1. Introduction

File System is used to control how data is stored and retrieved , consists of the following three parts: file system API, logical part (data + information about data management structure), and the physical storage of data.

A FS provides a generalized structure over persistent storage, allowing the low-level structure of the devices (e.g., disk, tape, flash memory storage) to be abstracted away. Generally speaking, the goal of a filesystem is allow logical groups of data to be organized into files, which can be manipulated as a unit. In order to do this, the filesystem must provide some sort of index of the locations of files in the actual secondary storage. The fundamental operations of any filesystem are:

Tracking the available storage space
Tracking which block or blocks of data belong to which files
Creating new files
Reading data from existing files into memory
Updating the data in the files
Deleting existing files

2. File System Theory

Metadata is data about data. For example a file named 'A.txt' , so 'A.txt' is data , and the owner , write/read rules , stored path of this file all called metadata.

2.1 Physical Storage of data

Whatever NAND or SSD , all the file system stored on physical storage are serial structure , such like memory address.

2.2 File System Structure

超级块superblock

超级块记录了关于文件系统本身的关键信息（metadata of metadata），包括文件系统的类型，大小，状态以及inode的结构（metadata）。由于超级块非常的重要，因此在文件系统中超级块存在着一定的冗余。

i-节点区i-node

i代表index，i-node也叫索引节点。一个i-node存储一个文件的元数据，i-node实际上存储了关于所有权（用户、组），访问权限（可读可写可执行），数据的存储位置等等元数据，所以i-node就是一个文件的化身，一个文件对应一个i-node。

值得注意的是：i-node中并不存储文件名这一信息，关于这一点原因将在后面解释。

还有很重要的一点是所有的i-node组成一个线性结构，并且通过下标来标识，下标也叫做i-node号，这个和磁盘盘块号不一样，inode是建立在盘块上的。

数据区data

最后一个部分是数据区，文件的内容（数据）就保存在这个区域。一个磁盘上的块大小是固定的，如果一个文件的大小大于一个磁盘块的话，数据会分布在多个磁盘块之中，文件系统（i-node）负责追踪这些数据块。

2.3 i-node

i-node stores metadata for every file on your system in a table like structure usually located near the beginning of a partition. They store all the information except the file name and the data.

i-node numbers are unique at the partition level. Every partition as its own i-node table. When a new file is created, it is assigned an i-node number and a file name. The i-node number is a unique number within that file system. Both name and i-node number are stored as entry in a directory. If you run out of i-node, you cannot create new files even if you have space left on the given partition.

( Note : Linux和Unix将文件的数据和元数据分开存放，具体来讲inode存储元数据，数据区存储数据。在 i-node 中不必存储真正的数据，但是需要存储数据被放在了数据区的哪个位置。inode保存着文件在数据区的盘块列表，这也是索引结点名称 i-node 的由来。)

3. JFFS2

4. Squashfs

Squashfs is a compressed read-only file system for Linux. It uses zlib/lzo/xz compression to compress files, inodes and directories.

A squashfs filesystem consists of a maximum of nine parts, packed together on a byte alignment:

     ---------------
    |  superblock   |
    |---------------|
    |  compression  |
    |    options    |
    |---------------|
    |  datablocks   |
    |  & fragments  |
    |---------------|
    |  inode table  |
    |---------------|
    |   directory   |
    |     table     |
    |---------------|
    |   fragment    |
    |    table      |
    |---------------|
    |    export     |
    |    table      |
    |---------------|
    |    uid/gid    |
    |  lookup table |
    |---------------|
    |     xattr     |
    |     table     |
     ---------------

Compressed data blocks are written to the filesystem as files are read from the source directory, and checked for duplicates. Once all file data has been written the completed inode, directory, fragment, export, uid/gid lookup and xattr tables are written.

5. YAFFS

Yaffs (Yet Another Flash File System) is a NAND-flash specific file system.( ref: How Yaffs Works . )

The Yaffs Direct Interface (YDI) which allows Yaffs to be simply integrated with embedded systems, with or without an RTOS.

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