【CHICHI ETUTOR@ARM/MIPS/PPC】FILE SYSTEM 1 _ INTRODUCTION TO File System

1. What is "File System"

• A filesystem is the methods and data structures that an operating system uses to keep track of files on a disk or partition; that is, the way the files are organized on the disk. The word is also used to refer to a partition or disk that is used to store the files or the type of the filesystem.

2. Where it stores

• File systems are based on block device e.g.  Hard disks , Flash
• File system is stored on physical devices in partitions 

3. File system type

• For Desktops  , EXT2 / EXT3 / EXT4 / FAT / NTFS
• For embedded system , YAFFS2 / JFFS2 / CRAMFS / UBIFS
• Pseudo file system , Proc / sysfs 
• For HADOOP , HDFS
• For google distributed system , GFS

4. Mounting a File system

• Kernel must have been compiled to support file system type
• have permission/right to access hardware device for mount
• using mount for file system mounting. 

                   mount   /dev/sdb1 /mnt 

5. Root File System

• mounted at the base of the file system hierarchy (FHS), designed as  /
• contain programs and utilities to boot system initialize service, load device driver and mount additional file system

6. File System Hierarchy Standard (FHS)

• establishes a minimum baseline of compatibility between Linux distributions and application program

• allows your application software and developer to predict where certain system elements, including file and directories, can be found on the file system

7. Minimal File System

|-- bin

        |-- busybox

        |-- sh -> busybox

|-- dev

        |-- console

|-- etc

        |-- init.d

        |-- rcS

|-- lib

        |-- ld-2.3.2.so

        |-- ld-linux.so.2 -> ld-2.3.2.so

        |-- libc-2.3.2.so

        |-- libc.so.6 -> libc-2.3.2.so

8. Setting Up Root File System

• Step 1: create file system hierarchy

        - mkdir all the folders

• Step 2: create device files

        - mknod 

        - All the device files in a Linux root filesystem are located in the /dev directory

        - conserved system device node

Name |Descrption              |Type |Major |Minor |Permission

mem        |Physical memory access    |Char |1     |1        |600

null         |Null device              |Char |1        |3        |666

zero |Null byte source              |Char |1     |5        |666

Random   |random number generator |Char |1     |8        |664

tty0         |Current virtual console        |Char    |4       |0        |600

tty1           |First virtual console            |Char    |4        |1        |600

ttyS0        |First UART serial port         |Char    |4        |64      |600

tty             |Current TTY device            |Char    |4        |0        |666

console    |System console              |Char    |5        |1        |600

• ™Step 3: install libraries and kernel modules

       -download library source code , compile , and copy to /lib 

       -download drivers for devices  , compile ,and copy to /lib/modules

• ™Step 4: install system utilities

       -make busybox and copy to /bin

• ™Step 5: make root file system image

       -mkfs.jffs2

       -mkfs.cramfs

       

9. What kernel did before "jump" to file system

• Free the memory
• Open a system console device
• Start the first user space program

kernel code example

10. "init" Process in File system

• "Init" is the ultimate parent of all userspace processes in a Linux system
• "init" runs with scripts (/etc/rc.X) commonly called system V init
• "init" has the concept of a runlevel

• folders corresponding to RunLevel  (/etc)

./rcS.d

./rc0.d

./rc6.d

./rc5.d

./rc.local

./rc1.d

./rc2.d

./rc3.d

./rc4.d

• script files in "rc0.d"

         -  basically soft link to files in /etc/init.d

         -  files with S point to service scripts, which invoked with startup instruction

         -  files with K point to service scripts, which invoked with shutdown   instruction

/etc/rc2.d$ ls -l

total 4

lrwxrwxrwx 1 root root  29 Aug  5 19:29 K10unattended-upgrades -> ../init.d/unattended-upgrades

lrwxrwxrwx 1 root root  20 Aug  5 19:29 K20kerneloops -> ../init.d/kerneloops

lrwxrwxrwx 1 root root  18 Oct  1 16:42 K20openipmi -> ../init.d/openipmi

lrwxrwxrwx 1 root root  15 Aug  5 19:29 K20rsync -> ../init.d/rsync

lrwxrwxrwx 1 root root  27 Aug  5 19:29 K20speech-dispatcher -> ../init.d/speech-dispatcher

lrwxrwxrwx 1 root root  27 Oct 13 15:56 K80nfs-kernel-server -> ../init.d/nfs-kernel-server

-rw-r--r-- 1 root root 353 Mar 13  2014 README

lrwxrwxrwx 1 root root  18 Aug  5 19:29 S20sendsigs -> ../init.d/sendsigs

lrwxrwxrwx 1 root root  17 Aug  5 19:29 S30urandom -> ../init.d/urandom

lrwxrwxrwx 1 root root  22 Aug  5 19:29 S31umountnfs.sh -> ../init.d/umountnfs.sh

lrwxrwxrwx 1 root root  18 Aug  5 19:29 S40umountfs -> ../init.d/umountfs

lrwxrwxrwx 1 root root  26 Aug  5 15:41 S59cryptdisks-early -> ../init.d/cryptdisks-early

lrwxrwxrwx 1 root root  20 Aug  5 19:29 S60umountroot -> ../init.d/umountroot

lrwxrwxrwx 1 root root  14 Aug  5 19:29 S90halt -> ../init.d/halt