diff --git a/src/sedpack/io/dataset_filler.py b/src/sedpack/io/dataset_filler.py
index 52ee104a..e20bf196 100644
--- a/src/sedpack/io/dataset_filler.py
+++ b/src/sedpack/io/dataset_filler.py
@@ -1,4 +1,4 @@
-# Copyright 2023-2024 Google LLC
+# Copyright 2023-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -15,6 +15,7 @@
 number."""
 
 from __future__ import annotations
+import concurrent.futures
 import dataclasses
 from pathlib import Path
 from types import TracebackType
@@ -31,6 +32,13 @@
     from sedpack.io.dataset_writing import DatasetWriting
 
 
+def _close_shard(shard: Shard) -> ShardInfo:
+    """Helper function to close a shard. This can be pickled and thus sent to
+    another process.
+    """
+    return shard.close()
+
+
 @dataclasses.dataclass
 class ShardProgress:
     """Internal information about a shard progress. Since we do not want to
@@ -47,11 +55,15 @@ class DatasetFillerContext:
     group_number.
     """
 
-    def __init__(self,
-                 dataset_root_path: Path,
-                 dataset_structure: DatasetStructure,
-                 relative_path_from_split: Path,
-                 write_updates: bool = True) -> None:
+    def __init__(
+        self,
+        dataset_root_path: Path,
+        dataset_structure: DatasetStructure,
+        relative_path_from_split: Path,
+        *,
+        concurrent_pool: concurrent.futures.Executor,
+        write_updates: bool = True,
+    ) -> None:
         """Initialize a dataset filler context which writes examples and
         automatically opens and closes shards (except possibly not closing the
         last shard which is closed by `DatasetFiller`).
@@ -67,6 +79,8 @@ def __init__(self,
           `dataset_root_path / split / relative_path_from_split`. May not
           contain "..".
 
+          concurrent_pool (concurrent.futures.Executor): Shard file writing.
+
           write_updates (bool): Whether to save progress of written shard info
           files. Defaults to `True`. In theory when set to `False` writing
           could be faster (not writing the info file after each shard). But
@@ -84,6 +98,7 @@ def __init__(self,
         self._dataset_root_path: Path = dataset_root_path
         self._dataset_structure: DatasetStructure = dataset_structure
         self._relative_path_from_split: Path = relative_path_from_split
+        self._concurrent_pool: concurrent.futures.Executor = concurrent_pool
         self._write_updates: bool = write_updates
 
         # Constants.
@@ -94,6 +109,8 @@ def __init__(self,
 
         # Cumulated shard infos.
         self._shards_lists: dict[SplitT, ShardsList] = {}
+        self._future_shard_infos: dict[
+            SplitT, list[concurrent.futures.Future[ShardInfo]]] = {}
 
         # Random path generator.
         self._path_generator = PathGenerator()
@@ -103,6 +120,34 @@ def shard_lists(self) -> dict[SplitT, ShardsList]:
         """Return information about all shards written by this
         DatasetFillerContext.
         """
+        if not self._future_shard_infos:
+            return self._shards_lists
+
+        for split, shard_info_futures in self._future_shard_infos.items():
+            for future_info in shard_info_futures:
+                shard_info: ShardInfo = future_info.result()
+
+                # Remember this shard info.
+                if split not in self._shards_lists:
+                    # Load if exists.
+                    self._shards_lists[split] = ShardsList.load_or_create(
+                        dataset_root_path=self._dataset_root_path,
+                        relative_path_self=shard_info.file_infos[0].file_path.
+                        parent / "shards_list.json",
+                    )
+                self._shards_lists[split].shard_files.append(shard_info)
+                self._shards_lists[
+                    split].number_of_examples += shard_info.number_of_examples
+
+        # Write down which shard files have been saved.
+        if self._write_updates:
+            for shards_list in self._shards_lists.values():
+                shards_list.write_config(
+                    dataset_root_path=self._dataset_root_path,
+                    hashes=(),  # We forget these now.
+                )
+
+        self._future_shard_infos = {}
         return self._shards_lists
 
