File Systems Basics

Overview

• storage stack

• file system
• abstraction on top of disk blocks
• organize persistent data into files and directories
• provide names and permissions to persistent data
• provide simple APIs to modify persistent data
• read, write, create, delete
• hide data placement (where data is stored on disk) details from users
• main abstractions
• file
• a container of persistent data
• consists of metadata and data
• metadata: size, owner, permission, type, where data locates on disk
• metadata for file is also known as inode, file header, file record
• directory
• a way to group and organize files
• if we implement directory as a file, what would its metadata & data track?
• metadata: track same set of information as file metadata
• data: track files within the directory
• any benefit to treating directory as a file?
• directory data format
• consists of an array of directory entries
• each directory entry contains 1. file name 2. file's metadata location
• each directory has two default directory entries: . (current dir) and .. (parent dir)
• path
• namespace convention
• /separated path consists of all directories leading to the file
• all files can be found starting at root direcotry /
• to read /home/tom/foo.txt

1. read in root's metadata, locate the data blocks for root
2. read in root's data block, search for directory entry for "home"
3. read in home's metadata, locate the data blocks for home
4. read in home's data block, search for directory entry for "tom"
5. read in tom's metadata, locate the data blocks for tom
6. read in tom's data block, search for directory entry for "foo.txt"
7. read in foo.txt's metadata, locate the data blocks for foo.txt
8. read foo.txt's data block(s)!
• how do we know where to find root directory's metadata?