diff --git a/crates/loro-internal/src/kv_store/sstable.rs b/crates/loro-internal/src/kv_store/sstable.rs
index 3990706a..32364d31 100644
--- a/crates/loro-internal/src/kv_store/sstable.rs
+++ b/crates/loro-internal/src/kv_store/sstable.rs
@@ -101,10 +101,22 @@ impl NormalBlock {
 
     fn first_key(&self)->Bytes{
         let mut buf = self.data.as_ref();
-        // skip common prefix of the first key
         buf.get_u8();
         let key_len = buf.get_u16() as usize;
-        Bytes::from(buf[..key_len].to_vec())
+        self.data.slice(SIZE_OF_U8 + SIZE_OF_U16..SIZE_OF_U8 + SIZE_OF_U16 + key_len)
+    }
+
+    fn last_key(&self)->Bytes{
+        let offset = self.offsets.last().map_or(0, |&o| o as usize);
+        let mut buf = self.data.slice(offset..);
+        let common = buf.get_u8();
+        let key_len = buf.get_u16() as usize;
+        let key = buf.slice(.. key_len);
+        let prefix = self.first_key().slice(..common as usize);
+        let mut last_key = Vec::with_capacity(prefix.len() + key.len());
+        last_key.extend(prefix);
+        last_key.extend(key);
+        last_key.into()
     }
 }
 
@@ -133,6 +145,13 @@ impl Block{
         }
     }
 
+    pub fn last_key(&self)->Bytes{
+        match self{
+            Block::Normal(block)=>block.last_key(),
+            Block::Large(block)=>block.key(),
+        }
+    }
+
     fn encode(&self)->Bytes{
         match self{
             Block::Normal(block)=>block.encode(),
@@ -267,7 +286,7 @@ pub(crate) struct BlockMeta{
     offset: usize,
     is_large: bool,
     first_key: Bytes,
-    last_key: Option<Bytes>,
+    last_key: Bytes,
 }
 
 impl BlockMeta{
@@ -281,23 +300,7 @@ impl BlockMeta{
     fn encode_meta(meta: &[BlockMeta], buf: &mut Vec<u8>){
         // the number of blocks
         let mut estimated_size = SIZE_OF_U32;
-        for m in meta{
-            // offset
-            estimated_size += SIZE_OF_U32;
-            // first key length
-            estimated_size += SIZE_OF_U16;
-            // first key
-            estimated_size += m.first_key.len();
-            // is large
-            estimated_size += SIZE_OF_U8;
-            if m.is_large{
-                continue;
-            }
-            // last key length
-            estimated_size += SIZE_OF_U16;
-            // last key
-            estimated_size += m.last_key.as_ref().unwrap().len();
-        }
+        estimated_size += meta.len() * (SIZE_OF_U32 + SIZE_OF_U8);
         // checksum
         estimated_size += SIZE_OF_U32;
 
@@ -306,16 +309,9 @@ impl BlockMeta{
         buf.put_u32(meta.len() as u32);
         for m in meta{
             buf.put_u32(m.offset as u32);
-            buf.put_u16(m.first_key.len() as u16);
-            buf.put_slice(&m.first_key);
             buf.put_u8(m.is_large as u8);
-            if m.is_large{
-                continue;
-            }
-            buf.put_u16(m.last_key.as_ref().unwrap().len() as u16);
-            buf.put_slice(m.last_key.as_ref().unwrap());
         }
-        let checksum = crc32fast::hash(&buf[ori_length+4..]);
+        let checksum = crc32fast::hash(&buf[ori_length+SIZE_OF_U32..]);
         buf.put_u32(checksum);
     }
 
