diff --git a/src/api2/tape/backup.rs b/src/api2/tape/backup.rs
index 17eed23e..9f1167fe 100644
--- a/src/api2/tape/backup.rs
+++ b/src/api2/tape/backup.rs
@@ -1,5 +1,5 @@
 use std::path::Path;
-use std::sync::Arc;
+use std::sync::{Mutex, Arc};
 
 use anyhow::{bail, format_err, Error};
 use serde_json::Value;
@@ -508,33 +508,48 @@ pub fn backup_snapshot(
         }
     };
 
-    let mut chunk_iter = snapshot_reader.chunk_iterator()?.peekable();
+    let snapshot_reader = Arc::new(Mutex::new(snapshot_reader));
+
+    let (reader_thread, chunk_iter) = pool_writer.spawn_chunk_reader_thread(
+        datastore.clone(),
+        snapshot_reader.clone(),
+    )?;
+
+    let mut chunk_iter = chunk_iter.peekable();
 
     loop {
         worker.check_abort()?;
 
         // test is we have remaining chunks
-        if chunk_iter.peek().is_none() {
-            break;
+        match chunk_iter.peek() {
+            None => break,
+            Some(Ok(_)) => { /* Ok */ },
+            Some(Err(err)) => bail!("{}", err),
         }
 
         let uuid = pool_writer.load_writable_media(worker)?;
 
         worker.check_abort()?;
 
-        let (leom, _bytes) = pool_writer.append_chunk_archive(worker, &datastore, &mut chunk_iter)?;
+        let (leom, _bytes) = pool_writer.append_chunk_archive(worker, &mut chunk_iter)?;
 
         if leom {
             pool_writer.set_media_status_full(&uuid)?;
         }
     }
 
+    if let Err(_) = reader_thread.join() {
+        bail!("chunk reader thread failed");
+    }
+
     worker.check_abort()?;
 
     let uuid = pool_writer.load_writable_media(worker)?;
 
     worker.check_abort()?;
 
+    let snapshot_reader = snapshot_reader.lock().unwrap();
+
     let (done, _bytes) = pool_writer.append_snapshot_archive(worker, &snapshot_reader)?;
 
     if !done {
diff --git a/src/tape/pool_writer.rs b/src/tape/pool_writer.rs
index 105fe695..2b21c374 100644
--- a/src/tape/pool_writer.rs
+++ b/src/tape/pool_writer.rs
@@ -1,8 +1,9 @@
 use std::collections::HashSet;
 use std::path::Path;
 use std::time::SystemTime;
+use std::sync::{Arc, Mutex};
 
-use anyhow::{bail, Error};
+use anyhow::{bail, format_err, Error};
 
 use proxmox::tools::Uuid;
 
@@ -10,6 +11,7 @@ use crate::{
     task_log,
     backup::{
         DataStore,
+        DataBlob,
     },
     server::WorkerTask,
     tape::{
@@ -18,7 +20,6 @@ use crate::{
         COMMIT_BLOCK_SIZE,
         TapeWrite,
         SnapshotReader,
-        SnapshotChunkIterator,
         MediaPool,
         MediaId,
         MediaCatalog,
@@ -38,32 +39,192 @@ use crate::{
     config::tape_encryption_keys::load_key_configs,
 };
 
