diff --git a/rust/hg-core/src/dirstate_tree/dirstate_map.rs b/rust/hg-core/src/dirstate_tree/dirstate_map.rs
--- a/rust/hg-core/src/dirstate_tree/dirstate_map.rs
+++ b/rust/hg-core/src/dirstate_tree/dirstate_map.rs
@@ -4,17 +4,15 @@
 use std::convert::TryInto;
 use std::path::PathBuf;
 
-use super::on_disk::V2_FORMAT_MARKER;
+use super::on_disk;
 use super::path_with_basename::WithBasename;
 use crate::dirstate::parsers::clear_ambiguous_mtime;
 use crate::dirstate::parsers::pack_entry;
 use crate::dirstate::parsers::packed_entry_size;
 use crate::dirstate::parsers::parse_dirstate_entries;
 use crate::dirstate::parsers::Timestamp;
-use crate::errors::HgError;
 use crate::matchers::Matcher;
 use crate::utils::hg_path::{HgPath, HgPathBuf};
-use crate::utils::SliceExt;
 use crate::CopyMapIter;
 use crate::DirstateEntry;
 use crate::DirstateError;
@@ -30,16 +28,16 @@
 
 pub struct DirstateMap<'on_disk> {
     /// Contents of the `.hg/dirstate` file
-    on_disk: &'on_disk [u8],
+    pub(super) on_disk: &'on_disk [u8],
 
     pub(super) root: ChildNodes<'on_disk>,
 
     /// Number of nodes anywhere in the tree that have `.entry.is_some()`.
-    nodes_with_entry_count: usize,
+    pub(super) nodes_with_entry_count: u32,
 
     /// Number of nodes anywhere in the tree that have
     /// `.copy_source.is_some()`.
-    nodes_with_copy_source_count: usize,
+    pub(super) nodes_with_copy_source_count: u32,
 }
 
 /// Using a plain `HgPathBuf` of the full path from the repository root as a
@@ -62,7 +60,7 @@
     pub(super) children: ChildNodes<'on_disk>,
 
     /// How many (non-inclusive) descendants of this node are tracked files
-    tracked_descendants_count: usize,
+    pub(super) tracked_descendants_count: u32,
 }
 
 impl<'on_disk> Node<'on_disk> {
@@ -89,32 +87,27 @@
 );
 
 impl<'on_disk> DirstateMap<'on_disk> {
+    pub(super) fn empty(on_disk: &'on_disk [u8]) -> Self {
+        Self {
+            on_disk,
+            root: ChildNodes::default(),
+            nodes_with_entry_count: 0,
+            nodes_with_copy_source_count: 0,
+        }
+    }
+
     #[timed]
     pub fn new_v2(
         on_disk: &'on_disk [u8],
     ) -> Result<(Self, Option<DirstateParents>), DirstateError> {
-        if let Some(rest) = on_disk.drop_prefix(V2_FORMAT_MARKER) {
-            Self::new_v1(rest)
-        } else if on_disk.is_empty() {
-            Self::new_v1(on_disk)
-        } else {
-            return Err(HgError::corrupted(
-                "missing dirstate-v2 magic number",
-            )
-            .into());
-        }
+        on_disk::read(on_disk)
     }
 
     #[timed]
     pub fn new_v1(
         on_disk: &'on_disk [u8],
     ) -> Result<(Self, Option<DirstateParents>), DirstateError> {
-        let mut map = Self {
-            on_disk,
-            root: ChildNodes::default(),
-            nodes_with_entry_count: 0,
-            nodes_with_copy_source_count: 0,
-        };
+        let mut map = Self::empty(on_disk);
         if map.on_disk.is_empty() {
             return Ok((map, None));
         }
@@ -565,10 +558,7 @@
         parents: DirstateParents,
         now: Timestamp,
     ) -> Result<Vec<u8>, DirstateError> {
-        // Inefficient but temporary
-        let mut v2 = V2_FORMAT_MARKER.to_vec();
-        v2.append(&mut self.pack_v1(parents, now)?);
-        Ok(v2)
+        on_disk::write(self, parents, now)
     }
 
