Author		:	Raghavendra Suvvari
SBU ID		:	109596686

Files included	:	wrapfs.h, file.c, inode.c, copyup.c, dentry.c
			lookup.c, mmap.c, super.c, Makefile, ReadMe



ABOUT u2fs:
-----------

u2fs is a stackable, fan-out, unification file system.
u2fs takes two branches - left branch(LB) and right branch (RB)
and while mounting it combines these two branches and 
presents them as a single branch to the user.

Here LB has higher priority than RB and LB had R/W permissions where as
RB had just READ permissions. So if we have same files/directories names
in both LB and RB then after mounting we can see only the contents of the 
LB with that same name as it has higher priority.

 		u2fs
                 /\
                /  \
               /    \
             LB      RB

Here the files in LB and RB will physically be staying in the same underlying 
location as they were before. The advantage of u2fs is that we can keep the 
underlying directories separate, which is convenient for software management 
as well as Live-CDs, but at the same time provide a convenient merged view 
of the two directories to users.


USAGE : 
-------

Mounting:	mount -t u2fs -o ldir=./left/dir,rdir=./right/dir null ./mnt/u2fs
Umounting:	umount mnt/u2fs

where './mnt/u2fs' is the mount point and 'ldir=./left/dir,rdir=./right/dir' 
are the two directories 'ldir' and 'rdir' which are to be combined and mounted 
in new file system.

Here the device name will be 'null' and we pass the directories as options to 
'mount' command which will parse the options and then pass the directories to 
the file system.

Inside the mounted directory (mnt/u2fs) we can use normal file/dir manipulation 
commands like cp,mv,mkdir,ls etc.


Compiling and executing :
------------------------
'make' will compile and create the 'img' file for the files system which is a loadable module.

'insmod ./fs/wrapfs/wrapfs.ko' will insert this module.

And then we can mount the u2fs and use it.
mount -t u2fs -o ldir=./left/dir,rdir=./right/dir null ./mnt/u2fs

now by going to ./mnt/u2fs directory we can see the contents of ./left/dir and ./right/dir
directories.
Now we can perform normal operations here in this directory.

Unmounting can be done by using 'umount mnt/u2fs'

'rmmod wrapfs' will remove the module.

Approach for coding:
-------------------
u2fs was developed on top of wrapfs(which is a stackable file system for single
branch) code. I have also referenced unionfs code and mirrored some functionalities
in u2fs.

u2fs was coded by taking the existing wrapfs code and adding functionalities of u2fs
in an incremental way.

The directories for the branching will be identified by parsing the options.
Inode numbers for the files in u2fs will be uniquely generated by using 'iunique' function.
we can create/edit the files in filesystem by using any of the file manipulation commands
like touch, echo, vim etc.
The new files created will be physically created in the LB (as RB has just Read permissions).

If we try to create a new file inside the directory of RB then the new file will be created
in the LB replacing the existing file in the RB in u2fs. This is called copyup operation.


And if we try to delete a file in the RB then the file will be marked as whiteout and it
will not be visible to the user in u2fs.
---------------------------------------------------------------------------------------------

Below are some of more in detail(in the format of unionfs documentation):

Branch Priority:
================

Left branch has higher priority than the right branch.

Branch Mode:
============

Left branch has R/W permissions where as Right branch has only Read permissions.

Whiteouts:
==========

A whiteout removes a file name from the namespace. Whiteouts are needed when
one attempts to remove a file on a read-only branch.

After unioning both the branches and if anyone tries to delete a file in the right branch
which is read only then a whiteout will be created in the left branch and the file in the right branch 
will be hidden.

Duplicate Elimination:
======================

If both the branches have files with the same names then the file in the left branch will
be displayed to the user as LB had higher priority than RB.

Copyup:
=======

If a user tries tries to create newfiles in the RB in the mount point then the file in RB wil be hidden
and a newopy of the file will be created in the LB physically.