diff --git a/src/api2/backup/environment.rs b/src/api2/backup/environment.rs
index 99d885e2..19624fae 100644
--- a/src/api2/backup/environment.rs
+++ b/src/api2/backup/environment.rs
@@ -1,4 +1,4 @@
-use anyhow::{bail, format_err, Error};
+use anyhow::{bail, format_err, Context, Error};
 use nix::dir::Dir;
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
@@ -72,8 +72,14 @@ struct FixedWriterState {
     incremental: bool,
 }
 
-// key=digest, value=length
-type KnownChunksMap = HashMap<[u8; 32], u32>;
+#[derive(Copy, Clone)]
+struct KnownChunkInfo {
+    uploaded: bool,
+    length: u32,
+}
+
+// key=digest, value=KnownChunkInfo
+type KnownChunksMap = HashMap<[u8; 32], KnownChunkInfo>;
 
 struct SharedBackupState {
     finished: bool,
@@ -159,7 +165,13 @@ impl BackupEnvironment {
 
         state.ensure_unfinished()?;
 
-        state.known_chunks.insert(digest, length);
+        state.known_chunks.insert(
+            digest,
+            KnownChunkInfo {
+                uploaded: false,
+                length,
+            },
+        );
 
         Ok(())
     }
@@ -213,7 +225,13 @@ impl BackupEnvironment {
         }
 
         // register chunk
-        state.known_chunks.insert(digest, size);
+        state.known_chunks.insert(
+            digest,
+            KnownChunkInfo {
+                uploaded: true,
+                length: size,
+            },
+        );
 
         Ok(())
     }
@@ -248,7 +266,13 @@ impl BackupEnvironment {
         }
 
         // register chunk
-        state.known_chunks.insert(digest, size);
+        state.known_chunks.insert(
+            digest,
+            KnownChunkInfo {
+                uploaded: true,
+                length: size,
+            },
+        );
 
         Ok(())
     }
@@ -256,7 +280,23 @@ impl BackupEnvironment {
     pub fn lookup_chunk(&self, digest: &[u8; 32]) -> Option<u32> {
         let state = self.state.lock().unwrap();
 
-        state.known_chunks.get(digest).copied()
+        state
+            .known_chunks
+            .get(digest)
+            .map(|known_chunk_info| known_chunk_info.length)
+    }
+
+    /// stat known chunks from previous backup, so excluding newly uploaded ones
+    pub fn stat_prev_known_chunks(&self) -> Result<(), Error> {
+        let state = self.state.lock().unwrap();
+        for (digest, known_chunk_info) in &state.known_chunks {
+            if !known_chunk_info.uploaded {
+                self.datastore
+                    .stat_chunk(digest)
+                    .with_context(|| format!("stat failed on {}", hex::encode(digest)))?;
+            }
+        }
+        Ok(())
     }
 
     /// Store the writer with an unique ID
diff --git a/src/api2/backup/mod.rs b/src/api2/backup/mod.rs
index ea0d0292..63c49f65 100644
--- a/src/api2/backup/mod.rs
+++ b/src/api2/backup/mod.rs
@@ -1,6 +1,6 @@
 //! Backup protocol (HTTP2 upgrade)
 
-use anyhow::{bail, format_err, Error};
+use anyhow::{bail, format_err, Context, Error};
 use futures::*;
 use hex::FromHex;
 use hyper::header::{HeaderValue, CONNECTION, UPGRADE};
@@ -785,6 +785,26 @@ fn finish_backup(
 ) -> Result<Value, Error> {
     let env: &BackupEnvironment = rpcenv.as_ref();
 
+    if let Err(err) = env.stat_prev_known_chunks() {
+        env.debug(format!("stat registered chunks failed - {err:?}"));
+
+        if let Some(last) = env.last_backup.as_ref() {
+            // No need to acquire snapshot lock, already locked when starting the backup
+            let verify_state = SnapshotVerifyState {
+                state: VerifyState::Failed,
+                upid: env.worker.upid().clone(), // backup writer UPID
+            };
+            let verify_state = serde_json::to_value(verify_state)?;
+            last.backup_dir
+                .update_manifest(|manifest| {
+                    manifest.unprotected["verify_state"] = verify_state;
+                })
+                .with_context(|| "manifest update failed")?;
+        }
+
+        bail!("stat known chunks failed - {err:?}");
+    }
+
     env.finish_backup()?;
     env.log("successfully finished backup");