+/// Helper to build and query sets of catalogs
+pub struct CatalogBuilder {
+    // read only part
+    media_set_catalog: MediaSetCatalog,
+    // catalog to modify (latest in  set)
+    catalog: Option<MediaCatalog>,
+}
+
+impl CatalogBuilder {
+
+    /// Test if the catalog already contains a snapshot
+    pub fn contains_snapshot(&self, snapshot: &str) -> bool {
+        if let Some(ref catalog) = self.catalog {
+            if catalog.contains_snapshot(snapshot) {
+                return true;
+            }
+        }
+        self.media_set_catalog.contains_snapshot(snapshot)
+    }
+
+    /// Test if the catalog already contains a chunk
+    pub fn contains_chunk(&self, digest: &[u8;32]) -> bool {
+        if let Some(ref catalog) = self.catalog {
+            if catalog.contains_chunk(digest) {
+                return true;
+            }
+        }
+        self.media_set_catalog.contains_chunk(digest)
+    }
+
+    /// Add a new catalog, move the old on to the read-only set
+    pub fn append_catalog(&mut self, new_catalog: MediaCatalog) -> Result<(), Error> {
+
+        // append current catalog to read-only set
+        if let Some(catalog) = self.catalog.take() {
+            self.media_set_catalog.append_catalog(catalog)?;
+        }
+
+        // remove read-only version from set (in case it is there)
+        self.media_set_catalog.remove_catalog(&new_catalog.uuid());
+
+        self.catalog = Some(new_catalog);
+
+        Ok(())
+    }
+
+    /// Register a snapshot
+    pub fn register_snapshot(
+        &mut self,
+        uuid: Uuid, // Uuid form MediaContentHeader
+        file_number: u64,
+        snapshot: &str,
+    )  -> Result<(), Error> {
+        match self.catalog {
+            Some(ref mut catalog) => {
+                catalog.register_snapshot(uuid, file_number, snapshot)?;
+            }
+            None => bail!("no catalog loaded - internal error"),
+        }
+        Ok(())
+    }
+
+    /// Register a chunk archive
+    pub fn register_chunk_archive(
+        &mut self,
+        uuid: Uuid, // Uuid form MediaContentHeader
+        file_number: u64,
+        chunk_list: &[[u8; 32]],
+    ) -> Result<(), Error> {
+        match self.catalog {
+            Some(ref mut catalog) => {
+                catalog.start_chunk_archive(uuid, file_number)?;
+                for digest in chunk_list {
+                    catalog.register_chunk(digest)?;
+                }
+                catalog.end_chunk_archive()?;
+            }
+            None => bail!("no catalog loaded - internal error"),
+        }
+        Ok(())
+    }
+
+    /// Commit the catalog changes
+    pub fn commit(&mut self) -> Result<(), Error> {
+        if let Some(ref mut catalog) = self.catalog {
+            catalog.commit()?;
+        }
+        Ok(())
+    }
+}
+
+/// Chunk iterator which use a separate thread to read chunks
+///
+/// The iterator skips duplicate chunks and chunks already in the
+/// catalog.
+pub struct NewChunksIterator {
+    rx: std::sync::mpsc::Receiver<Result<Option<([u8; 32], DataBlob)>, Error>>,
+}
+
+impl NewChunksIterator {
+
+    /// Creates the iterator, spawning a new thread
+    ///
+    /// Make sure to join() the returnd thread handle.
+    pub fn spawn(
+        datastore: Arc<DataStore>,
+        snapshot_reader: Arc<Mutex<SnapshotReader>>,
+        catalog_builder: Arc<Mutex<CatalogBuilder>>,
+    ) -> Result<(std::thread::JoinHandle<()>, Self), Error> {
+
+        let (tx, rx) = std::sync::mpsc::sync_channel(3);
+
+        let reader_thread = std::thread::spawn(move || {
+
+            let snapshot_reader = snapshot_reader.lock().unwrap();
+
+            let mut chunk_index: HashSet<[u8;32]> = HashSet::new();
+
+            let result: Result<(), Error> = proxmox::try_block!({
+
+                let mut chunk_iter = snapshot_reader.chunk_iterator()?;
+
+                loop {
+                    let digest = match chunk_iter.next() {
+                        None => {
+                            tx.send(Ok(None)).unwrap();
+                            break;
+                        }
+                        Some(digest) => digest?,
+                    };
+
+                    if chunk_index.contains(&digest) {
+                        continue;
+                    }
+
+                    if catalog_builder.lock().unwrap().contains_chunk(&digest) {
+                        continue;
+                    };
+
+                    let blob = datastore.load_chunk(&digest)?;
+                    //println!("LOAD CHUNK {}", proxmox::tools::digest_to_hex(&digest));
+                    tx.send(Ok(Some((digest, blob)))).unwrap();
+
+                    chunk_index.insert(digest);
+                }
+
+                Ok(())
+            });
+            if let Err(err) = result {
+                tx.send(Err(err)).unwrap();
+            }
+        });
+
+        Ok((reader_thread, Self { rx }))
+    }
+}
+
+// We do not use Receiver::into_iter(). The manual implementation
+// returns a simpler type.
+impl Iterator for NewChunksIterator {
+    type Item = Result<([u8; 32], DataBlob), Error>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.rx.recv() {
+            Ok(Ok(None)) => None,
+            Ok(Ok(Some((digest, blob)))) => Some(Ok((digest, blob))),
+            Ok(Err(err)) => Some(Err(err)),
+            Err(_) => Some(Err(format_err!("reader thread failed"))),
+        }
+    }
+}
 
 struct PoolWriterState {
     drive: Box<dyn TapeDriver>,
-    catalog: MediaCatalog,
     // tell if we already moved to EOM
     at_eom: bool,
     // bytes written after the last tape fush/sync
     bytes_written: usize,
 }
 
