diff --git a/seaweed-volume/src/storage/mod.rs b/seaweed-volume/src/storage/mod.rs
index 125b0db14..e24141156 100644
--- a/seaweed-volume/src/storage/mod.rs
+++ b/seaweed-volume/src/storage/mod.rs
@@ -2,3 +2,4 @@ pub mod types;
 pub mod needle;
 pub mod super_block;
 pub mod idx;
+pub mod needle_map;
diff --git a/seaweed-volume/src/storage/needle_map.rs b/seaweed-volume/src/storage/needle_map.rs
new file mode 100644
index 000000000..09dc9d07d
--- /dev/null
+++ b/seaweed-volume/src/storage/needle_map.rs
@@ -0,0 +1,332 @@
+//! NeedleMapper: in-memory index mapping NeedleId → (Offset, Size).
+//!
+//! Loaded from .idx file on volume mount. Supports Get, Put, Delete with
+//! metrics tracking (file count, byte count, deleted count, deleted bytes).
+
+use std::collections::HashMap;
+use std::io::{self, Read, Seek, Write};
+use std::sync::atomic::{AtomicI64, AtomicU64, Ordering};
+
+use crate::storage::idx;
+use crate::storage::types::*;
+
+// ============================================================================
+// NeedleValue
+// ============================================================================
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct NeedleValue {
+    pub offset: Offset,
+    pub size: Size,
+}
+
+// ============================================================================
+// NeedleMapMetric
+// ============================================================================
+
+/// Metrics tracking for needle map operations.
+#[derive(Debug, Default)]
+pub struct NeedleMapMetric {
+    pub file_count: AtomicI64,
+    pub file_byte_count: AtomicU64,
+    pub deletion_count: AtomicI64,
+    pub deletion_byte_count: AtomicU64,
+    pub max_file_key: AtomicU64,
+}
+
+impl NeedleMapMetric {
+    /// Update metrics based on a Put operation.
+    fn on_put(&self, key: NeedleId, old: Option<&NeedleValue>, new_size: Size) {
+        if new_size.is_valid() {
+            if old.is_none() || !old.unwrap().size.is_valid() {
+                self.file_count.fetch_add(1, Ordering::Relaxed);
+            }
+            self.file_byte_count.fetch_add(new_size.0 as u64, Ordering::Relaxed);
+            if let Some(old_val) = old {
+                if old_val.size.is_valid() {
+                    self.file_byte_count.fetch_sub(old_val.size.0 as u64, Ordering::Relaxed);
+                }
+            }
+        }
+        let key_val: u64 = key.into();
+        loop {
+            let current = self.max_file_key.load(Ordering::Relaxed);
+            if key_val <= current {
+                break;
+            }
+            if self.max_file_key.compare_exchange(current, key_val, Ordering::Relaxed, Ordering::Relaxed).is_ok() {
+                break;
+            }
+        }
+    }
+
+    /// Update metrics based on a Delete operation.
+    fn on_delete(&self, old: &NeedleValue) {
+        if old.size.is_valid() {
+            self.deletion_count.fetch_add(1, Ordering::Relaxed);
+            self.deletion_byte_count.fetch_add(old.size.0 as u64, Ordering::Relaxed);
+            self.file_count.fetch_sub(1, Ordering::Relaxed);
+            self.file_byte_count.fetch_sub(old.size.0 as u64, Ordering::Relaxed);
+        }
+    }
+}
+
+// ============================================================================
+// NeedleMapKind
+// ============================================================================
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum NeedleMapKind {
+    InMemory,
+    LevelDb,
+    LevelDbMedium,
+    LevelDbLarge,
+}
+
+// ============================================================================
+// CompactNeedleMap (in-memory)
+// ============================================================================
+
+/// In-memory needle map backed by a HashMap.
+/// The .idx file is kept open for append-only writes.
+pub struct CompactNeedleMap {
+    map: HashMap<NeedleId, NeedleValue>,
+    metric: NeedleMapMetric,
+    idx_file: Option<Box<dyn IdxFileWriter>>,
+    idx_file_offset: u64,
+}
+
+/// Trait for appending to an index file.
+pub trait IdxFileWriter: Write + Send + Sync {
+    fn sync_all(&self) -> io::Result<()>;
+}
+
+impl IdxFileWriter for std::fs::File {
+    fn sync_all(&self) -> io::Result<()> {
+        std::fs::File::sync_all(self)
+    }
+}
+
+impl CompactNeedleMap {
+    /// Create a new empty in-memory map.