     fn set_all_dirs(&mut self) -> Result<(), DirstateMapError> {
@@ -595,7 +585,7 @@
     }
 
     fn copy_map_len(&self) -> usize {
-        self.nodes_with_copy_source_count
+        self.nodes_with_copy_source_count as usize
     }
 
     fn copy_map_iter(&self) -> CopyMapIter<'_> {
@@ -646,7 +636,7 @@
     }
 
     fn len(&self) -> usize {
-        self.nodes_with_entry_count
+        self.nodes_with_entry_count as usize
     }
 
     fn contains_key(&self, key: &HgPath) -> bool {
diff --git a/rust/hg-core/src/dirstate_tree/on_disk.rs b/rust/hg-core/src/dirstate_tree/on_disk.rs
--- a/rust/hg-core/src/dirstate_tree/on_disk.rs
+++ b/rust/hg-core/src/dirstate_tree/on_disk.rs
@@ -1,4 +1,335 @@
+//! The "version 2" disk representation of the dirstate
+//!
+//! # File format
+//!
+//! The file starts with a fixed-sized header, whose layout is defined by the
+//! `Header` struct. Its `root` field contains the slice (offset and length) to
+//! the nodes representing the files and directories at the root of the
+//! repository. Each node is also fixed-size, defined by the `Node` struct.
+//! Nodes in turn contain slices to variable-size paths, and to their own child
+//! nodes (if any) for nested files and directories.
+
+use crate::dirstate::parsers::clear_ambiguous_mtime;
+use crate::dirstate::parsers::Timestamp;
+use crate::dirstate_tree::dirstate_map::{self, DirstateMap};
+use crate::dirstate_tree::path_with_basename::WithBasename;
+use crate::errors::HgError;
+use crate::utils::hg_path::HgPath;
+use crate::DirstateEntry;
+use crate::DirstateError;
+use crate::DirstateParents;
+use bytes_cast::unaligned::{I32Be, U32Be, U64Be};
+use bytes_cast::BytesCast;
+use std::borrow::Cow;
+use std::convert::{TryFrom, TryInto};
+
 /// Added at the start of `.hg/dirstate` when the "v2" format is used.
-/// Acts like a "magic number". This is a sanity check, not strictly necessary
-/// since `.hg/requires` already governs which format should be used.
+/// This a redundant sanity check more than an actual "magic number" since
+/// `.hg/requires` already governs which format should be used.
 pub const V2_FORMAT_MARKER: &[u8; 12] = b"dirstate-v2\n";
+
+#[derive(BytesCast)]
+#[repr(C)]
+struct Header {
+    marker: [u8; V2_FORMAT_MARKER.len()],
+
+    /// `dirstatemap.parents()` in `mercurial/dirstate.py` relies on this
+    /// `parents` field being at this offset, immediately after `marker`.
+    parents: DirstateParents,
+
+    root: ChildNodes,
+    nodes_with_entry_count: Size,
+    nodes_with_copy_source_count: Size,
+}
+
+#[derive(BytesCast)]
+#[repr(C)]
+struct Node {
+    full_path: PathSlice,
+
+    /// In bytes from `self.full_path.start`
+    base_name_start: Size,
+
+    copy_source: OptPathSlice,
+    entry: OptEntry,
+    children: ChildNodes,
+    tracked_descendants_count: Size,
+}
+
+/// Either nothing if `state == b'\0'`, or a dirstate entry like in the v1
+/// format
+#[derive(BytesCast)]
+#[repr(C)]
+struct OptEntry {
+    state: u8,
+    mode: I32Be,
+    mtime: I32Be,
+    size: I32Be,
+}
+
+/// Counted in bytes from the start of the file
+///
+/// NOTE: If we decide to never support `.hg/dirstate` files larger than 4 GiB
+/// we could save space by using `U32Be` instead.
+type Offset = U64Be;
+
+/// Counted in number of items
+///
+/// NOTE: not supporting directories with more than 4 billion direct children,
+/// or filenames more than 4 GiB.
+type Size = U32Be;
+
+/// Location of consecutive, fixed-size items.
+///
+/// An item can be a single byte for paths, or a struct with
+/// `derive(BytesCast)`.
+#[derive(BytesCast, Copy, Clone)]
+#[repr(C)]
+struct Slice {
+    start: Offset,
+    len: Size,
+}
+
+/// A contiguous sequence of `len` times `Node`, representing the child nodes
+/// of either some other node or of the repository root.
+///
+/// Always sorted by ascending `full_path`, to allow binary search.
+/// Since nodes with the same parent nodes also have the same parent path,
+/// only the `base_name`s need to be compared during binary search.
+type ChildNodes = Slice;
+
+/// A `HgPath` of `len` bytes
+type PathSlice = Slice;
+
+/// Either nothing if `start == 0`, or a `HgPath` of `len` bytes