-impl PoolWriterState {
-
-    fn commit(&mut self) -> Result<(), Error> {
-        self.drive.sync()?; // sync all data to the tape
-        self.catalog.commit()?; // then commit the catalog
-        self.bytes_written = 0;
-        Ok(())
-    }
-}
-
 /// Helper to manage a backup job, writing several tapes of a pool
 pub struct PoolWriter {
     pool: MediaPool,
     drive_name: String,
     status: Option<PoolWriterState>,
-    media_set_catalog: MediaSetCatalog,
+    catalog_builder: Arc<Mutex<CatalogBuilder>>,
     notify_email: Option<String>,
 }
 
@@ -97,11 +258,13 @@ impl PoolWriter {
             media_set_catalog.append_catalog(media_catalog)?;
         }
 
+        let catalog_builder = CatalogBuilder { media_set_catalog, catalog: None };
+
         Ok(Self {
             pool,
             drive_name: drive_name.to_string(),
             status: None,
-            media_set_catalog,
+            catalog_builder: Arc::new(Mutex::new(catalog_builder)),
             notify_email,
          })
     }
@@ -117,12 +280,7 @@ impl PoolWriter {
     }
 
     pub fn contains_snapshot(&self, snapshot: &str) -> bool {
-        if let Some(PoolWriterState { ref catalog, .. }) = self.status {
-            if catalog.contains_snapshot(snapshot) {
-                return true;
-            }
-        }
-        self.media_set_catalog.contains_snapshot(snapshot)
+        self.catalog_builder.lock().unwrap().contains_snapshot(snapshot)
     }
 
     /// Eject media and drop PoolWriterState (close drive)
@@ -188,16 +346,17 @@ impl PoolWriter {
     /// This is done automatically during a backupsession, but needs to
     /// be called explicitly before dropping the PoolWriter
     pub fn commit(&mut self) -> Result<(), Error> {
-        if let Some(ref mut status) = self.status {
-            status.commit()?;
+         if let Some(PoolWriterState {ref mut drive, .. }) = self.status {
+            drive.sync()?; // sync all data to the tape
         }
+        self.catalog_builder.lock().unwrap().commit()?; // then commit the catalog
         Ok(())
     }
 
     /// Load a writable media into the drive
     pub fn load_writable_media(&mut self, worker: &WorkerTask) -> Result<Uuid, Error> {
-        let last_media_uuid = match self.status {
-            Some(PoolWriterState { ref catalog, .. }) => Some(catalog.uuid().clone()),
+        let last_media_uuid = match self.catalog_builder.lock().unwrap().catalog {
+            Some(ref catalog) => Some(catalog.uuid().clone()),
             None => None,
         };
 
@@ -217,13 +376,11 @@ impl PoolWriter {
 
         task_log!(worker, "allocated new writable media '{}'", media.label_text());
 
-        // remove read-only catalog (we store a writable version in status)
-        self.media_set_catalog.remove_catalog(&media_uuid);
-
-        if let Some(PoolWriterState {mut drive, catalog, .. }) = self.status.take() {
-            self.media_set_catalog.append_catalog(catalog)?;
-            task_log!(worker, "eject current media");
-            drive.eject_media()?;
+        if let Some(PoolWriterState {mut drive, .. }) = self.status.take() {
+            if last_media_uuid.is_some() {
+                task_log!(worker, "eject current media");
+                drive.eject_media()?;
+            }
         }
 
         let (drive_config, _digest) = crate::config::drive::config()?;
@@ -249,6 +406,8 @@ impl PoolWriter {
             media.id(),
         )?;
 
+        self.catalog_builder.lock().unwrap().append_catalog(catalog)?;
+
         let media_set = media.media_set_label().clone().unwrap();
 
         let encrypt_fingerprint = media_set
@@ -258,19 +417,11 @@ impl PoolWriter {
 
         drive.set_encryption(encrypt_fingerprint)?;
 
-        self.status = Some(PoolWriterState { drive, catalog, at_eom: false, bytes_written: 0 });
+        self.status = Some(PoolWriterState { drive, at_eom: false, bytes_written: 0 });
 
         Ok(media_uuid)
     }
 
-    /// uuid of currently loaded BackupMedia
-    pub fn current_media_uuid(&self) -> Result<&Uuid, Error> {
-        match self.status {
-            Some(PoolWriterState { ref catalog, ..}) => Ok(catalog.uuid()),
-            None => bail!("PoolWriter - no media loaded"),
-        }
-    }
-
     /// Move to EOM (if not already there), then creates a new snapshot
     /// archive writing specified files (as .pxar) into it. On
     /// success, this return 'Ok(true)' and the media catalog gets
@@ -308,7 +459,7 @@ impl PoolWriter {
 
             match tape_write_snapshot_archive(writer.as_mut(), snapshot_reader)? {
                 Some(content_uuid) => {
-                    status.catalog.register_snapshot(
+                    self.catalog_builder.lock().unwrap().register_snapshot(
                         content_uuid,
                         current_file_number,
                         &snapshot_reader.snapshot().to_string(),
@@ -324,7 +475,7 @@ impl PoolWriter {
         let request_sync = status.bytes_written >= COMMIT_BLOCK_SIZE;
 
         if !done || request_sync {
-            status.commit()?;
+            self.commit()?;
         }
 
