diff --git a/src/backup/async_index_reader.rs b/src/backup/async_index_reader.rs
index c954b1bf..535e1678 100644
--- a/src/backup/async_index_reader.rs
+++ b/src/backup/async_index_reader.rs
@@ -45,7 +45,7 @@ impl<S: AsyncReadChunk, I: IndexFile> AsyncIndexReader<S, I> {
 }
 
 impl<S, I> AsyncRead for AsyncIndexReader<S, I> where
-S: AsyncReadChunk + Unpin + 'static,
+S: AsyncReadChunk + Unpin + Sync + 'static,
 I: IndexFile + Unpin
 {
     fn poll_read(
@@ -74,7 +74,7 @@ I: IndexFile + Unpin
 
                     this.current_chunk_digest = digest;
 
-                    let mut store = match this.store.take() {
+                    let store = match this.store.take() {
                         Some(store) => store,
                         None => {
                             return Poll::Ready(Err(io_format_err!("could not find store")));
diff --git a/src/backup/read_chunk.rs b/src/backup/read_chunk.rs
index 9ad3478e..b4890674 100644
--- a/src/backup/read_chunk.rs
+++ b/src/backup/read_chunk.rs
@@ -11,10 +11,10 @@ use super::datastore::DataStore;
 /// The ReadChunk trait allows reading backup data chunks (local or remote)
 pub trait ReadChunk {
     /// Returns the encoded chunk data
-    fn read_raw_chunk(&mut self, digest: &[u8; 32]) -> Result<DataBlob, Error>;
+    fn read_raw_chunk(&self, digest: &[u8; 32]) -> Result<DataBlob, Error>;
 
     /// Returns the decoded chunk data
-    fn read_chunk(&mut self, digest: &[u8; 32]) -> Result<Vec<u8>, Error>;
+    fn read_chunk(&self, digest: &[u8; 32]) -> Result<Vec<u8>, Error>;
 }
 
 #[derive(Clone)]
@@ -33,7 +33,7 @@ impl LocalChunkReader {
 }
 
 impl ReadChunk for LocalChunkReader {
-    fn read_raw_chunk(&mut self, digest: &[u8; 32]) -> Result<DataBlob, Error> {
+    fn read_raw_chunk(&self, digest: &[u8; 32]) -> Result<DataBlob, Error> {
         let (path, _) = self.store.chunk_path(digest);
         let raw_data = proxmox::tools::fs::file_get_contents(&path)?;
         let chunk = DataBlob::from_raw(raw_data)?;
@@ -42,7 +42,7 @@ impl ReadChunk for LocalChunkReader {
         Ok(chunk)
     }
 
-    fn read_chunk(&mut self, digest: &[u8; 32]) -> Result<Vec<u8>, Error> {
+    fn read_chunk(&self, digest: &[u8; 32]) -> Result<Vec<u8>, Error> {
         let chunk = ReadChunk::read_raw_chunk(self, digest)?;
 
         let raw_data = chunk.decode(self.crypt_config.as_ref().map(Arc::as_ref))?;
@@ -56,20 +56,20 @@ impl ReadChunk for LocalChunkReader {
 pub trait AsyncReadChunk: Send {
     /// Returns the encoded chunk data
     fn read_raw_chunk<'a>(
-        &'a mut self,
+        &'a self,
         digest: &'a [u8; 32],
     ) -> Pin<Box<dyn Future<Output = Result<DataBlob, Error>> + Send + 'a>>;
 
     /// Returns the decoded chunk data
     fn read_chunk<'a>(
-        &'a mut self,
+        &'a self,
         digest: &'a [u8; 32],
     ) -> Pin<Box<dyn Future<Output = Result<Vec<u8>, Error>> + Send + 'a>>;
 }
 
 impl AsyncReadChunk for LocalChunkReader {
     fn read_raw_chunk<'a>(
-        &'a mut self,
+        &'a self,
         digest: &'a [u8; 32],
     ) -> Pin<Box<dyn Future<Output = Result<DataBlob, Error>> + Send + 'a>> {
         Box::pin(async move{
@@ -84,7 +84,7 @@ impl AsyncReadChunk for LocalChunkReader {
     }
 
     fn read_chunk<'a>(
-        &'a mut self,
+        &'a self,
         digest: &'a [u8; 32],
     ) -> Pin<Box<dyn Future<Output = Result<Vec<u8>, Error>> + Send + 'a>> {
         Box::pin(async move {
diff --git a/src/bin/proxmox-backup-client.rs b/src/bin/proxmox-backup-client.rs
index ff83240b..d10146cd 100644
--- a/src/bin/proxmox-backup-client.rs
+++ b/src/bin/proxmox-backup-client.rs
@@ -1119,7 +1119,7 @@ async fn dump_image<W: Write>(
 
     let most_used = index.find_most_used_chunks(8);
 