     def _get_new_shard(self, split: SplitT) -> Shard:
@@ -184,26 +229,13 @@ def write_example(self,
     def close_shard(self, shard: Shard, split: SplitT) -> None:
         """Close shard. Called automatically by DatasetFiller.__exit__."""
         # Finish writing the shard.
-        shard_info: ShardInfo = shard.close()
-
-        # Remember this shard info.
-        if split not in self._shards_lists:
-            # Load if exists.
-            self._shards_lists[split] = ShardsList.load_or_create(
-                dataset_root_path=self._dataset_root_path,
-                relative_path_self=shard_info.file_infos[0].file_path.parent /
-                "shards_list.json",
-            )
-        self._shards_lists[split].shard_files.append(shard_info)
-        self._shards_lists[
-            split].number_of_examples += shard_info.number_of_examples
-
-        # Write down which shard files have been saved.
-        if self._write_updates:
-            self._shards_lists[split].write_config(
-                dataset_root_path=self._dataset_root_path,
-                hashes=(),  # We forget these now.
-            )
+        if split not in self._future_shard_infos:
+            self._future_shard_infos[split] = []
+        self._future_shard_infos[split].append(
+            self._concurrent_pool.submit(
+                _close_shard,
+                shard=shard,
+            ))
 
 
 class DatasetFiller:
@@ -223,6 +255,7 @@ class DatasetFiller:
 
     def __init__(self,
                  dataset: DatasetWriting,
+                 concurrency: int = 1,
                  relative_path_from_split: Path = Path("."),
                  auto_update_dataset: bool = True) -> None:
         """Context manager for writing examples into a dataset.
@@ -232,6 +265,10 @@ def __init__(self,
           dataset (DatasetWriting): Dataset object representing the dataset we
           are filling.
 
+          concurrency (int): Setting to a positive integer allows writing shard
+          files in parallel. Defaults to 1 (sequential writes in another thread
+          or process). Thread is used for "tfrec" otherwise process.
+
           relative_path_from_split (Path): New shards are created inside
           `dataset_root_path / split / relative_path_from_split` or children.
           It will be created if it does not exist yet. Useful for
@@ -244,11 +281,20 @@ def __init__(self,
           `DatasetFiller` objects it is advised to set this parameter to
           `False` since this is not thread safe.
         """
+        self._concurrent_pool: concurrent.futures.Executor
+        if dataset.dataset_structure.shard_file_type == "tfrec":
+            self._concurrent_pool = concurrent.futures.ThreadPoolExecutor(
+                max_workers=1)
+        else:
+            self._concurrent_pool = concurrent.futures.ProcessPoolExecutor(
+                max_workers=max(1, concurrency),)
+
         self._dataset_filler_context: DatasetFillerContext
         self._dataset_filler_context = DatasetFillerContext(
             dataset_root_path=dataset.path,
             dataset_structure=dataset.dataset_structure,
             relative_path_from_split=relative_path_from_split,
+            concurrent_pool=self._concurrent_pool,
         )
         self._auto_update_dataset: bool = auto_update_dataset
         self._dataset: DatasetWriting = dataset
@@ -286,6 +332,13 @@ def __exit__(self, exc_type: Type[BaseException] | None,
                     split=split,
                 )
 
+        # Wait to finish writing of all files (after closing all shards but
+        # before updating metadata files).
+        self._concurrent_pool.shutdown(
+            wait=True,
+            cancel_futures=False,
+        )
+
         # Write updated info files.
         self._update_infos()
 
diff --git a/src/sedpack/io/dataset_writing.py b/src/sedpack/io/dataset_writing.py
index e9f30084..5aab8d0f 100644
--- a/src/sedpack/io/dataset_writing.py
+++ b/src/sedpack/io/dataset_writing.py
@@ -140,9 +140,14 @@ def write_multiprocessing(  # pylint: disable=too-many-arguments
         # Return user-defined results.
         return results
 
-    def filler(self) -> DatasetFiller:
+    def filler(self, concurrency: int = 1) -> DatasetFiller:
         """Return a dataset filler context manager for writing examples.
 
+        Args:
+
+          concurrency (int): Setting to a positive integer allows writing shard
+          files in parallel. Defaults to 1 (sequential writes).
+
         Example use:
         # Context manager properly opens and closes shards.
         with dataset.filler() as dataset_filler:
@@ -153,7 +158,7 @@ def filler(self) -> DatasetFiller:
                     split=split,
                 )
         """
-        return DatasetFiller(self)
+        return DatasetFiller(dataset=self, concurrency=concurrency)
 
     def write_config(
         self,
@@ -279,7 +284,8 @@ def check(
                     if real_hashes != file_info.hash_checksums:
                         raise ValueError(
                             f"Hash checksum miss-match in {file_info.file_path}"
-                        )
+                            f"got: {real_hashes} but expected "
+                            f"{file_info.hash_checksums}")
 
 
 # We want to get results back.
diff --git a/src/sedpack/io/shard/shard.py b/src/sedpack/io/shard/shard.py
index 407dd691..a6522c6c 100644
--- a/src/sedpack/io/shard/shard.py
+++ b/src/sedpack/io/shard/shard.py
@@ -1,4 +1,4 @@
-# Copyright 2023-2024 Google LLC
+# Copyright 2023-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -16,7 +16,6 @@
 
 from pathlib import Path
 
-import sedpack
 from sedpack.io.metadata import DatasetStructure
 from sedpack.io.shard_file_metadata import ShardInfo
 from sedpack.io.types import ExampleT
@@ -65,16 +64,16 @@ def write(self, values: ExampleT) -> None:
         self.shard_info.number_of_examples += 1
 
     def close(self) -> ShardInfo:
-        """Close shard and return statistics."""
+        """Close shard and return statistics.
+        """
         if self._shard_writer is None:
             raise ValueError("Closing a shard which has not been open.")
 
-        self._shard_writer.close()
+        hash_checksums: tuple[str, ...] = self._shard_writer.close()
         self._shard_writer = None
 
         # Compute sha256 checksum.
-        self.shard_info.file_infos[
-            0].hash_checksums = self._compute_file_hash_checksums()
+        self.shard_info.file_infos[0].hash_checksums = hash_checksums
 
         # Return shard info.
         return self.shard_info
@@ -83,16 +82,3 @@ def _get_full_path(self) -> Path:
         """Return full path to the shard file.
         """
         return self._dataset_path / self.shard_info.file_infos[0].file_path
-
-    def _compute_file_hash_checksums(self) -> tuple[str, ...]:
-        """Compute hash checksums of the shard file(-s).
-
-        TODO This method should return a list of checksums defined by the user
-        in `self.dataset_structure`.
-        """
-        # Compute sha256 checksum.
-        shard_path = self._get_full_path()
-        return sedpack.io.utils.hash_checksums(
-            file_path=shard_path,
-            hashes=self.dataset_structure.hash_checksum_algorithms,
-        )
diff --git a/src/sedpack/io/shard/shard_writer_base.py b/src/sedpack/io/shard/shard_writer_base.py
index b59ae68b..64f5476f 100644
--- a/src/sedpack/io/shard/shard_writer_base.py
+++ b/src/sedpack/io/shard/shard_writer_base.py
@@ -1,4 +1,4 @@
-# Copyright 2024 Google LLC
+# Copyright 2024-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
 
 import numpy as np
 
+import sedpack
 from sedpack.io.metadata import DatasetStructure
 from sedpack.io.types import ExampleT, CompressionT
 
@@ -81,7 +82,7 @@ def _write(self, values: ExampleT) -> None:
         """
 
     @abstractmethod
-    def close(self) -> None:
+    def close(self) -> tuple[str, ...]:
         """Close the shard file(-s).
         """
 
@@ -90,3 +91,10 @@ def close(self) -> None:
     def supported_compressions() -> list[CompressionT]:
         """Return a list of supported compression types.
         """
+
+    def _compute_file_hash_checksums(self) -> tuple[str, ...]:
+        """Compute hash checksums of the shard file(-s). """
+        return sedpack.io.utils.hash_checksums(
+            file_path=self._shard_file,
+            hashes=self.dataset_structure.hash_checksum_algorithms,
+        )
diff --git a/src/sedpack/io/shard/shard_writer_flatbuffer.py b/src/sedpack/io/shard/shard_writer_flatbuffer.py
index 0a8ad7c1..cb86919d 100644
--- a/src/sedpack/io/shard/shard_writer_flatbuffer.py
+++ b/src/sedpack/io/shard/shard_writer_flatbuffer.py
@@ -1,4 +1,4 @@
-# Copyright 2024 Google LLC
+# Copyright 2024-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -26,6 +26,7 @@
 from sedpack.io.metadata import Attribute, DatasetStructure
 from sedpack.io.types import AttributeValueT, CompressionT, ExampleT
 from sedpack.io.shard.shard_writer_base import ShardWriterBase
+from sedpack.io.utils import hash_checksums_from_bytes
 
