buffer header writes. fallocate is a future.

Buffering headers won't work in incremental mode, so preserve the old
path

tensorizer/serialization.py

@@ -4449,7 +4449,7 @@ def do_commit(
             write_specs: Sequence[TensorSerializer._WriteSpec],
             dependencies: Sequence[_Future],
         ):
-
+            # Fast version: makes one buffer containing the size, metadata, and headers, and writes it one go
             header_block_size = self._header_cur - self._metadata_start
             header_buffer = bytearray(header_block_size)
 
@@ -4478,15 +4478,50 @@ def do_commit(
                 header_buffer, metadata_start, verify=header_block_size
             )
 
-        deps = [
-            w.tensor_data_task
-            for w in write_specs_
-            if w.tensor_data_task is not None
-        ]
-        commit_header_task = self._header_writer_pool.submit(
-            do_commit, list(write_specs_), deps
-        )
-        self._jobs.append(commit_header_task)
+        def do_commit_incremental(write_spec, dependency: _Future):
+            # Slow version: issues one write for each metadata and one write for each header
+            if dependency is not None:
+                dependency.result(_TIMEOUT)
+            self._pwrite(
+                write_spec.header.metadata_entry,
+                write_spec.metadata_pos,
+                verify=len(write_spec.header.metadata_entry),
+            )
+            self._pwrite(
+                write_spec.header.buffer,
+                write_spec.header.file_offset,
+                verify=write_spec.header.size,
+            )
+
+        if write_specs_[0].metadata_pos == self._metadata_start + 8:
+            deps = [
+                w.tensor_data_task
+                for w in write_specs_
+                if w.tensor_data_task is not None
+            ]
+            commit_header_task = self._header_writer_pool.submit(
+                do_commit, list(write_specs_), deps
+            )
+
+            self._jobs.append(commit_header_task)
+        else:
+            # We've already written headers (we're in incremental mode)
+            # So we can't just batch up and write all of them at once
+            for w in write_specs_:
+                # Note this does _not_ set w.tensor_data_task, as committing headers is safe
+                self._header_writer_pool.submit(
+                    do_commit_incremental, w, w.tensor_data_task
+                )
+            metadata_size = (
+                self._metadata_cur - self._metadata_start - 8
+            )  # 8 bytes for metadata length field
+            metadata_size_task = self._header_writer_pool.submit(
+                self._pwrite,
+                struct.pack("<Q", metadata_size),
+                self._metadata_start,
+                verify=8,
+            )
+            self._jobs.append(metadata_size_task)
 
     def _do_commit_tensor_data(self, write_specs: Sequence[_WriteSpec]):
         def commit_tensor_data(