--- /dev/null
+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.
+