+type OptPathSlice = Slice;
+
+/// Make sure that size-affecting changes are made knowingly
+fn _static_assert_size_of() {
+    let _ = std::mem::transmute::<Header, [u8; 72]>;
+    let _ = std::mem::transmute::<Node, [u8; 57]>;
+}
+
+pub(super) fn read<'on_disk>(
+    on_disk: &'on_disk [u8],
+) -> Result<(DirstateMap<'on_disk>, Option<DirstateParents>), DirstateError> {
+    if on_disk.is_empty() {
+        return Ok((DirstateMap::empty(on_disk), None));
+    }
+    let (header, _) = Header::from_bytes(on_disk)
+        .map_err(|_| HgError::corrupted("truncated dirstate-v2"))?;
+    let Header {
+        marker,
+        parents,
+        root,
+        nodes_with_entry_count,
+        nodes_with_copy_source_count,
+    } = header;
+    if marker != V2_FORMAT_MARKER {
+        return Err(HgError::corrupted("missing dirstated-v2 marker").into());
+    }
+    let dirstate_map = DirstateMap {
+        on_disk,
+        root: read_nodes(on_disk, *root)?,
+        nodes_with_entry_count: nodes_with_entry_count.get(),
+        nodes_with_copy_source_count: nodes_with_copy_source_count.get(),
+    };
+    let parents = Some(parents.clone());
+    Ok((dirstate_map, parents))
+}
+
+impl Node {
+    pub(super) fn path<'on_disk>(
+        &self,
+        on_disk: &'on_disk [u8],
+    ) -> Result<dirstate_map::NodeKey<'on_disk>, HgError> {
+        let full_path = read_hg_path(on_disk, self.full_path)?;
+        let base_name_start = usize::try_from(self.base_name_start.get())
+            // u32 -> usize, could only panic on a 16-bit CPU
+            .expect("dirstate-v2 base_name_start out of bounds");
+        if base_name_start < full_path.len() {
+            Ok(WithBasename::from_raw_parts(full_path, base_name_start))
+        } else {
+            Err(HgError::corrupted(
+                "dirstate-v2 base_name_start out of bounds",
+            ))
+        }
+    }
+
+    pub(super) fn copy_source<'on_disk>(
+        &self,
+        on_disk: &'on_disk [u8],
+    ) -> Result<Option<Cow<'on_disk, HgPath>>, HgError> {
+        Ok(if self.copy_source.start.get() != 0 {
+            Some(read_hg_path(on_disk, self.copy_source)?)
+        } else {
+            None
+        })
+    }
+
+    pub(super) fn entry(&self) -> Result<Option<DirstateEntry>, HgError> {
+        Ok(if self.entry.state != b'\0' {
+            Some(DirstateEntry {
+                state: self.entry.state.try_into()?,
+                mode: self.entry.mode.get(),
+                mtime: self.entry.mtime.get(),
+                size: self.entry.size.get(),
+            })
+        } else {
+            None
+        })
+    }
+
+    pub(super) fn to_in_memory_node<'on_disk>(
+        &self,
+        on_disk: &'on_disk [u8],
+    ) -> Result<dirstate_map::Node<'on_disk>, HgError> {
+        Ok(dirstate_map::Node {
+            children: read_nodes(on_disk, self.children)?,
+            copy_source: self.copy_source(on_disk)?,
+            entry: self.entry()?,
+            tracked_descendants_count: self.tracked_descendants_count.get(),
+        })
+    }
+}
+
+fn read_nodes(
+    on_disk: &[u8],
+    slice: ChildNodes,
+) -> Result<dirstate_map::ChildNodes, HgError> {
+    read_slice::<Node>(on_disk, slice)?
+        .iter()
+        .map(|node| {
+            Ok((node.path(on_disk)?, node.to_in_memory_node(on_disk)?))
+        })
+        .collect()
+}
+
+fn read_hg_path(on_disk: &[u8], slice: Slice) -> Result<Cow<HgPath>, HgError> {
+    let bytes = read_slice::<u8>(on_disk, slice)?;
+    Ok(Cow::Borrowed(HgPath::new(bytes)))
+}
+
+fn read_slice<T>(on_disk: &[u8], slice: Slice) -> Result<&[T], HgError>
+where
+    T: BytesCast,
+{
+    // Either `usize::MAX` would result in "out of bounds" error since a single
+    // `&[u8]` cannot occupy the entire addess space.
+    let start = usize::try_from(slice.start.get()).unwrap_or(std::usize::MAX);
+    let len = usize::try_from(slice.len.get()).unwrap_or(std::usize::MAX);
+    on_disk
+        .get(start..)
+        .and_then(|bytes| T::slice_from_bytes(bytes, len).ok())
+        .map(|(slice, _rest)| slice)
+        .ok_or_else(|| {
+            HgError::corrupted("dirstate v2 slice is out of bounds")
