diff --git a/src/storage/src/hummock/event_handler/uploader.rs b/src/storage/src/hummock/event_handler/uploader.rs
index 1da93dae10f1f..bbfb932162552 100644
--- a/src/storage/src/hummock/event_handler/uploader.rs
+++ b/src/storage/src/hummock/event_handler/uploader.rs
@@ -428,10 +428,7 @@ impl EpochData {
 }
 
 #[derive(Default)]
-/// Data at the sealed stage. We will ensure that data in `imms` are newer than the data in the
-/// `spilled_data`, and that data in the `uploading_tasks` in `spilled_data` are newer than data in
-/// the `uploaded_data` in `spilled_data`.
-struct SealedData {
+struct SyncDataBuilder {
     // newer epochs come first
     epochs: VecDeque<HummockEpoch>,
 
@@ -440,8 +437,8 @@ struct SealedData {
     table_watermarks: HashMap<TableId, TableWatermarks>,
 }
 
-impl SealedData {
-    /// Add the data of a newly sealed epoch.
+impl SyncDataBuilder {
+    /// Add the data of a new epoch.
     ///
     /// Note: it may happen that, for example, currently we hold `imms` and `spilled_data` of epoch
     /// 3,  and after we add the spilled data of epoch 4, both `imms` and `spilled_data` hold data
@@ -451,9 +448,9 @@ impl SealedData {
     /// data of `imms` must not overlap with the epoch 4 data of `spilled_data`. The explanation is
     /// as followed:
     ///
-    /// First, unsealed data has 3 stages, from earlier to later, imms, uploading task, and
-    /// uploaded. When we try to spill unsealed data, we first pick the imms of older epoch until
-    /// the imms of older epoch are all picked. When we try to poll the uploading tasks of unsealed
+    /// First, unsync data has 3 stages, from earlier to later, imms, uploading task, and
+    /// uploaded. When we try to spill unsync data, we first pick the imms of older epoch until
+    /// the imms of older epoch are all picked. When we try to poll the uploading tasks of unsync
     /// data, we first poll the task of older epoch, until there is no uploading task in older
     /// epoch. Therefore, we can reach that, if two data are in the same stage, but
     /// different epochs, data in the older epoch will always enter the next stage earlier than data
@@ -467,17 +464,19 @@ impl SealedData {
     /// Based on the two points above, we can reach that, if two data of a same key appear in
     /// different epochs, the data of older epoch will not appear at a later stage than the data
     /// of newer epoch. Therefore, we can safely merge the data of each stage when we seal an epoch.
-    fn seal_new_epoch(&mut self, epoch: HummockEpoch, mut unseal_epoch_data: EpochData) {
-        if let Some(prev_max_sealed_epoch) = self.epochs.front() {
+    fn add_new_epoch(&mut self, epoch: HummockEpoch, mut unseal_epoch_data: EpochData) {
+        if let Some(prev_max_epoch) = self.epochs.front() {
             assert!(
-                epoch > *prev_max_sealed_epoch,
-                "epoch {} to seal not greater than prev max sealed epoch {}",
+                epoch > *prev_max_epoch,
+                "epoch {} to seal not greater than prev max epoch {}",
                 epoch,
-                prev_max_sealed_epoch
+                prev_max_epoch
             );
         }
 
         self.epochs.push_front(epoch);
+        // for each local instance, earlier data must be spilled at earlier epoch. Therefore, since we add spill data from old epoch
+        // to new epoch,
         unseal_epoch_data
             .spilled_data
             .uploading_tasks
@@ -692,6 +691,10 @@ impl LocalInstanceUnsyncData {
 }
 
 #[derive(Default)]
+/// Unsync data, can be either imm or spilled sst, and some aggregated epoch information.
+///
+/// `instance_data` holds the imm of each individual local instance, and data are first added here.
+/// The aggregated epoch information (table watermarks, etc.) and the spilled sst will be added to `epoch_data`.
 struct UnsyncData {
     instance_data: HashMap<LocalInstanceId, LocalInstanceUnsyncData>,
     // An index of `instance_data` that maintains the set of existing instance id of a table id
@@ -766,18 +769,15 @@ impl UnsyncData {
         }
     }
 
-    fn sync(
-        &mut self,
-        epoch: HummockEpoch,
-    ) -> (HashMap<LocalInstanceId, Vec<UploaderImm>>, SealedData) {
+    fn sync(&mut self, epoch: HummockEpoch, context: &UploaderContext) -> SyncDataBuilder {
         let mut sync_epoch_data = self.epoch_data.split_off(&(epoch + 1));
         swap(&mut sync_epoch_data, &mut self.epoch_data);
 
         // by now, epoch data inclusively before epoch is in sync_epoch_data
 
-        let mut sealed_data = SealedData::default();
+        let mut sync_data = SyncDataBuilder::default();
         for (epoch, epoch_data) in sync_epoch_data {
-            sealed_data.seal_new_epoch(epoch, epoch_data);
+            sync_data.add_new_epoch(epoch, epoch_data);
         }
 
         let mut flush_payload = HashMap::new();
@@ -787,7 +787,8 @@ impl UnsyncData {
                 flush_payload.insert(*instance_id, payload);
             }
         }
-        (flush_payload, sealed_data)
+        sync_data.flush(context, flush_payload);
+        sync_data
     }
 
     fn ack_flushed(&mut self, sstable_info: &StagingSstableInfo) {
@@ -876,14 +877,13 @@ enum UploaderState {
 