+    pub fn new() -> Self {
+        CompactNeedleMap {
+            map: HashMap::new(),
+            metric: NeedleMapMetric::default(),
+            idx_file: None,
+            idx_file_offset: 0,
+        }
+    }
+
+    /// Load from an .idx file, building the in-memory map.
+    pub fn load_from_idx<R: Read + Seek>(reader: &mut R) -> io::Result<Self> {
+        let mut nm = CompactNeedleMap::new();
+        idx::walk_index_file(reader, 0, |key, offset, size| {
+            if !offset.is_zero() || !size.is_deleted() {
+                nm.set(key, NeedleValue { offset, size });
+            } else {
+                nm.delete_from_map(key);
+            }
+            Ok(())
+        })?;
+        Ok(nm)
+    }
+
+    /// Set the index file for append-only writes.
+    pub fn set_idx_file(&mut self, file: Box<dyn IdxFileWriter>, offset: u64) {
+        self.idx_file = Some(file);
+        self.idx_file_offset = offset;
+    }
+
+    // ---- Map operations ----
+
+    /// Insert or update an entry. Appends to .idx file if present.
+    pub fn put(&mut self, key: NeedleId, offset: Offset, size: Size) -> io::Result<()> {
+        let old = self.map.get(&key).cloned();
+        let nv = NeedleValue { offset, size };
+        self.metric.on_put(key, old.as_ref(), size);
+        self.map.insert(key, nv);
+
+        // Append to index file
+        if let Some(ref mut idx_file) = self.idx_file {
+            idx::write_index_entry(idx_file, key, offset, size)?;
+            self.idx_file_offset += NEEDLE_MAP_ENTRY_SIZE as u64;
+        }
+        Ok(())
+    }
+
+    /// Look up a needle.
+    pub fn get(&self, key: NeedleId) -> Option<&NeedleValue> {
+        self.map.get(&key)
+    }
+
+    /// Mark a needle as deleted. Appends tombstone to .idx file.
+    pub fn delete(&mut self, key: NeedleId, offset: Offset) -> io::Result<Option<Size>> {
+        if let Some(old) = self.map.get(&key).cloned() {
+            if old.size.is_valid() {
+                self.metric.on_delete(&old);
+                let deleted_size = Size(-(old.size.0));
+                self.map.insert(key, NeedleValue { offset, size: deleted_size });
+
+                // Append tombstone to index file
+                if let Some(ref mut idx_file) = self.idx_file {
+                    idx::write_index_entry(idx_file, key, offset, TOMBSTONE_FILE_SIZE)?;
+                    self.idx_file_offset += NEEDLE_MAP_ENTRY_SIZE as u64;
+                }
+                return Ok(Some(old.size));
+            }
+        }
+        Ok(None)
+    }
+
+    // ---- Internal helpers ----
+
+    /// Insert into map during loading (no idx file write).
+    fn set(&mut self, key: NeedleId, nv: NeedleValue) {
+        let old = self.map.get(&key).cloned();
+        self.metric.on_put(key, old.as_ref(), nv.size);
+        self.map.insert(key, nv);
+    }
+
+    /// Remove from map during loading (handle deletions in idx walk).
+    fn delete_from_map(&mut self, key: NeedleId) {
+        if let Some(old) = self.map.get(&key).cloned() {
+            if old.size.is_valid() {
+                self.metric.on_delete(&old);
+            }
+        }
+        self.map.remove(&key);
+    }
+
+    // ---- Metrics accessors ----
+
+    pub fn content_size(&self) -> u64 {
+        self.metric.file_byte_count.load(Ordering::Relaxed)
+    }
+
+    pub fn deleted_size(&self) -> u64 {
+        self.metric.deletion_byte_count.load(Ordering::Relaxed)
+    }
+
+    pub fn file_count(&self) -> i64 {
+        self.metric.file_count.load(Ordering::Relaxed)
+    }
+
+    pub fn deleted_count(&self) -> i64 {
+        self.metric.deletion_count.load(Ordering::Relaxed)
+    }
+
+    pub fn max_file_key(&self) -> NeedleId {
+        NeedleId(self.metric.max_file_key.load(Ordering::Relaxed))
+    }
+
+    pub fn index_file_size(&self) -> u64 {
+        self.idx_file_offset
+    }
+
+    /// Sync index file to disk.
+    pub fn sync(&self) -> io::Result<()> {
+        if let Some(ref idx_file) = self.idx_file {
+            idx_file.sync_all()?;
+        }
+        Ok(())
+    }
+
+    /// Close index file.
