diff --git a/kevo/common/fencepointers.py b/kevo/common/fencepointers.py
index d32c835..eb7bc83 100644
--- a/kevo/common/fencepointers.py
+++ b/kevo/common/fencepointers.py
@@ -1,12 +1,5 @@
-'''
-This now is implemented as a sorted dictionary (because I need the bisect_left/right) with base64/json-based ser/der.
-A better implementation would be: two arrays (one for keys one for values) so that I can binary-search on the keys, and binary encoding for ser/der.
-TODO rebuilding from string could be done linearly if the serialization is sorted, right now the sorteddict is being rebuilt from scratch so that should be fixed
-'''
-
 from sys import getsizeof
-import json
-from base64 import b64encode, b64decode
+import struct
 
 from sortedcontainers import SortedDict
 
@@ -17,38 +10,44 @@ def __init__(self, density_factor=20, from_str: str | None = None):
 
         self.density_factor = density_factor
         self.counter = 0
-
-        if type(from_str) is str:
-            data = json.loads(from_str) 
-            for k, v in data['pointers'].items():
-                self.pointers[b64decode(k)] = v
-            self.density_factor = data['density_factor']
-            self.counter = data['counter']
+        self.incr = 0
 
     def add(self, key: bytes, offset: int):
-        if self.counter % self.density_factor == 0:
+        if self.incr % self.density_factor == 0:
             self.pointers[key] = offset
+            self.counter += 1
+
+        self.incr += 1
 
-        self.counter += 1
-    
     def bisect(self, key: bytes):
         return self.pointers.bisect(key)
     
     def peekitem(self, idx):
         return self.pointers.peekitem(idx)
 
-    def serialize(self):
-        pointers = {}
+    def to_file_as_blob(self, fd, enc_len):
+        fd.write(struct.pack('<QQ', self.density_factor, self.counter))
         for k, v in self.pointers.items():
-            pointers[b64encode(k).decode()] = v
-        return json.dumps({
-            'pointers': pointers,
-            'density_factor': self.density_factor,
-            'counter': self.counter
-        })
+            fd.write(len(k).to_bytes(enc_len, byteorder='little'))
+            fd.write(k)
+            fd.write(v.to_bytes(8, byteorder='little'))
+
+    def _read_kv_pair(self, fd, enc_len):
+        key_len = int.from_bytes(fd.read(enc_len), byteorder='little')
+        key = fd.read(key_len)
+        value = int.from_bytes(fd.read(8), byteorder='little')
+        self.pointers[key] = value
+
+    def from_file_descriptor(self, fd, enc_len):
+        metadata = fd.read(16)
+        self.density_factor, self.counter = struct.unpack('<QQ', metadata)
+        cnt = self.counter
+        while cnt > 0:
+            self._read_kv_pair(fd, enc_len)
+            cnt -= 1
 
     def __len__(self):
-        return self.counter
+        return self.incr
 
     def __str__(self) -> str:
         return self.serialize()
diff --git a/kevo/engines/lsmtree.py b/kevo/engines/lsmtree.py
index 2ef67de..c6c5b36 100644
--- a/kevo/engines/lsmtree.py
+++ b/kevo/engines/lsmtree.py
@@ -13,15 +13,6 @@
 Run = namedtuple('Run', ['filter', 'pointers', 'nr_records'])
 
 
-def append_indices(file_descriptor, fence_pointers, bloom_filter, nr_records):
-    pointers_offset = file_descriptor.tell()
-    file_descriptor.write(fence_pointers.serialize().encode())
-    bloom_offset = file_descriptor.tell()
-    file_descriptor.write(bloom_filter.serialize().encode())
-    # pack two 8 byte unsigned ints for the offsets of the pointers and the bloom filter
-    file_descriptor.write(struct.pack('<QQQ', pointers_offset, bloom_offset, nr_records))
-
-
 class LSMTree(KVStore):
     name = 'LSMTree'
 