         Ok((done, bytes_written))
@@ -337,8 +488,7 @@ impl PoolWriter {
     pub fn append_chunk_archive(
         &mut self,
         worker: &WorkerTask,
-        datastore: &DataStore,
-        chunk_iter: &mut std::iter::Peekable<SnapshotChunkIterator>,
+        chunk_iter: &mut std::iter::Peekable<NewChunksIterator>,
     ) -> Result<(bool, usize), Error> {
 
         let status = match self.status {
@@ -363,10 +513,7 @@ impl PoolWriter {
         let (saved_chunks, content_uuid, leom, bytes_written) = write_chunk_archive(
             worker,
             writer,
-            datastore,
             chunk_iter,
-            &self.media_set_catalog,
-            &status.catalog,
             MAX_CHUNK_ARCHIVE_SIZE,
         )?;
 
@@ -383,63 +530,57 @@ impl PoolWriter {
         let request_sync = status.bytes_written >= COMMIT_BLOCK_SIZE;
 
         // register chunks in media_catalog
-        status.catalog.start_chunk_archive(content_uuid, current_file_number)?;
-        for digest in saved_chunks {
-            status.catalog.register_chunk(&digest)?;
-        }
-        status.catalog.end_chunk_archive()?;
+        self.catalog_builder.lock().unwrap()
+            .register_chunk_archive(content_uuid, current_file_number, &saved_chunks)?;
 
         if leom || request_sync {
-            status.commit()?;
+            self.commit()?;
         }
 
         Ok((leom, bytes_written))
     }
+
+    pub fn spawn_chunk_reader_thread(
+        &self,
+        datastore: Arc<DataStore>,
+        snapshot_reader: Arc<Mutex<SnapshotReader>>,
+    ) -> Result<(std::thread::JoinHandle<()>, NewChunksIterator), Error> {
+        NewChunksIterator::spawn(
+            datastore,
+            snapshot_reader,
+            Arc::clone(&self.catalog_builder),
+        )
+    }
 }
 
 /// write up to <max_size> of chunks
 fn write_chunk_archive<'a>(
     _worker: &WorkerTask,
     writer: Box<dyn 'a + TapeWrite>,
-    datastore: &DataStore,
-    chunk_iter: &mut std::iter::Peekable<SnapshotChunkIterator>,
-    media_set_catalog: &MediaSetCatalog,
-    media_catalog: &MediaCatalog,
+    chunk_iter: &mut std::iter::Peekable<NewChunksIterator>,
     max_size: usize,
 ) -> Result<(Vec<[u8;32]>, Uuid, bool, usize), Error> {
 
     let (mut writer, content_uuid) = ChunkArchiveWriter::new(writer, true)?;
 
-    let mut chunk_index: HashSet<[u8;32]> = HashSet::new();
-
     // we want to get the chunk list in correct order
     let mut chunk_list: Vec<[u8;32]> = Vec::new();
 
     let mut leom = false;
 
     loop {
-        let digest = match chunk_iter.peek() {
+        let (digest, blob) = match chunk_iter.peek() {
             None => break,
-            Some(Ok(digest)) => *digest,
+            Some(Ok((digest, blob))) => (digest, blob),
             Some(Err(err)) => bail!("{}", err),
         };
 
-        if media_catalog.contains_chunk(&digest)
-            || chunk_index.contains(&digest)
-            || media_set_catalog.contains_chunk(&digest)
-        {
-            chunk_iter.next(); // consume
-            continue;
-        }
-
-        let blob = datastore.load_chunk(&digest)?;
-        //println!("CHUNK {} size {}", proxmox::tools::digest_to_hex(&digest), blob.raw_size());
+        //println!("CHUNK {} size {}", proxmox::tools::digest_to_hex(digest), blob.raw_size());
 
         match writer.try_write_chunk(&digest, &blob) {
             Ok(true) => {
+                chunk_list.push(*digest);
                 chunk_iter.next(); // consume
-                chunk_index.insert(digest);
-                chunk_list.push(digest);
             }
             Ok(false) => {
                 // Note; we do not consume the chunk (no chunk_iter.next())