Chapter 40 File System Implementation

1. Chapter 40File System Implementation Chien-Chung Shen CIS, UD cshen@cis.udel.edu

2. Learning by Example • A simple file system implementation • vsfs (Very Simple File System) • a simplified version of Unix file system • features include on-disk structures, access methods, and various policies • File system is pure software so that no special hardware are added for better performance; however, need to understand and take advantage of device characteristics to make sure file systems work well • e.g., AFS -> ZFS with different data structures and features (pros and cons): learning via case studies

3. Mental Model of FS • Mental models are what you are really trying to develop when learning about systems • Mental model of file systems include answers to questions like • what on-disk structures store the file system’s data and metadata? • what happens when a process opens a file? • which on-disk structures are accessed during a read or write?

4. The Way to Think • Two aspects: object-oriented view • data structures, e.g., what types of on-disk structures are utilized by file system to organize its data and metadata (e.g., array of blocks or tree-based structure) • access methods, e.g., how system calls are mapped onto the data structures (e.g., which structures are read during the execution of a particular system call)

5. Overall Organization (1) • Overall on-disk organization of the data structures of the vsfs file system • disk is divided into a serious of equal-sized blocks • e.g.,a small disk is divided into 64 “4KB blocks” • 56-block “data region” for user data

6. Overall Organization (2) • Metadata [stored in inode] tracks each file (its data blocks, owner, access rights, modify time, etc.) • inode table on disk (5*16 “256-byte inodes”) • Need to track whether inodesand data blocks are free or allocated (allocation structures) • freelist or bitmap(data bitmap and inode bitmap)

7. Overall Organization (3) • Superblock: contains information about this file system, includinghow many inodes and data blocks are in the file system (80 and 56, respectively), where the inode table begins (block 3), etc. • When mounting file system, OS reads the superblock first to initialize various parameters

8. File Organization: inode (1) • “Indexnode” holding a file’s metadata • Inode table of 5 “4KB blocks” • To read inode number 32, calculate offset into inode region (32*sizeof(inode) = 8192), add it to the start address of the inode table on disk (12KB) to get 20KB

9. File Organization: inode (2) • Disks are not byte addressable, but rather consist of many addressable sectors, usually 512 bytes. • Thus, to fetch the block of inodes that contains inode 32, file system issues a read to sector 40 (20*1024/512) to fetch the desired inode block blk= (inumber * sizeof(inode_t)) / blockSize; sector = ((blk * blockSize) + inodeStartAddr) / sectorSize;

10. Where Data Blocks Are • One option: have one or more direct pointers (disk addresses) inside the inode, each refers to one disk block • Limited file size • Multi-level index • indirect pointers: instead of pointing to a block that contains user data, it points to a block that contains more pointers, each of which point to user data • e.g., 4 KB block and 4-byte disk addresses (12+1024)*4K • double indirect pointers • triple indirect pointers

11. Multi-Level Index