+        })
+}
+
+pub(super) fn write(
+    dirstate_map: &mut DirstateMap,
+    parents: DirstateParents,
+    now: Timestamp,
+) -> Result<Vec<u8>, DirstateError> {
+    // TODO: how do we want to handle this in 2038?
+    let now: i32 = now.0.try_into().expect("time overflow");
+
+    let header_len = std::mem::size_of::<Header>();
+
+    // This ignores the space for paths, and for nodes without an entry.
+    // TODO: better estimate? Skip the `Vec` and write to a file directly?
+    let size_guess = header_len
+        + std::mem::size_of::<Node>()
+            * dirstate_map.nodes_with_entry_count as usize;
+    let mut out = Vec::with_capacity(size_guess);
+
+    // Keep space for the header. We’ll fill it out at the end when we know the
+    // actual offset for the root nodes.
+    out.resize(header_len, 0_u8);
+
+    let root = write_nodes(&mut dirstate_map.root, now, &mut out)?;
+
+    let header = Header {
+        marker: *V2_FORMAT_MARKER,
+        parents: parents,
+        root,
+        nodes_with_entry_count: dirstate_map.nodes_with_entry_count.into(),
+        nodes_with_copy_source_count: dirstate_map
+            .nodes_with_copy_source_count
+            .into(),
+    };
+    out[..header_len].copy_from_slice(header.as_bytes());
+    Ok(out)
+}
+
+/// Serialize the dirstate to the `v2` format after clearing ambigous `mtime`s.
+fn write_nodes(
+    nodes: &mut dirstate_map::ChildNodes,
+    now: i32,
+    out: &mut Vec<u8>,
+) -> Result<ChildNodes, DirstateError> {
+    // `dirstate_map::ChildNodes` is a `HashMap` with undefined iteration
+    // order. Sort to enable binary search in the written file.
+    let nodes = dirstate_map::Node::sorted(nodes);
+
+    // First accumulate serialized nodes in a `Vec`
+    let mut on_disk_nodes = Vec::with_capacity(nodes.len());
+    for (full_path, node) in nodes {
+        on_disk_nodes.push(Node {
+            children: write_nodes(&mut node.children, now, out)?,
+            tracked_descendants_count: node.tracked_descendants_count.into(),
+            full_path: write_slice::<u8>(
+                full_path.full_path().as_bytes(),
+                out,
+            ),
+            base_name_start: u32::try_from(full_path.base_name_start())
+                // Could only panic for paths over 4 GiB
+                .expect("dirstate-v2 offset overflow")
+                .into(),
+            copy_source: if let Some(source) = &node.copy_source {
+                write_slice::<u8>(source.as_bytes(), out)
+            } else {
+                Slice {
+                    start: 0.into(),
+                    len: 0.into(),
+                }
+            },
+            entry: if let Some(entry) = &mut node.entry {
+                clear_ambiguous_mtime(entry, now);
+                OptEntry {
+                    state: entry.state.into(),
+                    mode: entry.mode.into(),
+                    mtime: entry.mtime.into(),
+                    size: entry.size.into(),
+                }
+            } else {
+                OptEntry {
+                    state: b'\0',
+                    mode: 0.into(),
+                    mtime: 0.into(),
+                    size: 0.into(),
+                }
+            },
+        })
+    }
+    // … so we can write them contiguously
+    Ok(write_slice::<Node>(&on_disk_nodes, out))
+}
+
+fn write_slice<T>(slice: &[T], out: &mut Vec<u8>) -> Slice
+where
+    T: BytesCast,
+{
+    let start = u64::try_from(out.len())
+        // Could only panic on a 128-bit CPU with a dirstate over 16 EiB
+        .expect("dirstate-v2 offset overflow")
+        .into();
+    let len = u32::try_from(slice.len())
+        // Could only panic for paths over 4 GiB or nodes with over 4 billions
+        // child nodes
+        .expect("dirstate-v2 offset overflow")
+        .into();
+    out.extend(slice.as_bytes());
+    Slice { start, len }
+}
diff --git a/rust/hg-core/src/dirstate_tree/path_with_basename.rs b/rust/hg-core/src/dirstate_tree/path_with_basename.rs
--- a/rust/hg-core/src/dirstate_tree/path_with_basename.rs
+++ b/rust/hg-core/src/dirstate_tree/path_with_basename.rs
@@ -24,18 +24,29 @@
     }
 }
 
