@ -61,19 +61,19 @@ impl DataStore {
         Ok(list)
     }
 
-    fn mark_used_chunks(&mut self) -> Result<(), Error> {
+    fn mark_used_chunks(&self) -> Result<(), Error> {
 
         let image_list = self.list_images()?;
 
         for path in image_list {
-            let mut index = self.open_image_reader(path)?;
+            let index = self.open_image_reader(path)?;
             index.mark_used_chunks()?;
         }
 
         Ok(())
    }
 
-    pub fn garbage_collection(&mut self) -> Result<(), Error> {
+    pub fn garbage_collection(&self) -> Result<(), Error> {
 
         self.mark_used_chunks()?;
 
diff --git a/src/backup/image_index.rs b/src/backup/image_index.rs
index 25170dc5..fb521a61 100644
--- a/src/backup/image_index.rs
+++ b/src/backup/image_index.rs
@@ -22,7 +22,7 @@ pub struct ImageIndexHeader {
 // split image into fixed size chunks
 
 pub struct ImageIndexReader<'a> {
-    store: &'a mut ChunkStore,
+    store: &'a ChunkStore,
     filename: PathBuf,
     chunk_size: usize,
     size: usize,
@@ -42,7 +42,7 @@ impl <'a> Drop for ImageIndexReader<'a> {
 
 impl <'a> ImageIndexReader<'a> {
 
-    pub fn open(store: &'a mut ChunkStore, path: &Path) -> Result<Self, Error> {
+    pub fn open(store: &'a ChunkStore, path: &Path) -> Result<Self, Error> {
 
         let full_path = store.relative_path(path);
 
@@ -100,7 +100,7 @@ impl <'a> ImageIndexReader<'a> {
         Ok(())
     }
 
-    pub fn mark_used_chunks(&mut self) -> Result<(), Error> {
+    pub fn mark_used_chunks(&self) -> Result<(), Error> {
 
         if self.index == std::ptr::null_mut() { bail!("detected closed index file."); }