DOCS-7145: limits for sharding existing data

Signed-off-by: kay <kay.kim@10gen.com>

source/reference/limits.txt

@@ -322,42 +322,74 @@ Sharding Operational Restrictions
 
 .. limit:: Sharding Existing Collection Data Size
 
-   For existing collections that hold documents, MongoDB supports
-   enabling sharding on *any* collections that contains less than 256
-   gigabytes of data. MongoDB *may* be able to shard collections with as
-   many as 400 gigabytes depending on the distribution of document
-   sizes. The precise size of the limitation is a function of the
-   chunk size and the data size. Consider the following table:
+   An existing collection can only be sharded if its size does not exceed
+   specific limits. These limits can be estimated based on the average size of
+   all :term:`shard key` values, and the configured :term:`chunk` size.
+
+   .. important:: 
+
+      These limits only apply for the initial sharding operation. Sharded
+      collections can grow to *any* size after successfully enabling sharding.
+
+   Use the following formulas to calculate the *theoretical* maximum
+   collection size. 
+
+   .. code-block:: javascript
+
+      maxSplits = 16777216 (bytes) / <average size of shard key values in bytes>
+      maxCollectionSize (MB) = maxSplits * (chunkSize / 2)
+
+   .. note::
+
+      The maximum :term:`BSON` document size is 16MB or ``16777216`` bytes.
+
+      All conversions should use base-2 scale, e.g. 1024 kilobytes = 1
+      megabyte.
+
+   If ``maxCollectionSize`` is less than or nearly equal to the target
+   collection, increase the chunk size to ensure sucessful initial sharding.
+   If there is doubt as to whether the result of the calculation is too
+   'close' to the target collection size, it is likely better to increase the
+   chunk size.
+
+   After successful initial sharding, you can reduce the chunk size as needed.
+   If you later reduce the chunk size, it may take time for all chunks to
+   split to the new size. See
+   :doc:`/tutorial/modify-chunk-size-in-sharded-cluster` for instructions on
+   modifying chunk size.
+
+   This table illustrates the approximate maximum collection sizes
+   using the formulas described above:
 
    .. list-table::
       :header-rows: 1
       :stub-columns: 1
 
-      * - Shard Key Size
+      * - Average Size of Shard Key Values
         - 512 bytes
-        - 200 bytes
-      * - Number of Splits
-        - 31,558
-        - 85,196
-      * - Max Collection Size (1 MB Chunk Size)
-        - 31 GB
-        - 83 GB
+        - 256 bytes
+        - 128 bytes
+        - 64 bytes
+      * - Maximum Number of Splits
+        - 32,768
+        - 65,536
+        - 131,072
+        - 262,144
       * - Max Collection Size (64 MB Chunk Size)
-        - 1.9 TB
-        - 5.3 TB
+        - 1 TB
+        - 2 TB
+        - 4 TB
+        - 8 TB
+      * - Max Collection Size (128 MB Chunk Size)
+        - 2 TB
+        - 4 TB
+        - 8 TB
+        - 16 TB
       * - Max Collection Size (256 MB Chunk Size)
-        - 7.6 TB
-        - 21.2 TB
-      * - Max Collection Size (512 MB Chunk Size)
-        - 15.2 TB
-        - 42.4 TB
-
-   The data in this chart reflects documents with no
-   data other than the shard key values and therefore represents the
-   smallest possible data size that could reach this limit.
-
-   .. important:: Sharded collections may have *any* size, after
-      successfully enabling sharding.
+        - 4 TB
+        - 8 TB
+        - 16 TB
+        - 32 TB
 
 .. limit:: Single Document Modification Operations in Sharded Collections