diff --git a/eden/scm/lib/dag/src/idmap/indexedlog_idmap.rs b/eden/scm/lib/dag/src/idmap/indexedlog_idmap.rs
index 1e7064439e..571287d5df 100644
--- a/eden/scm/lib/dag/src/idmap/indexedlog_idmap.rs
+++ b/eden/scm/lib/dag/src/idmap/indexedlog_idmap.rs
@@ -11,6 +11,7 @@ use crate::errors::bug;
 use crate::errors::programming;
 use crate::id::{Group, Id, VertexName};
 use crate::ops::IdConvert;
+use crate::ops::Persist;
 use crate::ops::PrefixLookup;
 use crate::Result;
 use byteorder::{BigEndian, ReadBytesExt};
@@ -126,20 +127,10 @@ impl IdMap {
 
         // Take a filesystem lock. The file name 'lock' is taken by indexedlog
         // running on Windows, so we choose another file name here.
-        let lock_file = {
-            let mut path = self.path.clone();
-            path.push("wlock");
-            File::open(&path).or_else(|_| {
-                fs::OpenOptions::new()
-                    .write(true)
-                    .create_new(true)
-                    .open(&path)
-            })?
-        };
-        lock_file.lock_exclusive()?;
+        let lock_file = self.lock()?;
 
         // Reload. So we get latest data.
-        self.reload()?;
+        self.reload(&lock_file)?;
 
         Ok(SyncableIdMap {
             map: self,
@@ -147,15 +138,6 @@ impl IdMap {
         })
     }
 
-    /// Reload from the filesystem. Discard pending changes.
-    fn reload(&mut self) -> Result<()> {
-        self.log.clear_dirty()?;
-        self.log.sync()?;
-        // Invalidate the next free id cache.
-        self.cached_next_free_ids = Default::default();
-        Ok(())
-    }
-
     /// Find name by a specified integer id.
     pub fn find_name_by_id(&self, id: Id) -> Result<Option<&[u8]>> {
         let key = id.0.to_be_bytes();
@@ -391,6 +373,41 @@ impl IdMapWrite for IdMap {
     }
 }
 
+impl Persist for IdMap {
+    type Lock = File;
+
+    fn lock(&self) -> Result<Self::Lock> {
+        let lock_file = {
+            let mut path = self.path.clone();
+            path.push("wlock");
+            File::open(&path).or_else(|_| {
+                fs::OpenOptions::new()
+                    .write(true)
+                    .create_new(true)
+                    .open(&path)
+            })?
+        };
+        lock_file.lock_exclusive()?;
+        Ok(lock_file)
+    }
+
+    fn reload(&mut self, _lock: &Self::Lock) -> Result<()> {
+        self.log.clear_dirty()?;
+        self.log.sync()?;
+        // Invalidate the next free id cache.
+        self.cached_next_free_ids = Default::default();
+        Ok(())
+    }
+
+    fn persist(&mut self, _lock: &Self::Lock) -> Result<()> {
+        if self.need_rebuild_non_master {
+            return bug("cannot persist with re-assigned ids unresolved");
+        }
+        self.log.sync()?;
+        Ok(())
+    }
+}
+
 impl PrefixLookup for IdMap {
     fn vertexes_by_hex_prefix(&self, hex_prefix: &[u8], limit: usize) -> Result<Vec<VertexName>> {
         self.find_names_by_hex_prefix(hex_prefix, limit)
diff --git a/eden/scm/lib/dag/src/idmap/mem_idmap.rs b/eden/scm/lib/dag/src/idmap/mem_idmap.rs
index 4ddd44ae5a..2eb74037aa 100644
--- a/eden/scm/lib/dag/src/idmap/mem_idmap.rs
+++ b/eden/scm/lib/dag/src/idmap/mem_idmap.rs
@@ -8,6 +8,7 @@
 use super::IdMapWrite;
 use crate::id::{Group, Id, VertexName};
 use crate::ops::IdConvert;
+use crate::ops::Persist;
 use crate::ops::PrefixLookup;
 use crate::Result;
 use std::collections::{BTreeMap, HashMap};
@@ -91,6 +92,22 @@ impl IdMapWrite for MemIdMap {
     }
 }
 
+impl Persist for MemIdMap {
+    type Lock = ();
+
+    fn lock(&self) -> Result<Self::Lock> {
+        Ok(())
+    }
+
+    fn reload(&mut self, _lock: &Self::Lock) -> Result<()> {
+        Ok(())
+    }
+
+    fn persist(&mut self, _lock: &Self::Lock) -> Result<()> {
+        Ok(())
+    }
+}
+
 impl PrefixLookup for MemIdMap {
     fn vertexes_by_hex_prefix(&self, hex_prefix: &[u8], limit: usize) -> Result<Vec<VertexName>> {
         let start = VertexName::from_hex(hex_prefix)?;