diff --git a/rust/hg-core/src/revlog/nodemap.rs b/rust/hg-core/src/revlog/nodemap.rs
--- a/rust/hg-core/src/revlog/nodemap.rs
+++ b/rust/hg-core/src/revlog/nodemap.rs
@@ -17,8 +17,10 @@
 };
 
 use std::fmt;
+use std::mem;
 use std::ops::Deref;
 use std::ops::Index;
+use std::slice;
 
 #[derive(Debug, PartialEq)]
 pub enum NodeMapError {
@@ -175,6 +177,8 @@
 #[derive(Clone, PartialEq)]
 pub struct Block([RawElement; 16]);
 
+pub const BLOCK_SIZE: usize = mem::size_of::<Block>();
+
 impl Block {
     fn new() -> Self {
         Block([-1; 16])
@@ -262,6 +266,56 @@
         }
     }
 
+    /// Create from an opaque bunch of bytes
+    ///
+    /// The created `NodeTreeBytes` from `buffer`,
+    /// of which exactly `amount` bytes are used.
+    ///
+    /// - `buffer` could be derived from `PyBuffer` and `Mmap` objects.
+    /// - `offset` allows for the final file format to include fixed data
+    ///   (generation number, behavioural flags)
+    /// - `amount` is expressed in bytes, and is not automatically derived from
+    ///   `bytes`, so that a caller that manages them atomically can perform
+    ///   temporary disk serializations and still rollback easily if needed.
+    ///   First use-case for this would be to support Mercurial shell hooks.
+    ///
+    /// panics if `buffer` is smaller than `amount`
+    pub fn load_bytes(
+        bytes: Box<dyn Deref<Target = [u8]> + Send>,
+        amount: usize,
+    ) -> Self {
+        NodeTree::new(Box::new(NodeTreeBytes::new(bytes, amount)))
+    }
+
+    /// Retrieve added `Block` and the original immutable data
+    pub fn into_readonly_and_added(
+        self,
+    ) -> (Box<dyn Deref<Target = [Block]> + Send>, Vec<Block>) {
+        let mut vec = self.growable;
+        let readonly = self.readonly;
+        if readonly.last() != Some(&self.root) {
+            vec.push(self.root);
+        }
+        (readonly, vec)
+    }
+
+    /// Retrieve added `Blocks` as bytes, ready to be written to persistent
+    /// storage
+    pub fn into_readonly_and_added_bytes(
+        self,
+    ) -> (Box<dyn Deref<Target = [Block]> + Send>, Vec<u8>) {
+        let (readonly, vec) = self.into_readonly_and_added();
+        let bytes = unsafe {
+            Vec::from_raw_parts(
+                vec.as_ptr() as *mut u8,
+                vec.len() * BLOCK_SIZE,
+                vec.capacity() * BLOCK_SIZE,
+            )
+        };
+        mem::forget(vec);
+        (readonly, bytes)
+    }
+
     /// Total number of blocks
     fn len(&self) -> usize {
         self.readonly.len() + self.growable.len() + 1
@@ -410,6 +464,38 @@
     }
 }
 
+pub struct NodeTreeBytes {
+    buffer: Box<dyn Deref<Target = [u8]> + Send>,
+    len_in_blocks: usize,
+}
+
+impl NodeTreeBytes {
+    fn new(
+        buffer: Box<dyn Deref<Target = [u8]> + Send>,
+        amount: usize,
+    ) -> Self {
+        assert!(buffer.len() >= amount);
+        let len_in_blocks = amount / BLOCK_SIZE;
+        NodeTreeBytes {
+            buffer,
+            len_in_blocks,
+        }
+    }
+}
+
+impl Deref for NodeTreeBytes {
+    type Target = [Block];
+
+    fn deref(&self) -> &[Block] {
+        unsafe {
+            slice::from_raw_parts(
+                (&self.buffer).as_ptr() as *const Block,
+                self.len_in_blocks,
+            )
+        }
+    }
+}
+
 struct NodeTreeVisitor<'n, 'p> {
     nt: &'n NodeTree,
     prefix: NodePrefixRef<'p>,
@@ -786,4 +872,30 @@
 
         Ok(())
     }
+
+    #[test]
+    fn test_into_added_empty() {
+        assert!(sample_nodetree().into_readonly_and_added().1.is_empty());
+        assert!(sample_nodetree()
+            .into_readonly_and_added_bytes()
+            .1
+            .is_empty());
+    }
+
+    #[test]
+    fn test_into_added_bytes() -> Result<(), NodeMapError> {
+        let mut idx = TestNtIndex::new();
+        idx.insert(0, "1234")?;
+        let mut idx = idx.commit();
+        idx.insert(4, "cafe")?;
+        let (_, bytes) = idx.nt.into_readonly_and_added_bytes();
+
+        // only the root block has been changed
+        assert_eq!(bytes.len(), BLOCK_SIZE);
+        // big endian for -2
+        assert_eq!(&bytes[4..2 * 4], [255, 255, 255, 254]);
+        // big endian for -6
+        assert_eq!(&bytes[12 * 4..13 * 4], [255, 255, 255, 250]);
+        Ok(())
+    }
 }