diff --git a/src/api2/admin/datastore/pxar.rs b/src/api2/admin/datastore/pxar.rs
index f2c46bf4..88af9bdc 100644
--- a/src/api2/admin/datastore/pxar.rs
+++ b/src/api2/admin/datastore/pxar.rs
@@ -78,7 +78,7 @@ fn upload_pxar(
         bail!("got wrong content-type for pxar archive upload");
     }
 
-    let chunk_size = param["chunk-size"].as_u64().unwrap_or(4096*1024);
+    let chunk_size = param["chunk-size"].as_u64().unwrap_or(4096*1024) as usize;
     verify_chunk_size(chunk_size)?;
 
     let datastore = DataStore::lookup_datastore(store)?;
diff --git a/src/backup/chunk_store.rs b/src/backup/chunk_store.rs
index 33a50dde..b0945143 100644
--- a/src/backup/chunk_store.rs
+++ b/src/backup/chunk_store.rs
@@ -42,9 +42,9 @@ pub struct ChunkStore {
 
 // TODO: what about sysctl setting vm.vfs_cache_pressure (0 - 100) ?
 
-pub fn verify_chunk_size(size: u64) -> Result<(), Error> {
+pub fn verify_chunk_size(size: usize) -> Result<(), Error> {
 
-    static SIZES: [u64; 7] = [64*1024, 128*1024, 256*1024, 512*1024, 1024*1024, 2048*1024, 4096*1024];
+    static SIZES: [usize; 7] = [64*1024, 128*1024, 256*1024, 512*1024, 1024*1024, 2048*1024, 4096*1024];
 
     if !SIZES.contains(&size) {
         bail!("Got unsupported chunk size '{}'", size);
diff --git a/src/backup/chunk_stream.rs b/src/backup/chunk_stream.rs
index fb11ac8d..a8a77e09 100644
--- a/src/backup/chunk_stream.rs
+++ b/src/backup/chunk_stream.rs
@@ -15,8 +15,8 @@ pub struct ChunkStream<S> {
 }
 
 impl <S> ChunkStream<S> {
-    pub fn new(input: S) -> Self {
-        Self { input, chunker: Chunker::new(4 * 1024 * 1024), buffer: BytesMut::new(), scan_pos: 0}
+    pub fn new(input: S, chunk_size: Option<usize>) -> Self {
+        Self { input, chunker: Chunker::new(chunk_size.unwrap_or(4*1024*1024)), buffer: BytesMut::new(), scan_pos: 0}
     }
 }
 
diff --git a/src/bin/proxmox-backup-client.rs b/src/bin/proxmox-backup-client.rs
index 04eed517..f5ae7490 100644
--- a/src/bin/proxmox-backup-client.rs
+++ b/src/bin/proxmox-backup-client.rs
@@ -110,17 +110,13 @@ fn backup_directory<P: AsRef<Path>>(
     client: &BackupClient,
     dir_path: P,
     archive_name: &str,
-    chunk_size: Option<u64>,
+    chunk_size: Option<usize>,
     all_file_systems: bool,
     verbose: bool,
 ) -> Result<(), Error> {
 
-    if let Some(_size) = chunk_size {
-        unimplemented!();
-    }
-
     let pxar_stream = PxarBackupStream::open(dir_path.as_ref(), all_file_systems, verbose)?;
-    let chunk_stream = ChunkStream::new(pxar_stream);
+    let chunk_stream = ChunkStream::new(pxar_stream, chunk_size);
 
     let (tx, rx) = mpsc::channel(10); // allow to buffer 10 chunks
 
@@ -144,7 +140,7 @@ fn backup_image<P: AsRef<Path>>(
     image_path: P,
     archive_name: &str,
     image_size: u64,
-    chunk_size: Option<u64>,
+    chunk_size: Option<usize>,
     verbose: bool,
 ) -> Result<(), Error> {
 
@@ -155,7 +151,7 @@ fn backup_image<P: AsRef<Path>>(
     let stream = tokio::codec::FramedRead::new(file, tokio::codec::BytesCodec::new())
         .map_err(Error::from);
 
-    let stream = FixedChunkStream::new(stream, chunk_size.unwrap_or(4*1024*1024) as usize);
+    let stream = FixedChunkStream::new(stream, chunk_size.unwrap_or(4*1024*1024));
 
     client.upload_stream(archive_name, stream, "fixed", Some(image_size)).wait()?;
 
@@ -395,7 +391,7 @@ fn create_backup(
 
     let verbose = param["verbose"].as_bool().unwrap_or(false);
 
-    let chunk_size_opt = param["chunk-size"].as_u64().map(|v| v*1024);
+    let chunk_size_opt = param["chunk-size"].as_u64().map(|v| (v*1024) as usize);
 
     if let Some(size) = chunk_size_opt {
         verify_chunk_size(size)?;
diff --git a/src/bin/test_chunk_speed2.rs b/src/bin/test_chunk_speed2.rs
index c56694cb..d9ed3488 100644
--- a/src/bin/test_chunk_speed2.rs
+++ b/src/bin/test_chunk_speed2.rs
@@ -27,7 +27,7 @@ fn main() {
             let stream = tokio::codec::FramedRead::new(file, tokio::codec::BytesCodec::new())
                 .map(|bytes| bytes.to_vec()).map_err(Error::from);
             //let chunk_stream = FixedChunkStream::new(stream, 4*1024*1024);
-            let chunk_stream = ChunkStream::new(stream);
+            let chunk_stream = ChunkStream::new(stream, None);
 
             let start_time = std::time::Instant::now();