diff --git a/deepspeed/runtime/data_pipeline/data_sampling/data_analyzer.py b/deepspeed/runtime/data_pipeline/data_sampling/data_analyzer.py
index 5da11097319d..7088df223bd8 100644
--- a/deepspeed/runtime/data_pipeline/data_sampling/data_analyzer.py
+++ b/deepspeed/runtime/data_pipeline/data_sampling/data_analyzer.py
@@ -12,7 +12,7 @@
 import torch
 from torch.utils.data import BatchSampler, SequentialSampler, DataLoader, Subset
 
-from deepspeed.utils import logger, groups
+from deepspeed.utils import logger
 import deepspeed.comm as dist
 from deepspeed.runtime.data_pipeline.data_sampling.indexed_dataset import MMapIndexedDataset, valid_dtypes
 from deepspeed.runtime.data_pipeline.data_sampling.utils import split_dataset, split_index, create_mmap_dataset_builder, close_mmap_dataset_builder, find_fit_int_dtype
@@ -482,17 +482,17 @@ def __init__(
             dist.init_distributed()
 
         # comm_group and worker_id+num_workers are mutually exclusive
-        if comm_group is not None:
-            self.comm_group = comm_group
-            self.num_workers = self.comm_group.size()
-            self.worker_id = self.comm_group.rank()
+        self.comm_group = comm_group
+        if self.comm_group is None:
+            # self.comm_group = deepspeed.utils.groups._clone_world_group()
+            self.num_workers = num_workers
+            self.worker_id = worker_id
         else:
-            self.comm_group = groups._clone_world_group()
             self.num_workers = self.comm_group.size()
             self.worker_id = self.comm_group.rank()
 
         if self.worker_id == 0:
-            logger.info(f"Data analyzer initialized with {self.num_workers} workers.")
+            logger.info(f"Distributed data analyzer initialized with {self.num_workers} workers.")
 
     def run_map_reduce(self):
 
@@ -635,9 +635,18 @@ def file_write_ordered(self, tensor_list, fname, numpy_dtype):
         # method to deserializes a buffer into rows of different lengths and write them to file
         def write_buffer_to_file(buff, src, builder):
             assert self.worker_id == 0, "only rank 0 can write to file"
+
+            # # write one buffer at a time
+            # for row_len in row_lens[src]:
+            #     builder.add_item(buff[:row_len].cpu())
+            #     buff = buff[row_len:]
+
+            # collect all buffers and write them all at once
+            buffer_list = []
             for row_len in row_lens[src]:
-                builder.add_item(buff[:row_len].cpu())
+                buffer_list.append(buff[:row_len].cpu())
                 buff = buff[row_len:]
+            builder.add_items(buffer_list)
 
         # 5. rank 0 prepares output folder and file
         if self.worker_id == 0:
diff --git a/deepspeed/runtime/data_pipeline/data_sampling/indexed_dataset.py b/deepspeed/runtime/data_pipeline/data_sampling/indexed_dataset.py
index 7a6963bc27eb..453e6ba6039d 100644
--- a/deepspeed/runtime/data_pipeline/data_sampling/indexed_dataset.py
+++ b/deepspeed/runtime/data_pipeline/data_sampling/indexed_dataset.py
@@ -581,10 +581,18 @@ def __init__(self, out_file, dtype=np.int64):
         self._doc_idx = [0]
 
     def add_item(self, tensor):
+        """ write the tensor to the file and update its size in the index"""
         np_array = np.array(tensor.numpy(), dtype=self._dtype)
         self._data_file.write(np_array.tobytes(order='C'))
         self._sizes.append(np_array.size)
 
+    def add_items(self, tensor_list):
+        """ write a list of tensors to the file and update their sizes in the index"""
+        np_arrays = [np.array(t.numpy(), dtype=self._dtype) for t in tensor_list]
+        self._data_file.writelines([arr.tobytes(order='C') for arr in np_arrays])
+        for arr in np_arrays:
+            self._sizes.append(arr.size)
+
     def add_item_numpy(self, np_array):
         if np_array.dtype != self._dtype:
             np_array = np_array.astype(self._dtype)