Project

1. Understanding Relevant Code

Refer to operating system and the links given in reference section of each page.

LInk to the book

2. Design

There are two ways in which immediate files can be implemented in the minix operating system -
Static : In this approach, the maximum file size is specified at creation and you can't change the type of file once created. In case the immediate file size exceeds the specified size, it will report an error. This is can be implemented by creating a new file system.
Dynamic : A file is created as an immediate file and if size exceeds specified size, it becomes a regular file. To implement this approach, we need to make changes in the existing file system.
Implementation in Dynamic Approach: To include immediate file system in an existing file system, the following sections are involved - Data structures representing files Virtual File System Message Formats Minix File System : in the funtions of the regular file system of minix, there is almost no change in create, open and close functions. The changes in read, write, delete are made so that if the file is immediate (which can be deduced by comparing size), then there is the content is to be accessed from the inode.

3. Tutorials for Basic Understanding

4. Installation and setting up the system

Follow this link to install minix in virtual box
Vi editor can be used to edit the code but if you need any other editior you can install it using pkgin or pkgin_cd. Refer the link above
If installation is to be done by pkgin_cd, you must install virtual box extensions from here
You might need to edit the rc.conf, if pkgin_cdis not working.
To transfer your code to eclipse (most preferred), follow this link

5. Algorithm

6. Final Implementation - Coding

File System Basic Structure

The /usr/src/servers/mfs directory contains the source code for FS in minx3.2 operating system. Some of the important files in mfs are main.c, in- ode.h, open.c, write.c, buf.h, super.h, super.c etc. The main function of each file in mfs are listed below:

buf.h - Defines the block cache. It contains a union named fsdata u with following attributes:
- b data[MAX BLOCK SIZE], a character array, containing ordinary user data.
- b dir[NR DIR ENTRIES( MAX BLOCK SIZE)] - directory block
- b bitmap[[FS BITMAP CHUNKS( MAX BLOCK SIZE)]] - bitmap block
- direct and indirect inode blocks
cache.c - FS has a buffer cache to reduce the number of disk accesses. File contains 9 procedures, few of them are listed below.
- get block - Fetch a block for reading/writing.
- put block - Return a block
- rw block - Transfer block between disk and cache
- free zone - If file is deleted, free the zone
const.h - Defines constants, like flags, table size that will be used in the file system. Few constants are, IN CLEAN, IN DIRTY, ATIME, CTIME etc
fs.h - Master header for FS, includes all header files needed by the MFS source files
glo.h - Defines all the global variables. Few examples of global variables are fs m in, fs m out, err code, fs dev, user path[PATH MAX] etc.
inode.h - Contains the stucucture for inode and the inode table as in- ode[NR INODES]
inode.c - Contains functions which manages the inode table. The func- tions are get inode(), put inode(), rw inode(), alloc inode() etc
main.c - Contains the main routiene of the file system. The main loop does three activities

get work(&fs_m_in) - Gets a new work
Processes the work i.e. selects the fuction to be called to complete the work and calls it from the table of function pointers.
reply(src, &fs_m_out) - Sends a reply

open.c - Contains the codes for six system calls:open, close, mknode, mkdir, close, lseek
proto.h - Lists all function prototypes for all functions used in MFS
read.c - All functions that are used for reading or writing are present in read.c. Some of the functions include, fs readwrite, rw chunk, read map etc.
super.h - Contains the superblock table. Super block holds information about inode bitmap, zone bitmaps, inodes etc.
super.c - Handles the superblock table and other related data structures like zone bitmap, inode bitmap etc. Major functions in this file are: al- loc bit(), free bit(), get super(), read super() etc.
table.c - Contains the table that map system call numbers onto the rou- tines.
write.c - Contains files that are not in read.c but are necessary for writing in a file. Most important functions in write.c are write map, clear zone, new block and zero block

Data Structures Involved

Inode

we can see that there are 7 zones of 4 bytes each, an indirect zone of size 4 bytes, a double indirect node of size 4 bytes and an unused space of 4 bytes. Each zone points to a disk block where actual data gets stored. According to definition of immediate files we had to find a space in the inode where we can store the immediate files. These zones are apt place to store the immediate files because no data which is critical to the file is being affected. We calcualted maximum size of immediate files as 40 bytes by adding up sizes of all zones, indirect zones and unused space.

max size = zn sz X 7 + ud sz + idt sz + ddt sz
max size = 4 X 7 + 4 + 4 + 4 = 40bytes

buffer or block cache

This is a union of different types of blocks in the disk. Eg. normal data block, directory block, inode block, bitmap block etc. The design of buffer or block cache is given below.

b_data array is used to cache the data which is stored in the disk block, all the modifications by the user are done here and then the data is written back to the disk. b data(b) is a macro which returns the pointer to the first byte of b data array.

message

mess 1, mess 2, mess 3, . . . are different message types. In MFS, global variables, fs m in and fs m out, of type message are used to send and recieve messages from various servers like VFS. Following system calls are used for message passing: echo, notify, sendrec, receive, send.

Files Involved

List of all files in which codes are added or deleted.

/src/include/const.h - A new flag I IMMEDIATE is created to sup- port immediate files
src/sys/lib/libsa/minixfs3.h - A new flag I IMMEDIATE is created to support immediate files
/src/servers/vfs/open.c - When O CREATE flag is set. Set the file mode as immediate instead of regular. It will have size zero.

src/sys/sys/stat.h - Following additions are done in this file:
- S_IFIMMED - macro was defined, similar to regular files
  #define S_IFIMMED 0130000
- S_IFIMMED - macro redifined for easier usage, similar to regular files.
  #define S_IFIMMED S_IFIMMED
- S_ISIMMED(m) - macro defined which checks whether a files is immediate or not (similar to regular files)
  #define S_ISIMMED(m) ( ( (m) & S IFMT ) == S IFIMMED )
- There are 114 more files where we have located S ISREG(m) and added S ISIMMED(m) also in the code, because regular files and immediate files have same functionalities except that immediate files are stored in inode and regular files are stored in disk blocks pointed by zones in the inode. Few of the files are listed below:
  - /servers/vfs/select.c
  - /servers/vfs/link.c
  - /servers/vfs/read.c
  - /commands/grep/mmfile.c ... etc
- /src/severs/mfs/read.c - Major changes/addition for implementation of immediate file system is done in this file, in the fs readwrite() function
References
- Paper on immediate files
- Operating Systems: Design And Implementation 3rd Edition by Andrew Tanenbaum
- Major Project Report