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+MDS internal data structures
+==============================
+
+*CInode*
+  CInode contains the metadata of a file, there is one CInode for each file.
+  The CInode stores information like who owns the file, how big the file is.
+
+*CDentry*
+  CDentry is the glue that holds inodes and files together by relating inode to
+  file/directory names. A CDentry links to at most one CInode (it may not link
+  to any CInode). A CInode may be linked by multiple CDentries.
+
+*CDir*
+  CDir only exists for directory inode, it's used to link CDentries under the
+  directory. A CInode can have multiple CDir when the directory is fragmented.
+
+These data structures are linked together as::
+
+  CInode
+  CDir
+   |   \
+   |      \
+   |         \
+  CDentry   CDentry
+  CInode    CInode
+  CDir      CDir
+   |         |  \
+   |         |     \
+   |         |        \
+  CDentry   CDentry  CDentry
+  CInode    CInode   CInode
+
+As this doc is being written, size of CInode is about 1400 bytes, size of CDentry
+is about 400 bytes, size of CDir is about 700 bytes. These data structures are
+quite large. Please be careful if you want to add new fields to them.
+