@@ -74,7 +65,6 @@ def __init__(self,
             self._rebuild_indices()
 
     def _rebuild_indices(self):
-        # TODO set the global version to the max of the discovered ones
         self.levels.clear()
         self.rfds.clear()
 
@@ -92,7 +82,8 @@ def _rebuild_indices(self):
                 offsets = run_file.read()
                 pointers_offset, bloom_offset, nr_records = struct.unpack('<QQQ', offsets)
                 run_file.seek(pointers_offset)
-                pointers = FencePointers(from_str=run_file.read(bloom_offset - pointers_offset).decode())
+                pointers = FencePointers()
+                pointers.from_file_descriptor(run_file, self.key_enc_len)
                 run_file.seek(bloom_offset)
                 bloom_filter = BloomFilter(from_str=run_file.read(bloom_end_offset - bloom_offset).decode())
 
@@ -220,7 +211,7 @@ def _merge(self, level_idx: int):
                             keys[i], values[i] = self._read_kv_pair(fds[i])
                             counters[i] += 1
 
-            append_indices(run_file, fence_pointers, bloom_filter, nr_records)
+            self._append_indices(run_file, fence_pointers, bloom_filter, nr_records)
 
         if level_idx + 1 >= len(self.rfds):
             self.rfds.append([])
@@ -268,7 +259,7 @@ def _flush(self):
                 self._write_kv_pair(run_file, k, v)
                 bloom_filter.add(k)
                 nr_records += 1
-            append_indices(run_file, fence_pointers, bloom_filter, nr_records)
+            self._append_indices(run_file, fence_pointers, bloom_filter, nr_records)
 
         self.memtable_bytes_count = 0
 
@@ -301,6 +292,14 @@ def restore(self, version=None):
                     rfd.close()
             self._rebuild_indices()
 
+    def _append_indices(self, file_descriptor, fence_pointers, bloom_filter, nr_records):
+        pointers_offset = file_descriptor.tell()
+        fence_pointers.to_file_as_blob(file_descriptor, self.key_enc_len)
+        bloom_offset = file_descriptor.tell()
+        file_descriptor.write(bloom_filter.serialize().encode())
+        # pack two 8 byte unsigned ints for the offsets of the pointers and the bloom filter
+        file_descriptor.write(struct.pack('<QQQ', pointers_offset, bloom_offset, nr_records))
+
     def __sizeof__(self):
         memtable_size = sum((getsizeof(k) + getsizeof(v) for k, v in self.memtable.items()))
         bloom_filters_size = sum((getsizeof(run.filter) for level in self.levels for run in level))
diff --git a/tests/test_fencepointers.py b/tests/test_fencepointers.py
index ac050d9..fee0f6a 100644
--- a/tests/test_fencepointers.py
+++ b/tests/test_fencepointers.py
@@ -1,18 +1,35 @@
+import os.path
 import unittest
 
 from random import Random
 from kevo.common import FencePointers
+from tempfile import TemporaryFile
 
 
 class TestFencePointers(unittest.TestCase):
+    tmpfile = 'tmpfile'
+
+    def tearDown(self):
+        if os.path.exists(self.tmpfile):
+            os.remove(self.tmpfile)
+
     def test_ser_der(self):
         rng = Random(1)
+
         fp1 = FencePointers()
-        for i in range(100):
-            randbytes = rng.randbytes(rng.randint(50, 100))
-            randint = rng.randint(0, 1000)
+
+        for i in range(1_000):
+            randbytes = rng.randbytes(rng.randint(0, 100))
+            randint = rng.randint(0, 10_000_000_000)
             fp1.add(randbytes, randint)
-        fp2 = FencePointers(from_str=fp1.serialize())
+
+        with open(self.tmpfile, 'wb') as f:
+            fp1.to_file_as_blob(f, 2)
+
+        fp2 = FencePointers()
+        with open(self.tmpfile, 'rb') as f:
+            fp2.from_file_descriptor(f, 2)
+
         self.assertEqual(fp1.pointers, fp2.pointers)