File System Interface and Implementations

1. File System Interface and Implementations Fred Kuhns CS523 – Operating Systems CS523S: Operating Systems

2. FS Framework in UNIX • Provides persistent storage • Facilities for managing data • file - abstraction for data container, supports sequential and random access • file system - permits organizing, manipulating and accessing files • User interface specifies behavior and semantics of relevant system calls • Interface exported abstractions: files, directories, file descriptorsand differentfile systems CS523S: Operating Systems

3. Kernel, Files and Directories • kernel provides control operations to name, organize and control access to files but it does not interpret contents • Running programs have an associated current working directory. Permits use of relative pathnames. Otherwise complete pathnames are required. • File viewed as a collection of bytes • Applications requiring more structure must define and implement themselves CS523S: Operating Systems

4. Kernel, Files and Directories • files and directories form hierarchical tree structure name space. • tree forms a directed acyclic graph • Directory entry for a file is known as a hard link. • Files may also have symbolic links • File may have one or more links • POSIX defines library routines {opendir(), readdir(), rewinddir(), closedir()} struct dirent { ino_t d_ino; char d_name[NAME_MAX + 1]; } CS523S: Operating Systems

5. File and Directory Organization / (hard) links bin etc dev usr vmunix sh local etc /usr/local/bin/bash bin bash CS523S: Operating Systems

6. File Attributes • Type – directory, regular file, FIFO, symbolic link, special. • Reference count – number of hard links {link(), unlink()} • size in bytes • device id – device files resides on • inode number - one inode per file, inodes are unique within a disk partition (device id) • ownership - user and group id {chown()} • access modes - Permissions and modes {chmod()} • {read, write execute} for {owner, group or other} • timestamps – three different timestamps: last access, last modify, last attributes modified. {utime()} CS523S: Operating Systems

7. Permissions and Modes • Three Mode Flags = {suid, sgid and sticky} • suid – • File: if set and executable then set the user’s effective user id • Directory: Not used • sgid – • File: if set and executable then set the effective group id. If sgid is set but not executable then mandatory file/record locking • Directory: if set then new files inherit group of directory otherwise group or creator. • sticky – • File: if set and executable file then keep copy of program in swap area. • Directory: if set and directory writable then remove/rename if EUID = owner of file/directory or if process has write permission for file. Otherwise any process with write permission to directory may remove or rename. CS523S: Operating Systems

8. User View of Files • File Descriptors (open, dup, dup2, fork) • All I/O is through file descriptors • references the open file object • per process object • file descriptors may be dup’ed {dup(), dup2()}, copied on fork {fork()} or passed to unrelated process {(see ioctl() or sendmsg(), recvmsg()}permitting multiple descriptors to reference one object. • File Object - holds context • created by an open() system call • stores file offset • reference to vnode • vnode - abstract representation of a file CS523S: Operating Systems

9. How it works fd = open(path, oflag, mode); lseek(), read(), write() affect offset File Descriptors {{0, uf_ofile} {1, uf_ofile} {2 , uf_ofile} {3 , uf_ofile} {4 , uf_ofile} {5 , uf_ofile}} Open File Objects {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}, {*f_vnode,f_offset,f_count,...}} Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file Vnode/vfs In-memory representation of file CS523S: Operating Systems

10. File Systems • File hierarchy composed of one or more File Systems • One File System is designated the Root File System • Attached to mount points • File can not span multiple File Systems • Resides on one logical disk CS523S: Operating Systems

11. Logical Disks • Viewed as linear sequence of fixed sized, randomly accessible blocks. • device driver maps FS blocks to underlying storage device. • created using newfs or mkfs utilities • A file system must reside in a logical disk, however a logical disk need not contain a file system (for example the swap device). • Typically logical disk corresponds to partion of a physical disk. However, logical disk may: • map to multiple physical disks • be mirrored on several physical disks • striped across multiple disks or other RAID techniques. CS523S: Operating Systems

12. File Abstraction • Abstracts different types of I/O objects • for example directories, symbolic links, disks, terminals, printers, and pseudodevices (memory, pipes sockets etc). • Control interface includes fstat, ioctl, fcntl • Symbolic links: file contains a pathname to the linked file/directory. {lstat(), symlink(), readlink()} • