diff --git a/fast_llm/csrc/data.cpp b/fast_llm/csrc/data.cpp
index 59598eaf..a1a24c7c 100644
--- a/fast_llm/csrc/data.cpp
+++ b/fast_llm/csrc/data.cpp
@@ -27,7 +27,7 @@
 
 /*
  Helper methods for fast index mapping builds.
- Changes for Fast-LLM: Use int16 for dataset index, add verbose argument to build_sample_idx.
+ Changes for Fast-LLM: Use int16 for dataset index, add verbose argument to build_sample_idx, add build_sample_idx_padded
 */
 
 #include <iostream>
@@ -129,6 +129,65 @@ py::array build_sample_idx(const py::array_t<int32_t>& sizes_,
 
 }
 
+py::array build_padded_token_cumsum(const py::array_t<int32_t>& sizes_,
+                                const int32_t seq_length,
+                                const int32_t token_cumsum_rate,
+                                const int64_t offset
+                              ) {
+  /*
+  Build token cumsums at regular intervals from document sizes with padding in mind.
+  We inject 0 or more padding tokens at the end of every sequence to fill the sequence length.
+  */
+  int32_t seq_size = 0;
+  int64_t sizes_idx = 0;
+  int32_t samples = 0;
+  auto sizes = sizes_.unchecked<1>();
+  std::vector<int64_t> token_cumsum;
+
+  int64_t cumsum = offset;
+
+  while (sizes_idx < sizes.size()) {
+    int32_t size = sizes[sizes_idx];
+    if (size > seq_length) {
+      // Skip sequences that are too long, to avoid truncations
+      if (samples % token_cumsum_rate==0) token_cumsum.push_back(cumsum);
+      sizes_idx += 1;
+      samples += 1;
+    } else if (seq_size + size > seq_length) {
+      // add padded tokens if a document does not fit in current sequence and start a new sequence
+      cumsum += seq_length - seq_size;
+      seq_size = 0;
+    } else {
+      // Increment here to account for padding. This ensures that the stored values match the beginning of the next document.
+      if (samples % token_cumsum_rate==0) token_cumsum.push_back(cumsum);
+      seq_size += size;
+      cumsum += size;
+      sizes_idx += 1;
+      samples += 1;
+    }
+  }
+
+  // Add a final (padded) entry so we know how many tokens there are in total.
+  cumsum += seq_length - seq_size;
+  token_cumsum.push_back(cumsum);
+
+
+  int64_t* token_cumsum_result = new int64_t[token_cumsum.size()];
+  memcpy(token_cumsum_result, token_cumsum.data(), token_cumsum.size() * sizeof(int64_t));
+
+  py::capsule free_when_done(token_cumsum_result, [](void *mem_) {
+    int64_t *mem = reinterpret_cast<int64_t*>(mem_);
+    delete[] mem;
+  });
+
+  const auto byte_size = sizeof(int64_t);
+  return py::array(std::vector<int64_t>{token_cumsum.size()},
+                   {byte_size},
+                   token_cumsum_result,
+                   free_when_done);
+}
+
 PYBIND11_MODULE(data, m) {
     m.def("build_sample_idx", &build_sample_idx);
+    m.def("build_padded_token_cumsum", &build_padded_token_cumsum);
 }
diff --git a/fast_llm/data/data/gpt/config.py b/fast_llm/data/data/gpt/config.py
index 18b1eaac..c98a781e 100644
--- a/fast_llm/data/data/gpt/config.py
+++ b/fast_llm/data/data/gpt/config.py
@@ -57,6 +57,15 @@ class GPTDataConfig(DataConfig, GPTLegacyConfig):
         desc="Multiprocessing context. Do not touch.",
         hint=FieldHint.expert,
     )
+    truncate_documents: bool = Field(
+        default=True,
+        desc=(
+            "If enabled, documents may be truncated while being packed to fit the sequence length."
+            "Otherwise, sequences will be padded such that every document lies entirely within a sample"
+            " (and documents exceeding the sequence length will be skipped altogether)."
+        ),
+        hint=FieldHint.feature,
+    )
 
     def _validate(self) -> None:
         if not self.datasets:
diff --git a/fast_llm/data/data/gpt/data.py b/fast_llm/data/data/gpt/data.py
index 8fc33376..a0940e7c 100644
--- a/fast_llm/data/data/gpt/data.py
+++ b/fast_llm/data/data/gpt/data.py
@@ -121,6 +121,7 @@ def setup(
                     sequence_length=self._max_sequence_length,
                     vocab_size=self._vocab_size,
                     tokenizer=self._tokenizer,
+                    truncate_documents=self._config.truncate_documents,
                     cross_document_attention=self._cross_document_attention,
                 )
                 dataset = self._config.datasets[dataset_name].build_and_sample(sampling)
diff --git a/fast_llm/data/dataset/gpt/config.py b/fast_llm/data/dataset/gpt/config.py
index f91c537e..0f04884b 100644
--- a/fast_llm/data/dataset/gpt/config.py
+++ b/fast_llm/data/dataset/gpt/config.py
@@ -71,6 +71,7 @@ class GPTSamplingData(SamplingData):
     sequence_length: int
     vocab_size: int
     tokenizer: "Tokenizer"
+    truncate_documents: bool = True
     cross_document_attention: bool = True
 
 
diff --git a/fast_llm/data/dataset/gpt/sampled.py b/fast_llm/data/dataset/gpt/sampled.py
index f5d23031..dd449358 100644
--- a/fast_llm/data/dataset/gpt/sampled.py
+++ b/fast_llm/data/dataset/gpt/sampled.py
@@ -12,12 +12,12 @@
 from fast_llm.data.dataset.abstract import SampledDataset
 from fast_llm.data.dataset.gpt.config import GPTSamplingData, ShufflingType
 from fast_llm.data.dataset.gpt.indexed import GPTIndexedDataset
-from fast_llm.engine.config_utils.data_type import get_unsigned_integer_type
+from fast_llm.engine.config_utils.data_type import DataType, get_unsigned_integer_type
 from fast_llm.engine.config_utils.run import log_main_rank
 from fast_llm.utils import Assert
 
 try:
-    from fast_llm.csrc.data import build_sample_idx  # noqa
+    from fast_llm.csrc.data import build_padded_token_cumsum, build_sample_idx  # noqa
 
     _extension_available = True
 except ImportError:
@@ -89,6 +89,7 @@ def __init__(
         self._sequence_length = sampling.sequence_length
         self._cross_document_attention = sampling.cross_document_attention
         self._config = sampling.config
+        self._truncate_documents = sampling.truncate_documents
         self._device = torch.device("cuda" if self._config.gpu else "cpu")
 
         if sampling.cache_directory is None:
@@ -124,15 +125,35 @@ def _sample(self) -> None:
         """
         # Get the document sizes, the main information needed for sampling.
         document_sizes = torch.from_numpy(self._indexed_dataset.get_document_sizes()).to(self._device)
-
-        # Calculate basic stats.
         documents_per_epoch = document_sizes.numel()
         tokens_per_epoch = document_sizes.sum().item()
+
+        # Calculate basic stats.
+        if not self._truncate_documents:
+            assert _extension_available, (
+                "The C++ extension for dataset sampling is missing."
+                " Please make sure Fast-LLM is installed correctly."
+            )
+            long_docs_filter = document_sizes > self._sequence_length + 1
+            ignored_documents = sum(long_docs_filter)
+            if ignored_documents:
+                log_main_rank(
+                    f" > {ignored_documents}/{documents_per_epoch} documents are longer than {self._sequence_length+1} tokens and will be ignored.",
+                    log_fn=logger.warning,
+                )
+            tokens_per_epoch = document_sizes[~long_docs_filter].sum().item()
+            if tokens_per_epoch == 0:
+                raise RuntimeError(
+                    f" > No documents shorter than {self._sequence_length+1} tokens found in dataset {self._indexed_dataset.name}."
+                )
         # TODO MTP: Produce more labels to provide labels for the multi-token prediction heads?
         # We produce sequences of length `self._sequence_length + 1` so the last token has a label,
-        # but we also include that last label in the following sample,
+        # but in case of truncations we also include that last label in the following sample,
         # so we need `sequence_length * num_samples + 1` tokens in total.
-        num_epochs = math.ceil((self._sequence_length * self._num_samples + 1) / tokens_per_epoch)
+        num_epochs = math.ceil(
+            ((self._sequence_length + 1 - self._truncate_documents) * self._num_samples + 1 * self._truncate_documents)
+            / tokens_per_epoch
+        )
 
         # Prepare for shuffling.
         generator = torch.Generator(device=self._device)
@@ -154,6 +175,7 @@ def _sample(self) -> None:
             "num_samples": self._num_samples,
             "unshuffled_epochs": unshuffled_epochs,
             "sequence_length": self._sequence_length,
+            "truncate_documents": self._truncate_documents,
             "config": self._config.to_serialized(),
         }
         self._load_yaml_data(yaml_data)
@@ -161,6 +183,9 @@ def _sample(self) -> None:
         if self._yaml_path is not None:
             if self._yaml_path.is_file():
                 loaded_yaml_data = yaml.safe_load(self._yaml_path.open("r"))
+                unshuffled_tokens = loaded_yaml_data.pop("unshuffled_tokens", None)
+                if unshuffled_tokens is not None:
+                    self._unshuffled_tokens = unshuffled_tokens
                 if loaded_yaml_data != yaml_data:
                     raise RuntimeError(
                         f"Invalid dataset cache for dataset {self.name}."
@@ -172,9 +197,6 @@ def _sample(self) -> None:
                 # Dataset is already sampled, skip.
                 logger.info(f"Using existing sampling for dataset {self.name}")
                 return
-            else:
-                self._yaml_path.parent.mkdir(parents=True, exist_ok=True)
-                yaml.safe_dump(yaml_data, self._yaml_path.open("w"))
 
         if shuffled_documents > 1e8:
             warnings.warn(
@@ -232,51 +254,78 @@ def _sample(self) -> None:
         # So it is enough to pre-compute the (zero-padded) token cumsum at regular intervals `TOKEN_CUMSUM_RATE`.
         # Using `TOKEN_CUMSUM_RATE > 1` reduces pre-computation overhead at the cost of runtime computation.
         # Equivalent to `torch.hstack((0, document_sizes[all_document_index].cumsum()[::TOKEN_CUMSUM_RATE]))`
+        if unshuffled_epochs > 0:
+            token_cumsum_unshuffled, num_tokens_unshuffled = self._get_token_cumsum(
+                document_sizes,
+                offset=0,
+                # TODO: Allowing for max 100% extra tokens for padding, is that enough?
+                dtype=get_unsigned_integer_type((2 - self._truncate_documents) * tokens_per_epoch * num_epochs),
+            )
+            if self._truncate_documents:
+                num_tokens_unshuffled = tokens_per_epoch * unshuffled_epochs
+            self._token_cumsum_unshuffled.save(token_cumsum_unshuffled)
+        else:
+            num_tokens_unshuffled = 0
+        self._unshuffled_tokens = num_tokens_unshuffled
+
+        if self._yaml_path is not None:
+            yaml_data["unshuffled_tokens"] = num_tokens_unshuffled
+            self._yaml_path.parent.mkdir(parents=True, exist_ok=True)
+            yaml.safe_dump(yaml_data, self._yaml_path.open("w"))
+
         if shuffled_epochs > 0:
-            token_cumsum_shuffled = self._get_token_cumsum(
+            token_cumsum_shuffled, num_tokens_shuffled = self._get_token_cumsum(
                 document_sizes[
                     # Torch indexing only works with int32 or int64
                     document_shuffling.to(
                         dtype=torch.int64 if document_shuffling.dtype == torch.int64 else torch.int32
                     )
                 ],
-                offset=unshuffled_epochs * tokens_per_epoch,
-                dtype=get_unsigned_integer_type(tokens_per_epoch * num_epochs).torch,
+                offset=num_tokens_unshuffled,
+                # TODO: Allowing for max 100% extra tokens for padding, is that enough?
+                dtype=get_unsigned_integer_type((2 - self._truncate_documents) * tokens_per_epoch * num_epochs),
             )
-            self._token_cumsum_shuffled.save(token_cumsum_shuffled.numpy(force=self._config.gpu))
+            self._token_cumsum_shuffled.save(token_cumsum_shuffled)
             self._document_shuffling.save(
-                document_shuffling[: (token_cumsum_shuffled.numel() + 1) * TOKEN_CUMSUM_RATE].numpy(
+                document_shuffling[: (token_cumsum_shuffled.size + 1) * TOKEN_CUMSUM_RATE].numpy(
                     force=self._config.gpu
                 )
             )
             # Free memory
-            del token_cumsum_shuffled
             del document_shuffling
 
-        if unshuffled_epochs > 0:
-            token_cumsum_unshuffled = self._get_token_cumsum(
-                document_sizes, offset=0, dtype=get_unsigned_integer_type(tokens_per_epoch * num_epochs).torch
+    def _get_token_cumsum(self, sizes: torch.Tensor, offset: int, dtype: DataType) -> tuple[np.ndarray, int | None]:
+        if self._truncate_documents:
+            # Create the output tensor.
+            out = sizes.new_empty(sizes.numel() // TOKEN_CUMSUM_RATE + 1, dtype=dtype.torch)
+            # Get partial sums for regular intervals, excluding the last incomplete interval.
+            torch.sum(
+                sizes[: sizes.numel() - sizes.numel() % TOKEN_CUMSUM_RATE].view(-1, TOKEN_CUMSUM_RATE),
+                dim=1,
+                out=out[1:],
             )
-            self._token_cumsum_unshuffled.save(token_cumsum_unshuffled.numpy(force=self._config.gpu))
-
-    def _get_token_cumsum(self, sizes: torch.Tensor, offset: int, dtype: torch.dtype) -> torch.Tensor:
-        # Create the output tensor.
-        out = sizes.new_empty(sizes.numel() // TOKEN_CUMSUM_RATE + 1, dtype=dtype)
-        # Get partial sums for regular intervals, excluding the last incomplete interval.
-        torch.sum(
-            sizes[: sizes.numel() - sizes.numel() % TOKEN_CUMSUM_RATE].view(-1, TOKEN_CUMSUM_RATE), dim=1, out=out[1:]
-        )
-        # Pad with the begin offset
-        out[0] = offset
-        # Calculate the cumsum.
-        out.cumsum_(0)
-        # Crop unnecessary entries.
-        return out[
-            : torch.clamp_min_(
-                torch.searchsorted(out, self._num_samples * self._sequence_length, side="right"),
-                0,
+            # Pad with the begin offset
+            out[0] = offset
+            # Calculate the cumsum.
+            out.cumsum_(0)
+            # Crop unnecessary entries.
+            out = out[
+                : torch.clamp_min_(
+                    torch.searchsorted(out, self._num_samples * self._sequence_length, side="right"),
+                    0,
+                )
+            ]
+            return out.numpy(force=self._config.gpu), None
+        else:
+            # TODO: dynamically handle int64 or int32 in CPP
+            out = build_padded_token_cumsum(
+                sizes.cpu().numpy(), (self._sequence_length + 1), TOKEN_CUMSUM_RATE, offset
             )
-        ]
+            num_tokens = out[-1]
+            out = out[:-1][
+                : np.clip(np.searchsorted(out, self._num_samples * (self._sequence_length + 1), side="right"), 0, None)
+            ]
+            return out, num_tokens
 
     def __len__(self) -> int:
         return self._num_samples
@@ -288,7 +337,9 @@ def __getitem__(self, index: int) -> typing.Any:
         The returned sample is ready to be concatenated, then fed to a `GPTModel` (see `GPTModel.preprocess`).
         """
         self._lazy_load()
-        token_start = index * self._sequence_length
+        # tokens at the boundary are included in only one sample when we pack without truncations
+        # in case of packing with truncations, the last token from the previous sample is also the first token of the next sample
+        token_start = index * (self._sequence_length + 1 - self._truncate_documents)
         token_end = token_start + self._sequence_length + 1
 
         if token_start < self._unshuffled_tokens:
@@ -302,6 +353,7 @@ def __getitem__(self, index: int) -> typing.Any:
         token_start_cumsum_index = np.searchsorted(token_start_array, token_start, side="right").item() - 1
 
         document_sampling_index = token_start_cumsum_index * TOKEN_CUMSUM_RATE + token_start_array_document_offset
+
         token_count = token_start_array[token_start_cumsum_index]
 
         token_ids = []
@@ -314,6 +366,25 @@ def __getitem__(self, index: int) -> typing.Any:
                 document_index = self._document_shuffling[document_sampling_index - self._unshuffled_documents].item()
 
             document_size = self._indexed_dataset.get_document_size(document_index)
+
+            if not self._truncate_documents:
+                if document_size > self._sequence_length + 1:
+                    # Document too long, ignore
+                    document_sampling_index += 1
+                    continue
+                tokens_in_sample = token_count % (self._sequence_length + 1)
+                if document_size + tokens_in_sample > self._sequence_length + 1:
+                    # Document belongs to the next sample, need to account for padding.
+                    padding_size = self._sequence_length + 1 - tokens_in_sample
+                    if token_count > token_start:
+                        # Add padding tokens to current sample
+                        token_ids.append(np.full((padding_size,), -100, dtype=np.int64))
+                        Assert.eq(token_count + padding_size, token_end)
+                        break
+                    else:
+                        # Move on to the next sample.
+                        token_count += padding_size
+
             # Determine if the document belongs to the requested sample.
             if token_count + document_size >= token_start:
                 # Determine which part of the document belong to the sample, and add it to the list.
@@ -343,7 +414,9 @@ def __getitem__(self, index: int) -> typing.Any:
         )
         token_ids = np.concatenate(token_ids, dtype=np.int64)
         loss_masking_spans = (
-            np.stack(loss_masking_spans, dtype=np.int32) if self._config.use_loss_masking_spans else None
+            (np.stack(loss_masking_spans, dtype=np.int32) if loss_masking_spans else np.array([]))
+            if self._config.use_loss_masking_spans
+            else None
         )
         Assert.eq(len(token_ids), self._sequence_length + 1)
 
@@ -357,9 +430,12 @@ def _lazy_load(self):
         if not hasattr(self, "_documents_per_epoch"):
             self._load_yaml_data(yaml.safe_load(self._yaml_path.open("r")))
 
-    def _load_yaml_data(self, data: dict[str, typing.Any]):
+    def _load_yaml_data(self, data: dict[str, typing.Any]) -> None:
         self._documents_per_epoch = data["dataset"]["documents_per_epoch"]
-        self._unshuffled_tokens = data["unshuffled_epochs"] * data["dataset"]["tokens_per_epoch"]
+        if unshuffled_tokens := data.get("unshuffled_tokens") is not None:
+            self._unshuffled_tokens = unshuffled_tokens
+        else:
+            self._unshuffled_tokens = data["unshuffled_epochs"] * data["dataset"]["tokens_per_epoch"]
         self._unshuffled_documents = data["unshuffled_epochs"] * self._documents_per_epoch
 
 
@@ -380,9 +456,12 @@ def __init__(
         self._indexed_dataset = indexed_dataset
         self._num_samples = sampling.num_samples
         self._sequence_length = sampling.sequence_length
+        if not sampling.truncate_documents:
+            raise NotImplementedError(
+                "Legacy sampling only supports document truncation. Please use the latest dataset format."
+            )
         self._cross_document_attention = sampling.cross_document_attention
         self._config = sampling.config
-        self._tokenizer = sampling.tokenizer
 
         if sampling.cache_directory is None:
             log_main_rank(
@@ -498,7 +577,7 @@ def __getitem__(self, idx: int) -> typing.Any:
                 for span in sample.loss_masking_spans:
                     spans.append(span + offset)
                 offset += len(sample.token_ids)
-            spans = np.stack(spans, dtype=np.int32)
+            spans = np.stack(spans, dtype=np.int32) if spans else np.array([])
         else:
             spans = None
         sequence_lengths = (
diff --git a/tests/data/common.py b/tests/data/common.py
index 5177b1f1..b3f41a1a 100644
--- a/tests/data/common.py
+++ b/tests/data/common.py
@@ -36,6 +36,7 @@ def get_sampling_data(
     tokenizer: Tokenizer | None = None,
     gpu: bool = False,
     shuffle: ShufflingType = ShufflingType.epoch,
+    truncate_documents=True,
 ) -> GPTSamplingData:
     # Config with convenient defaults.
     return GPTSamplingData(
@@ -51,6 +52,7 @@ def get_sampling_data(
         sequence_length=sequence_length,
         vocab_size=vocab_size,
         tokenizer=tokenizer,
+        truncate_documents=truncate_documents,
     )
 
 
diff --git a/tests/data/test_sampling.py b/tests/data/test_sampling.py
index e622d118..38679582 100644
--- a/tests/data/test_sampling.py
+++ b/tests/data/test_sampling.py
@@ -15,6 +15,14 @@
     validate_indexed_dataset_sampling,
 )
 
+try:
+    from fast_llm.csrc.data import build_padded_token_cumsum  # noqa
+
+    _extension_available = True
+except ImportError:
+    _extension_available = False
+
+
 GPT_MEMMAP_SAMPLES = [
     [4709, 819, 79, 207, 277, 1790],
     [1790, 80, 6506, 1735, 542, 88],
@@ -125,3 +133,70 @@ def test_gpt_sample(seed, shuffle):
             # Check that the sequence is independent of `num_sample`.
             Assert.all_equal(samples, previous_samples[: len(samples)])
         previous_samples = samples
+
+
+@pytest.mark.skipif(not _extension_available, reason="CPP Extension not available")
+def test_build_padded_token_cumsum():
+    sizes = np.array([100, 256, 580, 600, 550, 89, 339, 430, 400, 795, 680, 50], dtype=np.int32)
+    sequence_length = 768
+    token_cumsum_rate = 4
+    offset = 0
+    # sequences with padding:
+    # [100, 256, 413 padded, 580, 189 padded, 600, 169 padded, 550, 89, 130 padded, 339, 430, 400, 369 padded, 680, 50, 39 padded]
+    # cumsums:
+    # [100, 356, 1349, 2307, 2857, 2946, 3415, 3845, 4245, 5294, 5344, 5383]
+    expected_cumsums = [0, 2307, 3845, 5383]
+    token_cumsum = build_padded_token_cumsum(sizes, sequence_length + 1, token_cumsum_rate, offset)
+    Assert.all_equal(token_cumsum, expected_cumsums)
+
+
+def get_test_seeds(num_seeds):
+    np.random.seed(42)
+    seeds = np.random.randint(0, num_seeds * 100, num_seeds)
+    return seeds.tolist()
+
+
+@pytest.mark.skipif(not _extension_available, reason="CPP Extension not available")
+def test_gpt_sample_padding():
+    for seed in get_test_seeds(100):
+        vocab_size = 30
+        np.random.seed(seed)
+        num_sequences = np.random.randint(1, 20)
+        sequence_length = np.random.randint(1, 20)
+        doc_sizes = np.random.randint(1, 2 * sequence_length, num_sequences)
+        samples = [np.random.randint(0, vocab_size, size) for size in doc_sizes]
+        expected_samples = []
+        seq_size = 0
+        token_ids = []
+        total_tokens = 0
+        for idx, sample in enumerate(samples):
+            doc_size = len(sample)
+            if doc_size > sequence_length + 1:
+                continue
+            elif doc_size + seq_size > sequence_length + 1:
+                padding_tokens = sequence_length + 1 - seq_size
+                token_ids.append([-100] * padding_tokens)
+                expected_samples.append(list(np.concatenate(token_ids)))
+                token_ids = [sample]
+                seq_size = doc_size
+                total_tokens += doc_size
+            else:
+                token_ids.append(sample)
+                seq_size += doc_size
+                total_tokens += doc_size
+        dataset = SimpleGPTIndexedDataset(samples)
+        sampling = get_sampling_data(
+            num_samples=len(expected_samples),
+            sequence_length=sequence_length,
+            vocab_size=vocab_size,
+            seed=seed,
+            shuffle=ShufflingType.disabled,
+            truncate_documents=False,
+        )
+        if total_tokens == 0:
+            with pytest.raises(RuntimeError):
+                dataset.sample(sampling)
+        else:
+            sampled = dataset.sample(sampling)
+            for idx in range(len(expected_samples)):
+                Assert.all_equal(sampled[idx].token_ids, np.array(expected_samples[idx]))