diff --git a/src/api2/admin/datastore/catar.rs b/src/api2/admin/datastore/catar.rs
index b2a60b12..982b8a15 100644
--- a/src/api2/admin/datastore/catar.rs
+++ b/src/api2/admin/datastore/catar.rs
@@ -76,7 +76,8 @@ fn upload_catar(
         bail!("got wrong content-type for catar archive upload");
     }
 
-    let chunk_size = 4*1024*1024;
+    let chunk_size = param["chunk-size"].as_u64().unwrap_or(4096*1024);
+    verify_chunk_size(chunk_size)?;
 
     let datastore = DataStore::lookup_datastore(store)?;
 
@@ -84,7 +85,7 @@ fn upload_catar(
 
     path.push(archive_name);
 
-    let index = datastore.create_dynamic_writer(path, chunk_size)?;
+    let index = datastore.create_dynamic_writer(path, chunk_size as usize)?;
 
     let upload = UploadCaTar { stream: req_body, index, count: 0};
 
@@ -112,7 +113,13 @@ pub fn api_method_upload_catar() -> ApiAsyncMethod {
             .required("id", StringSchema::new("Backup ID."))
             .required("time", IntegerSchema::new("Backup time (Unix epoch.)")
                       .minimum(1547797308))
-
+            .optional(
+                "chunk-size",
+                IntegerSchema::new("Chunk size in bytes. Must be a power of 2.")
+                    .minimum(64*1024)
+                    .maximum(4096*1024)
+                    .default(4096*1024)
+            )
     )
 }
 
diff --git a/src/backup/chunk_store.rs b/src/backup/chunk_store.rs
index e472ed3d..57e00b05 100644
--- a/src/backup/chunk_store.rs
+++ b/src/backup/chunk_store.rs
@@ -40,6 +40,16 @@ pub struct ChunkStore {
 
 // TODO: what about sysctl setting vm.vfs_cache_pressure (0 - 100) ?
 
+pub fn verify_chunk_size(size: u64) -> Result<(), Error> {
+
+    static SIZES: [u64; 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);
+    }
+    Ok(())
+}
+
 fn digest_to_prefix(digest: &[u8]) -> PathBuf {
 
     let mut buf = Vec::<u8>::with_capacity(2+1+2+1);
diff --git a/src/backup/dynamic_index.rs b/src/backup/dynamic_index.rs
index f5c2ddd4..cb203e60 100644
--- a/src/backup/dynamic_index.rs
+++ b/src/backup/dynamic_index.rs
@@ -329,6 +329,7 @@ pub struct DynamicIndexWriter {
     pub uuid: [u8; 16],
     pub ctime: u64,
 
+    chunk_count: usize,
     chunk_offset: usize,
     last_chunk: usize,
     chunk_buffer: Vec<u8>,
@@ -387,6 +388,7 @@ impl DynamicIndexWriter {
             ctime,
             uuid: *uuid.as_bytes(),
 
+            chunk_count: 0,
             chunk_offset: 0,
             last_chunk: 0,
             chunk_buffer: Vec::with_capacity(chunk_size*4),
@@ -405,6 +407,8 @@ impl DynamicIndexWriter {
 
         self.writer.flush()?;
 
+        let avg = ((self.chunk_offset as f64)/(self.chunk_count as f64)) as usize;
+        println!("Average chunk size {}", avg);
         // fixme:
 
         if let Err(err) = std::fs::rename(&self.tmp_filename, &self.filename) {
@@ -429,6 +433,8 @@ impl DynamicIndexWriter {
                 format!("wrong chunk size {} != {}", expected_chunk_size, chunk_size)));
         }
 
+        self.chunk_count += 1;
+
         self.last_chunk = self.chunk_offset;
 
         match self.store.insert_chunk(&self.chunk_buffer) {
diff --git a/src/bin/proxmox-backup-client.rs b/src/bin/proxmox-backup-client.rs
index 2ea87264..32bb42a4 100644
--- a/src/bin/proxmox-backup-client.rs
+++ b/src/bin/proxmox-backup-client.rs
@@ -8,7 +8,7 @@ use proxmox_backup::cli::command::*;
 use proxmox_backup::api_schema::*;
 use proxmox_backup::api_schema::router::*;
 use proxmox_backup::client::*;
-//use proxmox_backup::backup::chunk_store::*;
+use proxmox_backup::backup::*;
 //use proxmox_backup::backup::image_index::*;
 //use proxmox_backup::config::datastore;
 //use proxmox_backup::catar::encoder::*;
@@ -18,19 +18,31 @@ use serde_json::{Value};
 use hyper::Body;
 use std::sync::Arc;
 
-fn backup_directory(repo: &BackupRepository, body: Body, archive_name: &str) -> Result<(), Error> {
+fn backup_directory(
+    repo: &BackupRepository,
+    body: Body,
+    archive_name: &str,
+    chunk_size: Option<u64>,
+) -> Result<(), Error> {
 
     let client = HttpClient::new(&repo.host, &repo.user);
 
     let epoch = std::time::SystemTime::now().duration_since(
         std::time::SystemTime::UNIX_EPOCH)?.as_secs();
 
-    let query = url::form_urlencoded::Serializer::new(String::new())
+    let mut query = url::form_urlencoded::Serializer::new(String::new());
+
+    query
         .append_pair("archive_name", archive_name)
         .append_pair("type", "host")
         .append_pair("id", &tools::nodename())
-        .append_pair("time", &epoch.to_string())
-        .finish();
+        .append_pair("time", &epoch.to_string());
+
+    if let Some(size) = chunk_size {
+        query.append_pair("chunk-size", &size.to_string());
+    }
+
+    let query = query.finish();
 
     let path = format!("api2/json/admin/datastore/{}/catar?{}", repo.store, query);
 
@@ -96,16 +108,10 @@ fn create_backup(
 
     let repo = BackupRepository::parse(repo_url)?;
 
-    let mut _chunk_size = 4*1024*1024;
+    let chunk_size_opt = param["chunk-size"].as_u64().map(|v| v*1024);
 
-    if let Some(size) = param["chunk-size"].as_u64() {
-        static SIZES: [u64; 7] = [64, 128, 256, 512, 1024, 2048, 4096];
-
-        if SIZES.contains(&size) {
-            _chunk_size = (size as usize) * 1024;
-        } else {
-            bail!("Got unsupported chunk size '{}'", size);
-        }
+    if let Some(size) = chunk_size_opt {
+        verify_chunk_size(size)?;
     }
 
     let stat = match nix::sys::stat::stat(filename) {
@@ -120,7 +126,7 @@ fn create_backup(
 
         let body = Body::wrap_stream(stream);
 
-        backup_directory(&repo, body, target)?;
+        backup_directory(&repo, body, target, chunk_size_opt)?;
 
     } else if (stat.st_mode & (libc::S_IFREG|libc::S_IFBLK)) != 0 {
         println!("Backup image '{}' to '{:?}'", filename, repo);