Operating Systems 2017F Lecture 18

Additional Notes

Lec 18

• More on filesystems
  
• How can you recover a fs and how do you delete a file?
  

A filesystem is a:

• Persistent data structure
  
• Stored in fixed size blocks (at least 512 bytes in size)
  
• Maps hierarchical filenames to file contents
  
• Has metadata about files somehow
  

What's in a filesystem

• data blocks
  
• metadata blocks, you need someway to find the blocks
  

How do you organize metadata?

First identify basic characteristics of the filesystem

You need a "superblock" which is a "summary" block that tells you about everything else

Normally the superblock is the first block of the filesystem

In the superblock

• Type of filesystem
  
  • What filesystem magic number is there
    
  • file command to know file type
    
• Size of the filesystem
  
• How the filesystem is organized
  
• Where can I find the rest of the metadata
  

He opened a .jpg as a binary file to show us the magic number in a file, first several bytes identify type of file. Kernel does not care about file extension. Userspace programs may care about the extension.

POSIX is a standard for UNIX
For POSIX filesystems

• File metadata is stored in INODES
  
• most have pre-reserved inodes