diff --git a/src/bin/test_chunk_size.rs b/src/bin/test_chunk_size.rs
new file mode 100644
index 00000000..6abeed5f
--- /dev/null
+++ b/src/bin/test_chunk_size.rs
@@ -0,0 +1,115 @@
+extern crate proxmox_backup;
+
+// also see https://www.johndcook.com/blog/standard_deviation/
+
+use failure::*;
+use std::io::{Read, Write};
+
+use proxmox_backup::backup::*;
+
+struct ChunkWriter {
+    chunker: Chunker,
+    last_chunk: usize,
+    chunk_offset: usize,
+
+    chunk_count: usize,
+
+    M_old: f64,
+    M_new: f64,
+    S_old: f64,
+    S_new: f64,
+}
+
+impl ChunkWriter {
+
+    fn new(chunk_size: usize) -> Self {
+        ChunkWriter {
+            chunker: Chunker::new(chunk_size),
+            last_chunk: 0,
+            chunk_offset: 0,
+            chunk_count: 0,
+
+            M_old: 0.0,
+            M_new: 0.0,
+            S_old: 0.0,
+            S_new: 0.0,
+        }
+    }
+
+    fn record_stat(&mut self, chunk_size: f64) {
+
+        self.chunk_count += 1;
+
+        if self.chunk_count == 1 {
+            self.M_old = chunk_size;
+            self.M_new = chunk_size;
+            self.S_old = 0.0;
+        } else {
+            self.M_new = self.M_old + (chunk_size - self.M_old)/(self.chunk_count as f64);
+            self.S_new = self.S_old +
+                (chunk_size - self.M_old)*(chunk_size - self.M_new);
+            // set up for next iteration
+            self.M_old = self.M_new;
+            self.S_old = self.S_new;
+        }
+
+        let variance = if self.chunk_count > 1 {
+            self.S_new/((self.chunk_count -1)as f64)
+        } else { 0.0 };
+
+        let std_deviation = variance.sqrt();
+        let deviation_per = (std_deviation*100.0)/self.M_new;
+        println!("COUNT {:10} SIZE {:10} MEAN {:10} DEVIATION {:3}%", self.chunk_count, chunk_size, self.M_new as usize, deviation_per as usize);
+    }
+}
+
+impl Write for ChunkWriter {
+
+    fn write(&mut self, data: &[u8]) -> std::result::Result<usize, std::io::Error> {
+
+        let chunker = &mut self.chunker;
+
+        let pos = chunker.scan(data);
+
+        if pos > 0 {
+            self.chunk_offset += pos;
+
+            let chunk_size = self.chunk_offset - self.last_chunk;
+
+            self.record_stat(chunk_size as f64);
+
+            self.last_chunk = self.chunk_offset;
+            Ok(pos)
+
+        } else {
+            self.chunk_offset += data.len();
+            Ok(data.len())
+        }
+    }
+
+    fn flush(&mut self) -> std::result::Result<(), std::io::Error> {
+        Ok(())
+    }
+}
+
+fn main() -> Result<(), Error> {
+
+    let mut file = std::fs::File::open("/dev/urandom")?;
+
+    let mut bytes = 0;
+
+    let mut buffer = [0u8; 64*1024];
+
+    let mut writer = ChunkWriter::new(4096*1024);
+
+    loop {
+
+        file.read_exact(&mut buffer)?;
+        bytes += buffer.len();
+
+        writer.write_all(&buffer)?;
+
+    }
+
+    Ok(())
+}