 # Autogenerated from src/sedpack/io/flatbuffer/shard.fbs
 import sedpack.io.flatbuffer.shardfile.Attribute as fbapi_Attribute
@@ -253,13 +254,16 @@ def save_numpy_vector_as_bytearray(  # type: ignore[no-any-unimported]
         # Return the vector offset.
         return builder.EndVector()  # type: ignore[no-any-return]
 
-    def close(self) -> None:
+    def close(self) -> tuple[str, ...]:
         """Close the shard file(-s).
         """
         if not self._examples:
             # Nothing to save.
             assert not self._shard_file.is_file()
-            return
+            return hash_checksums_from_bytes(
+                file_content=b"",
+                hashes=self.dataset_structure.hash_checksum_algorithms,
+            )
 
         if not self._builder:
             raise ValueError("Attempting to close a closed shard")
@@ -278,13 +282,21 @@ def close(self) -> None:
         # Finish the builder.
         self._builder.Finish(shard)
 
+        # Hash check-sums must be computed from already compressed file.
+        file_content: bytes = CompressedFile(
+            self.dataset_structure.compression).compress(
+                bytes(self._builder.Output()))
+        self._builder = None
+        hash_checksums: tuple[str, ...] = hash_checksums_from_bytes(
+            file_content=file_content,
+            hashes=self.dataset_structure.hash_checksum_algorithms,
+        )
+
         # Write the buffer into a file.
         with open(self._shard_file, "wb") as file:
-            compressor = CompressedFile(self.dataset_structure.compression)
-            file.write(compressor.compress(bytes(self._builder.Output())))
+            file.write(file_content)
 
-        self._builder = None
-        assert self._shard_file.is_file()
+        return hash_checksums
 
     @staticmethod
     def supported_compressions() -> list[CompressionT]:
diff --git a/src/sedpack/io/shard/shard_writer_np.py b/src/sedpack/io/shard/shard_writer_np.py
index 7c8fcdf7..fa600e4c 100644
--- a/src/sedpack/io/shard/shard_writer_np.py
+++ b/src/sedpack/io/shard/shard_writer_np.py
@@ -1,4 +1,4 @@
-# Copyright 2024 Google LLC
+# Copyright 2024-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -23,8 +23,8 @@
 from numpy import typing as npt
 
 from sedpack.io.metadata import Attribute, DatasetStructure
-from sedpack.io.types import AttributeValueT, CompressionT, ExampleT
 from sedpack.io.shard.shard_writer_base import ShardWriterBase
+from sedpack.io.types import AttributeValueT, CompressionT, ExampleT
 
 
 class ShardWriterNP(ShardWriterBase):
@@ -117,12 +117,12 @@ def _write(self, values: ExampleT) -> None:
                                    )
                 self._buffer[name] = byte_list  # type: ignore[assignment]
 
-    def close(self) -> None:
-        """Close the shard file(-s).
+    def close(self) -> tuple[str, ...]:
+        """Close the shard file and return hash check-sums.
         """
         if not self._buffer:
             assert not self._shard_file.is_file()
-            return
+            return ()
 
         # Deal properly with "bytes" attributes.
         for attribute in self.dataset_structure.saved_data_description:
@@ -160,6 +160,7 @@ def close(self) -> None:
 
         self._buffer = {}
         assert self._shard_file.is_file()
+        return self._compute_file_hash_checksums()
 
