DOCS-773 faq on mongodb storage

source/faq/storage.txt

@@ -193,6 +193,46 @@ inserted into the database. Consider the following possible causes:
      running. Be aware that :dbcommand:`repairDatabase` will block
      all other operations and may take a long time to complete.
 
+After migration to MongoDB, why do my databases use more disk space?
+--------------------------------------------------------------------
+
+If you migrate existing databases to MongoDB, your MongoDB installation
+might take up significantly more disk space than your previous
+installation. There are a number of factors that can explain this
+difference, the most common of which is preallocation.
+
+The following factors affect database instance size:
+
+- Preallocation. MongoDB preallocates disk space for databases upon
+  creation to facilitate responsive write performance. For an
+  explanation of preallocation, see :ref:`faq-disk-size`.
+
+  To see how much space MongoDB has allocated for document storage for a
+  database, use the :dbcommand:`dbStats` command.
+
+- Document update patterns. To minimize document movements, which can
+  adversely affect performance, MongoDB adds :ref:`padding
+  <write-operations-padding-factor>` to documents in anticipation of
+  updates that change the sizes of documents. MongoDB adaptively learns
+  if documents in a collection tend to grow, and if your update patterns
+  often change document sizes, then MongoDB increases the factor used to
+  allocate padding, thus increasing disk storage. For more information,
+  see :ref:`write-operations-padding-factor`.
+
+  To see the padding factor for a given collection, use the
+  :dbcommand:`collStats` command.
+
+  For an approach to document growth, see
+  :doc:`/use-cases/pre-aggregated-reports`.
+
+- Data model. In small databases especially, document structure and
+  database organization affect the amount of disk space used. See
+  :ref:`faq-small-documents`.
+
+To compact storage space for a collection, run the :dbcommand:`compact`
+command, but only run it during scheduled maintenance periods. The
+:dbcommand:`compact` command is slow and blocks other database use.
+
 How can I check the size of a collection?
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