@@ -325,16 +321,8 @@ impl BlockMeta{
         let checksum = crc32fast::hash(&buf[..buf.remaining() - SIZE_OF_U32]);
         for _ in 0..num{
             let offset = buf.get_u32() as usize;
-            let first_key_len = buf.get_u16() as usize;
-            let first_key = buf.copy_to_bytes(first_key_len);
             let is_large = buf.get_u8() == 1;
-            if is_large{
-                ans.push(BlockMeta{offset, is_large, first_key, last_key: None});
-                continue;
-            }
-            let last_key_len = buf.get_u16() as usize;
-            let last_key = buf.copy_to_bytes(last_key_len);
-            ans.push(BlockMeta{offset, is_large, first_key, last_key: Some(last_key)});
+            ans.push(BlockMeta{offset, is_large, first_key: Bytes::new(), last_key: Bytes::new()});
         }
         let checksum_read = buf.get_u32();
         if checksum != checksum_read{
@@ -397,11 +385,17 @@ impl SsTableBuilder{
         let block = builder.build();
         let encoded_bytes = block.encode();
         let is_large = block.is_large();
+        let first_key = std::mem::take(&mut self.first_key);
+        let last_key = std::mem::take(&mut self.last_key);
         let meta = BlockMeta{
             offset: self.data.len(),
             is_large,
-            first_key: std::mem::take(&mut self.first_key),
-            last_key: if is_large{None}else{Some(std::mem::take(&mut self.last_key))} ,
+            first_key: first_key.clone(),
+            last_key: if is_large{
+                first_key
+            }else{
+               last_key
+            } ,
         };
         self.meta.push(meta);
         self.data.extend_from_slice(&encoded_bytes);
@@ -421,7 +415,7 @@ impl SsTableBuilder{
         BlockMeta::encode_meta(&self.meta, &mut buf);
         buf.put_u32(meta_offset);
         let first_key = self.meta.first().map(|m|m.first_key.clone()).unwrap_or_default();
-        let last_key = self.meta.last().map(|m|m.last_key.clone().unwrap_or(self.meta.last().map(|m|m.first_key.clone()).unwrap_or_default())).unwrap_or_default();
+        let last_key = self.meta.last().map(|m|m.last_key.clone()).unwrap_or_default();
         SsTable { 
             data: Bytes::from(buf),
             first_key, 
@@ -492,10 +486,10 @@ impl SsTable{
         let data_len = bytes.len();
         let meta_offset = (&bytes[data_len-SIZE_OF_U32..]).get_u32() as usize;
         let raw_meta = &bytes[meta_offset..data_len-SIZE_OF_U32];
-        let meta = BlockMeta::decode_meta(raw_meta)?;
-        Self::check_block_checksum(&meta, &bytes, meta_offset)?;
+        let mut meta = BlockMeta::decode_meta(raw_meta)?;
+        Self::check_block_checksum_first_last_key(&mut meta, &bytes, meta_offset)?;
         let first_key = meta.first().map(|m|m.first_key.clone()).unwrap_or_default();
-        let last_key = meta.last().map(|m|m.last_key.clone().unwrap_or(meta.last().map(|m|m.first_key.clone()).unwrap_or_default())).unwrap_or_default();
+        let last_key = meta.last().map(|m|m.last_key.clone()).unwrap_or_default();
         let ans = Self { 
             data: bytes, 
             first_key,
@@ -508,7 +502,7 @@ impl SsTable{
         Ok(ans)
     }
 
-    fn check_block_checksum(meta: &[BlockMeta], bytes: &Bytes, meta_offset: usize)->LoroResult<()>{
+    fn check_block_checksum_first_last_key(meta: &mut [BlockMeta], bytes: &Bytes, meta_offset: usize)->LoroResult<()>{
         for i in 0..meta.len(){
             let offset = meta[i].offset;
             let offset_end = meta.get(i+1).map_or(meta_offset, |m| m.offset);
@@ -517,6 +511,11 @@ impl SsTable{
             if checksum != crc32fast::hash(&raw_block_and_check[..raw_block_and_check.len() - SIZE_OF_U32]){
                 return Err(LoroError::DecodeChecksumMismatchError);
             }
+        
+            let meta_mut = &mut meta[i];
+            let block = Block::decode(raw_block_and_check, meta_mut.is_large);
+            meta_mut.first_key = block.first_key();
+            meta_mut.last_key = block.last_key();
         }
         Ok(())
     }
@@ -529,7 +528,7 @@ impl SsTable{
 
     pub fn find_prev_block_idx(&self, key: &[u8])->usize{
         self.meta
-            .partition_point(|meta| meta.last_key.as_ref().unwrap_or(&meta.first_key) <= key)
+            .partition_point(|meta| meta.last_key.as_ref() <= key)
     }
 
     fn read_block(&self, block_idx: usize)->Arc<Block>{
@@ -719,7 +718,7 @@ impl<'a> SsTableIter<'a>{
             }
         }
         if let Some(key) = end_excluded {
-            if ans.is_prev_valid() &&  ans.prev_key() == key{
+            if ans.is_prev_valid() && ans.prev_key() == key{
                 ans.prev();
             }
         }
@@ -735,6 +734,7 @@ impl<'a> SsTableIter<'a>{
     }
 
     pub fn is_next_valid(&self)->bool{
+        // TODO: the next iter maybe not valid too
         self.next_block_iter.next_is_valid() || (self.next_block_idx as isize) < self.prev_block_idx
     }
 
@@ -750,6 +750,7 @@ impl<'a> SsTableIter<'a>{
         if self.next_first{
             self.next_block_iter.prev_is_valid()
         }else{
+            // TODO: the next prev iter maybe not valid too
             self.prev_block_iter.prev_is_valid() || (self.next_block_idx as isize) < self.prev_block_idx
         }
     }