 /// An uploader for hummock data.
 ///
-/// Data have 4 sequential stages: unsealed, sealed, syncing, synced.
+/// Data have 3 sequential stages: unsync (inside each local instance, data can be unsealed, sealed), syncing, synced.
 ///
-/// The 4 stages are divided by 3 marginal epochs: `max_sealed_epoch`, `max_syncing_epoch`,
+/// The 3 stages are divided by 2 marginal epochs: `max_syncing_epoch`,
 /// `max_synced_epoch`. Epochs satisfy the following inequality.
 ///
 /// (epochs of `synced_data`) <= `max_synced_epoch` < (epochs of `syncing_data`) <=
-/// `max_syncing_epoch` < (epochs of `sealed_data`) <= `max_sealed_epoch` < (epochs of
-/// `unsealed_data`)
+/// `max_syncing_epoch` < (epochs of `unsync_data`)
 ///
 /// Data are mostly stored in `VecDeque`, and the order stored in the `VecDeque` indicates the data
 /// order. Data at the front represents ***newer*** data.
@@ -1005,11 +1005,9 @@ impl HummockUploader {
 
         self.max_syncing_epoch = epoch;
 
-        let (payload, mut sealed_data) = data.unsync_data.sync(epoch);
+        let sync_data = data.unsync_data.sync(epoch, &self.context);
 
-        sealed_data.flush(&self.context, payload);
-
-        let SealedData {
+        let SyncDataBuilder {
             spilled_data:
                 SpilledData {
                     uploading_tasks,
@@ -1017,7 +1015,7 @@ impl HummockUploader {
                 },
             table_watermarks,
             ..
-        } = sealed_data;
+        } = sync_data;
 
         let try_join_all_upload_task = if uploading_tasks.is_empty() {
             None
@@ -1067,11 +1065,15 @@ impl HummockUploader {
         }
         if self.max_syncing_epoch < max_committed_epoch {
             self.max_syncing_epoch = max_committed_epoch;
-            // // there must not be any sealed data below MCE
-            // if let Some(&epoch) = self.sealed_data.epochs.back() {
-            //     assert_gt!(epoch, max_committed_epoch);
-            // }
-            // TODO: may check each instance
+            if let UploaderState::Working(data) = &self.state {
+                for instance_data in data.unsync_data.instance_data.values() {
+                    if let Some(oldest_epoch) = instance_data.sealed_data.back() {
+                        assert_gt!(oldest_epoch.epoch, max_committed_epoch);
+                    } else if let Some(current_epoch) = &instance_data.current_epoch_data {
+                        assert_gt!(current_epoch.epoch, max_committed_epoch);
+                    }
+                }
+            }
         }
     }
 
@@ -1178,17 +1180,14 @@ impl UploaderData {
         }
     }
 
-    /// Poll the success of the oldest spilled task of unsealed data. Return `Poll::Ready(None)` if
+    /// Poll the success of the oldest spilled task of unsync spill data. Return `Poll::Ready(None)` if
     /// there is no spilling task.
-    fn poll_unsealed_spill_task(
-        &mut self,
-        cx: &mut Context<'_>,
-    ) -> Poll<Option<StagingSstableInfo>> {
+    fn poll_spill_task(&mut self, cx: &mut Context<'_>) -> Poll<Option<StagingSstableInfo>> {
         // iterator from older epoch to new epoch so that the spill task are finished in epoch order
-        for unsealed_data in self.unsync_data.epoch_data.values_mut() {
-            // if None, there is no spilling task. Search for the unsealed data of the next epoch in
+        for epoch_data in self.unsync_data.epoch_data.values_mut() {
+            // if None, there is no spilling task. Search for the unsync data of the next epoch in
             // the next iteration.
-            if let Some(sstable_info) = ready!(unsealed_data.spilled_data.poll_success_spill(cx)) {
+            if let Some(sstable_info) = ready!(epoch_data.spilled_data.poll_success_spill(cx)) {
                 self.unsync_data.ack_flushed(&sstable_info);
                 return Poll::Ready(Some(sstable_info));
             }
@@ -1242,7 +1241,7 @@ impl HummockUploader {
                 };
             }
 
-            if let Some(sstable_info) = ready!(data.poll_unsealed_spill_task(cx)) {
+            if let Some(sstable_info) = ready!(data.poll_spill_task(cx)) {
                 return Poll::Ready(UploaderEvent::DataSpilled(sstable_info));
             }
 
@@ -1651,9 +1650,7 @@ pub(crate) mod tests {
         assert!(poll_fn(|cx| data.poll_syncing_task(cx, &uploader.context))
             .await
             .is_none());
-        assert!(poll_fn(|cx| data.poll_unsealed_spill_task(cx))
-            .await
-            .is_none());
+        assert!(poll_fn(|cx| data.poll_spill_task(cx)).await.is_none());
     }
 
     #[tokio::test]