The Way to Think

To think about file systems, think of:

1. Data structures of the file system

• Should the file system use a simple structure like an array or a complicated one?

2. Access methods

• How does this map onto system calls like open(), read(), write()?

Overall Organization

We divide the disk into 4 KB blocks. Imagine there are 64 blocks (0 - 63):

0-7, 8-15, 16-23, 24-31, 32-39, 40-47, 48-55, 56-63

The first chunk of these blocks (0-7) is reserved for the system’s use.

We’ll use 5 blocks (3-7) to hold inodes, which comprise a file, the size of the file, owner and access rights, access and modify times, and other information.

We need a way to index into the data and the inodes, so one block is used for a data bitmap, and another is used for an inode bitmap.

Finally, the first block is the superblock, which contains information about the particular file system, like how many inodes and data blocks are in the system, where the inode table begins, and where the data begins, as well as a magic number to identify the file system type.

File Organization: The Inode

The inode is short for index node, because these nodes were arranged in an array, and indexed into when accessing a particular inode.

To read an inode (like number 32), the file system would calculate the offset into the inode region (32 * sizeof(inode)), which is 8192, add it to the start of the inode table (12KB), and then find out it was located at the 20KB.

Then it would issue a read to a sector on disk that corresponds to it (512KB blocks, so sector 40).