/***********************************************************/
/*  This is the readme for the testfs filesystem           */
/*  Author : Manish Katiyar <mkatiyar@gmail.com>           */
/*  Description : A simple disk based filesystem for linux */
/*  Date   : 08/01/09                                      */
/*  Version : 0.01                                         */
/*  Distributed under GPL                                  */
/***********************************************************/

Testfs is based on minix/ext2 filesystem and most of the code,logic, structures have been
duplicated from them. I started writing this fs just for fun and to learn linux
filesystems. Feel free to modify if you wish.

About testfs :
--------------

Testfs has a very simple layout on disk. Currently I plan to have limitation on the size of
files and directories. Inorder to keep things simple at start I don't do any special allocation
for blocks. Each inode has a fixed block number where it should look for its data and that is
its inode number. So an inode number of 6 will have its data in the 6th block of filesystem. And
that is the reason I have a limit that no file can grow more than the blocksize. If you try to write
more than that you should be getting an ENOSPC error.

Inode blocks span over 3 blocks in our filesystem and start from the 3rd block ie... 3rd, 4th and 5th 
blocks are reserved for the inode table. So the maximum number of inodes we can have in the filesystem
is (3*blocksize)/(size of inode).

0th block in testfs is free, superblock resides at the first block. And block number 1, holds the inode
bitmap.
Inode allocation block and block allocation map are same because once you reserve an inode, you automatically
also reserve the block because of the 1:1 mapping between them.

So the overall layout looks something like below :

______________________________________ _ _ _ _ _ _ _ _ ________________________
|       |        |      |      |     |       |         |        |       | 
|  0    |   1    |  2   |  3   |  4  |  5    |    6    |   7    |   8   | 
|       |        |      |      |     |       |         |        |       | ---->  blocks  
|_______|________|______|______|_____|_ _ _ _|_ _ _ _ _|________|_______|_______
    ^       ^        ^   <----- -^---------->     ^        ^
 Free    superblock inode     inode table      root dir   1st user
 block              bitmap      block                      file


Directory entry layout of the testfs is exactly same as ext2 except that fact that I haven't optimised
on size of fields and all of them are 4 byte aligned. The max name length of a file that testfs can
support is based on TESTFS_MAX_NAME_LEN which is 12 currently.

Some fields :

sb_first_nonmeta_inode : This is a #defined value which is the first inode number any file
can have in our filesystem. ie... All the inodes below are reserved and should not ever appear
in the fs listing.

How to Use :
-------------

Inorder to test and use testfs do the following steps.

a) Compile util/mktestfs.c. Just a "gcc -o mktestfs mktestfs.c" from util directory will work.
b) Create an empty directory where you need to test. "mkdir testdir"
c) Create an empty file . "cd testdir;dd if=/dev/zero of=mytestfile bs=4096 count=30"
d) Create "testfs" filesystem on mytestfile". Run "mktestfs" and give argument as mytestfile when it asks
for a filename.
e) Compile testfs source code with your kernel source. I do it with my UML . Change the pathname in Makefile
appropriately. If you don't want debug messages to be flooded on your screen you can change the build flags,
but probably you should keep it so that you know what is happening if you are using testfs for learning.
f) Inser the testfs module. As root do "insmod testfs.ko"
g) You should see an entry for testfs in "lsmod"
h) Go to testdir and create a directory "cd testdir;mkdir mnt"
i) Mount testfs over mnt. " mount -t testfs mytestfile mnt"
j) Verify that it has been mounted using "mount"

k) Now you can go into mnt and try to do some fs operations like creating, deleting file etc. Not all the operations
are supported currently so you might get errors.
l) You can also see testfs inode cache utilization by doing "cat /proc/slabinfo|grep testfs"
l) After that unmount the mnt directory and do a "rmmod testfs.ko" to unregister testfs filesystem.

Feel free to play and learn with testfs. Send any bugs/queries to mkatiyar@gmail.com !!!