+fn find_base_name_start(full_path: &HgPath) -> usize {
+    if let Some(last_slash_position) =
+        full_path.as_bytes().iter().rposition(|&byte| byte == b'/')
+    {
+        last_slash_position + 1
+    } else {
+        0
+    }
+}
+
 impl<T: AsRef<HgPath>> WithBasename<T> {
     pub fn new(full_path: T) -> Self {
-        let base_name_start = if let Some(last_slash_position) = full_path
-            .as_ref()
-            .as_bytes()
-            .iter()
-            .rposition(|&byte| byte == b'/')
-        {
-            last_slash_position + 1
-        } else {
-            0
-        };
+        Self {
+            base_name_start: find_base_name_start(full_path.as_ref()),
+            full_path,
+        }
+    }
+
+    pub fn from_raw_parts(full_path: T, base_name_start: usize) -> Self {
+        debug_assert_eq!(
+            base_name_start,
+            find_base_name_start(full_path.as_ref())
+        );
         Self {
             base_name_start,
             full_path,
@@ -47,6 +58,10 @@
             &self.full_path.as_ref().as_bytes()[self.base_name_start..],
         )
     }
+
+    pub fn base_name_start(&self) -> usize {
+        self.base_name_start
+    }
 }
 
 impl<T: AsRef<HgPath>> Borrow<HgPath> for WithBasename<T> {