     @staticmethod
     def supported_compressions() -> list[CompressionT]:
diff --git a/src/sedpack/io/shard/shard_writer_tfrec.py b/src/sedpack/io/shard/shard_writer_tfrec.py
index a51b4b45..0df0aa7f 100644
--- a/src/sedpack/io/shard/shard_writer_tfrec.py
+++ b/src/sedpack/io/shard/shard_writer_tfrec.py
@@ -1,4 +1,4 @@
-# Copyright 2024 Google LLC
+# Copyright 2024-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -80,13 +80,14 @@ def _write(self, values: ExampleT) -> None:
         )
         self._tf_shard_writer.write(example)
 
-    def close(self) -> None:
+    def close(self) -> tuple[str, ...]:
         """Close the shard file(-s).
         """
         if not self._tf_shard_writer:
             raise ValueError("Trying to close a shard that was not open")
         self._tf_shard_writer.close()
         self._tf_shard_writer = None
+        return self._compute_file_hash_checksums()
 
     @staticmethod
     def supported_compressions() -> list[CompressionT]:
diff --git a/src/sedpack/io/utils.py b/src/sedpack/io/utils.py
index 0590b377..8067955d 100644
--- a/src/sedpack/io/utils.py
+++ b/src/sedpack/io/utils.py
@@ -60,6 +60,35 @@ def _get_hash_function(name: HashChecksumT) -> HashProtocol:
             return hashlib.new(name)
 
 
+def hash_checksums_from_bytes(
+    file_content: bytes,
+    hashes: tuple[HashChecksumT, ...],
+) -> tuple[str, ...]:
+    """Compute the hex-encoded hash checksums. An alternative to
+    `hash_checksums` to avoid reading the file again.
+
+    Args:
+
+        file_content (bytes): The whole file content.
+
+        hashes (tuple[HashChecksumT, ...]): A tuple of hash algorithm names to
+        be computed.
+
+    Returns: hex-encoded hash checksums of the file in the order given by
+    `hashes`.
+    """
+    # Actual hash functions, same order as hashes.
+    hash_functions = tuple(
+        _get_hash_function(hash_name) for hash_name in hashes)
+
+    # Update all hashes.
+    for hash_function in hash_functions:
+        hash_function.update(file_content)
+
+    # Hex-encoded results, same order as hashes.
+    return tuple(hash_function.hexdigest() for hash_function in hash_functions)
+
+
 def hash_checksums(file_path: Path, hashes: tuple[HashChecksumT,
                                                   ...]) -> tuple[str, ...]:
     """Compute the hex-encoded hash checksums.
diff --git a/tests/io/iteration/test_rust_batched_generator.py b/tests/io/iteration/test_rust_batched_generator.py
index bfcd83a2..7fd98f15 100644
--- a/tests/io/iteration/test_rust_batched_generator.py
+++ b/tests/io/iteration/test_rust_batched_generator.py
@@ -107,7 +107,7 @@ def dataset_and_values(tmpdir_factory) -> None:
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for i in range(data_points):
             filler.write_example(
                 values={
diff --git a/tests/io/test_bytes.py b/tests/io/test_bytes.py
index ce57f66f..39cbb606 100644
--- a/tests/io/test_bytes.py
+++ b/tests/io/test_bytes.py
@@ -62,7 +62,7 @@ def test_attribute_bytes(tmpdir: Union[str, Path]) -> None:
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for attribute_value in array_of_values:
             filler.write_example(
                 values={"attribute_name": attribute_value},
diff --git a/tests/io/test_check_consistency.py b/tests/io/test_check_consistency.py
index 3a4165b7..e2c519dc 100644
--- a/tests/io/test_check_consistency.py
+++ b/tests/io/test_check_consistency.py
@@ -61,7 +61,7 @@ def get_dataset(tmp_path: Union[str, Path], shard_file_type: ShardFileTypeT,
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for attribute_value in array_of_values:
             filler.write_example(
                 values={"attribute_name": attribute_value},
diff --git a/tests/io/test_custom_metadata_type_limit.py b/tests/io/test_custom_metadata_type_limit.py
index c422e2cc..2d2633a4 100644
--- a/tests/io/test_custom_metadata_type_limit.py
+++ b/tests/io/test_custom_metadata_type_limit.py
@@ -61,7 +61,7 @@ def end2end(tmpdir: Union[str, Path], dtype: npt.DTypeLike, method: str,
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for attribute_value in array_of_values:
             filler.write_example(
                 values={"attribute_name": attribute_value},
diff --git a/tests/io/test_end2end.py b/tests/io/test_end2end.py
index 3aa54e08..8dd5268d 100644
--- a/tests/io/test_end2end.py
+++ b/tests/io/test_end2end.py
@@ -64,7 +64,7 @@ def end2end(tmpdir: Union[str, Path], dtype: npt.DTypeLike, method: str,
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for attribute_value in array_of_values:
             filler.write_example(
                 values={"attribute_name": attribute_value},
diff --git a/tests/io/test_end2end_dtypes.py b/tests/io/test_end2end_dtypes.py
index 6d7944f0..6e11efda 100644
--- a/tests/io/test_end2end_dtypes.py
+++ b/tests/io/test_end2end_dtypes.py
@@ -97,7 +97,7 @@ def dataset_and_values_dynamic_shape(
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for i in range(items):
             filler.write_example(
                 values={
diff --git a/tests/io/test_end2end_shuffled.py b/tests/io/test_end2end_shuffled.py
index 67b5e506..84b1cda2 100644
--- a/tests/io/test_end2end_shuffled.py
+++ b/tests/io/test_end2end_shuffled.py
@@ -58,7 +58,7 @@ def end2end(tmpdir: Union[str, Path], dtype: npt.DTypeLike, method: str,
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for attribute_value in array_of_values:
             filler.write_example(
                 values={"attribute_name": attribute_value},
diff --git a/tests/io/test_hash_checksums.py b/tests/io/test_hash_checksums.py
index f2e78757..c64272e9 100644
--- a/tests/io/test_hash_checksums.py
+++ b/tests/io/test_hash_checksums.py
@@ -1,4 +1,4 @@
-# Copyright 2024 Google LLC
+# Copyright 2024-2025 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,7 +14,10 @@
 
