Add info about journal file drive space needs (#6837) (#6910)

* Add info about journal fiole drive space needs

* review changes

* refactor for clarity

* drive space => disk space

Author: MongoCaleb

source/core/journaling.txt

@@ -122,14 +122,29 @@ For details, see :ref:`manage-journaling-change-wt-journal-compressor`.
 Journal File Size Limit
 ```````````````````````
 
-WiredTiger journal files for MongoDB have a maximum size limit of
-approximately 100 MB.
+WiredTiger journal files have a maximum size limit of approximately 100 MB. 
+Once the file exceeds that limit, WiredTiger creates a new journal file.
 
-- Once the file exceeds that limit, WiredTiger creates a new journal
-  file.
+WiredTiger automatically removes old journal files and maintains only
+the files needed to recover from the last checkpoint.  To determine how much 
+disk space to set aside for journal files, consider the following: 
+
+- The default maximum size for a checkpoint is 2 GB
+- Additional space may be required for MongoDB to write new journal 
+  files while recovering from a checkpoint
+- MongoDB compresses journal files
+- The time it takes to restore a checkpoint is specific to your use case
+- If you override the maximum checkpoint size or disable compression, your 
+  calculations may be significantly different 
+
+For these reasons, it is difficult to calculate exactly how much additional 
+space you need. Over-estimating disk space is always a safer approach. 
+
+.. important::
+
+   If you do not set aside enough disk space for your journal 
+   files, the MongoDB server will crash. 
 
-- WiredTiger automatically removes old journal files to maintain only
-  the files needed to recover from last checkpoint.
 
 Pre-Allocation
 ``````````````