+    pub fn close(&mut self) {
+        let _ = self.sync();
+        self.idx_file = None;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Cursor;
+
+    #[test]
+    fn test_needle_map_put_get() {
+        let mut nm = CompactNeedleMap::new();
+        nm.put(NeedleId(1), Offset::from_actual_offset(0), Size(100)).unwrap();
+        nm.put(NeedleId(2), Offset::from_actual_offset(128), Size(200)).unwrap();
+
+        let v1 = nm.get(NeedleId(1)).unwrap();
+        assert_eq!(v1.size, Size(100));
+
+        let v2 = nm.get(NeedleId(2)).unwrap();
+        assert_eq!(v2.size, Size(200));
+
+        assert!(nm.get(NeedleId(99)).is_none());
+    }
+
+    #[test]
+    fn test_needle_map_delete() {
+        let mut nm = CompactNeedleMap::new();
+        nm.put(NeedleId(1), Offset::from_actual_offset(0), Size(100)).unwrap();
+
+        assert_eq!(nm.file_count(), 1);
+        assert_eq!(nm.content_size(), 100);
+
+        let deleted = nm.delete(NeedleId(1), Offset::from_actual_offset(0)).unwrap();
+        assert_eq!(deleted, Some(Size(100)));
+
+        assert_eq!(nm.file_count(), 0);
+        assert_eq!(nm.deleted_count(), 1);
+        assert_eq!(nm.deleted_size(), 100);
+    }
+
+    #[test]
+    fn test_needle_map_metrics() {
+        let mut nm = CompactNeedleMap::new();
+        nm.put(NeedleId(1), Offset::from_actual_offset(0), Size(100)).unwrap();
+        nm.put(NeedleId(2), Offset::from_actual_offset(128), Size(200)).unwrap();
+        nm.put(NeedleId(3), Offset::from_actual_offset(384), Size(300)).unwrap();
+
+        assert_eq!(nm.file_count(), 3);
+        assert_eq!(nm.content_size(), 600);
+        assert_eq!(nm.max_file_key(), NeedleId(3));
+
+        // Update existing
+        nm.put(NeedleId(2), Offset::from_actual_offset(700), Size(250)).unwrap();
+        assert_eq!(nm.file_count(), 3); // still 3
+        assert_eq!(nm.content_size(), 650); // 100 + 250 + 300
+
+        // Delete
+        nm.delete(NeedleId(1), Offset::from_actual_offset(0)).unwrap();
+        assert_eq!(nm.file_count(), 2);
+        assert_eq!(nm.deleted_count(), 1);
+    }
+
+    #[test]
+    fn test_needle_map_load_from_idx() {
+        // Build an idx file in memory
+        let mut idx_data = Vec::new();
+        idx::write_index_entry(&mut idx_data, NeedleId(1), Offset::from_actual_offset(0), Size(100)).unwrap();
+        idx::write_index_entry(&mut idx_data, NeedleId(2), Offset::from_actual_offset(128), Size(200)).unwrap();
+        idx::write_index_entry(&mut idx_data, NeedleId(3), Offset::from_actual_offset(384), Size(300)).unwrap();
+        // Delete needle 2
+        idx::write_index_entry(&mut idx_data, NeedleId(2), Offset::default(), TOMBSTONE_FILE_SIZE).unwrap();
+
+        let mut cursor = Cursor::new(idx_data);
+        let nm = CompactNeedleMap::load_from_idx(&mut cursor).unwrap();
+
+        assert!(nm.get(NeedleId(1)).is_some());
+        assert!(nm.get(NeedleId(2)).is_none()); // deleted
+        assert!(nm.get(NeedleId(3)).is_some());
+        assert_eq!(nm.file_count(), 2);
+    }
+
+    #[test]
+    fn test_needle_map_double_delete() {
+        let mut nm = CompactNeedleMap::new();
+        nm.put(NeedleId(1), Offset::from_actual_offset(0), Size(100)).unwrap();
+
+        let r1 = nm.delete(NeedleId(1), Offset::from_actual_offset(0)).unwrap();
+        assert_eq!(r1, Some(Size(100)));
+
+        // Second delete should return None (already deleted)
+        let r2 = nm.delete(NeedleId(1), Offset::from_actual_offset(0)).unwrap();
+        assert_eq!(r2, None);
+        assert_eq!(nm.deleted_count(), 1); // not double counted
+    }
+}