 from pathlib import Path
 
-from sedpack.io.utils import hash_checksums
+import numpy as np
+import pytest
+
+from sedpack.io.utils import hash_checksums, hash_checksums_from_bytes
 
 
 def test_compress_gzip_write(tmpdir: str | Path) -> None:
@@ -31,3 +34,39 @@ def test_compress_gzip_write(tmpdir: str | Path) -> None:
         "b7f783baed8297f0db917462184ff4f08e69c2d5e5f79a942600f9725f58ce1f29c18139bf80b06c0fff2bdd34738452ecf40c488c22a7e3d80cdf6f9c1c0d47",
         "9203b0c4439fd1e6ae5878866337b7c532acd6d9260150c80318e8ab8c27ce330189f8df94fb890df1d298ff360627e1",
     )
+
+
+@pytest.mark.parametrize("size", [1, 3, 7, 8, 13, 17, 33, 67])
+def test_equivalent(size: int, tmp_path: Path) -> None:
+    file_content = np.random.randint(
+        0,
+        256,
+        size=size,
+        dtype=np.uint8,
+    ).tobytes()
+    hashes = (
+        "md5",
+        "sha256",
+        "sha512",
+        "sha384",
+        "xxh32",
+        "xxh64",
+        "xxh128",
+    )
+
+    tmp_path = tmp_path / "shard_file.extension"
+
+    with open(tmp_path, "wb") as f:
+        f.write(file_content)
+
+    # Redundant, but read again.
+    with open(tmp_path, "rb") as f:
+        assert f.read() == file_content
+
+    assert hash_checksums_from_bytes(
+        file_content=file_content,
+        hashes=hashes,
+    ) == hash_checksums(
+        file_path=tmp_path,
+        hashes=hashes,
+    )
diff --git a/tests/io/test_rust_iter.py b/tests/io/test_rust_iter.py
index c227cac6..f694ab88 100644
--- a/tests/io/test_rust_iter.py
+++ b/tests/io/test_rust_iter.py
@@ -61,7 +61,7 @@ def end2end(tmpdir: Union[str, Path], dtype: npt.DTypeLike,
 
     # Fill data in the dataset
 
-    with dataset.filler() as filler:
+    with dataset.filler(concurrency=np.random.randint(0, 4),) as filler:
         for attribute_value in array_of_values:
             filler.write_example(
                 values={"attribute_name": attribute_value},