-    let mut chunk_reader = RemoteChunkReader::new(client.clone(), crypt_config, most_used);
+    let chunk_reader = RemoteChunkReader::new(client.clone(), crypt_config, most_used);
 
     // Note: we avoid using BufferedFixedReader, because that add an additional buffer/copy
     // and thus slows down reading. Instead, directly use RemoteChunkReader
diff --git a/src/client/remote_chunk_reader.rs b/src/client/remote_chunk_reader.rs
index 51c80a56..978c6b34 100644
--- a/src/client/remote_chunk_reader.rs
+++ b/src/client/remote_chunk_reader.rs
@@ -1,7 +1,7 @@
 use std::future::Future;
 use std::collections::HashMap;
 use std::pin::Pin;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 
 use anyhow::Error;
 
@@ -14,7 +14,7 @@ pub struct RemoteChunkReader {
     client: Arc<BackupReader>,
     crypt_config: Option<Arc<CryptConfig>>,
     cache_hint: HashMap<[u8; 32], usize>,
-    cache: HashMap<[u8; 32], Vec<u8>>,
+    cache: Mutex<HashMap<[u8; 32], Vec<u8>>>,
 }
 
 impl RemoteChunkReader {
@@ -30,11 +30,11 @@ impl RemoteChunkReader {
             client,
             crypt_config,
             cache_hint,
-            cache: HashMap::new(),
+            cache: Mutex::new(HashMap::new()),
         }
     }
 
-    pub async fn read_raw_chunk(&mut self, digest: &[u8; 32]) -> Result<DataBlob, Error> {
+    pub async fn read_raw_chunk(&self, digest: &[u8; 32]) -> Result<DataBlob, Error> {
         let mut chunk_data = Vec::with_capacity(4 * 1024 * 1024);
 
         self.client
@@ -49,12 +49,12 @@ impl RemoteChunkReader {
 }
 
 impl ReadChunk for RemoteChunkReader {
-    fn read_raw_chunk(&mut self, digest: &[u8; 32]) -> Result<DataBlob, Error> {
+    fn read_raw_chunk(&self, digest: &[u8; 32]) -> Result<DataBlob, Error> {
         block_on(Self::read_raw_chunk(self, digest))
     }
 
-    fn read_chunk(&mut self, digest: &[u8; 32]) -> Result<Vec<u8>, Error> {
-        if let Some(raw_data) = self.cache.get(digest) {
+    fn read_chunk(&self, digest: &[u8; 32]) -> Result<Vec<u8>, Error> {
+        if let Some(raw_data) = (*self.cache.lock().unwrap()).get(digest) {
             return Ok(raw_data.to_vec());
         }
 
@@ -66,7 +66,7 @@ impl ReadChunk for RemoteChunkReader {
 
         let use_cache = self.cache_hint.contains_key(digest);
         if use_cache {
-            self.cache.insert(*digest, raw_data.to_vec());
+            (*self.cache.lock().unwrap()).insert(*digest, raw_data.to_vec());
         }
 
         Ok(raw_data)
@@ -75,18 +75,18 @@ impl ReadChunk for RemoteChunkReader {
 
 impl AsyncReadChunk for RemoteChunkReader {
     fn read_raw_chunk<'a>(
-        &'a mut self,
+        &'a self,
         digest: &'a [u8; 32],
     ) -> Pin<Box<dyn Future<Output = Result<DataBlob, Error>> + Send + 'a>> {
         Box::pin(Self::read_raw_chunk(self, digest))
     }
 
     fn read_chunk<'a>(
-        &'a mut self,
+        &'a self,
         digest: &'a [u8; 32],
     ) -> Pin<Box<dyn Future<Output = Result<Vec<u8>, Error>> + Send + 'a>> {
         Box::pin(async move {
-            if let Some(raw_data) = self.cache.get(digest) {
+            if let Some(raw_data) = (*self.cache.lock().unwrap()).get(digest) {
                 return Ok(raw_data.to_vec());
             }
 
@@ -98,7 +98,7 @@ impl AsyncReadChunk for RemoteChunkReader {
 
             let use_cache = self.cache_hint.contains_key(digest);
             if use_cache {
-                self.cache.insert(*digest, raw_data.to_vec());
+                (*self.cache.lock().unwrap()).insert(*digest, raw_data.to_vec());
             }
 
             Ok(raw_data)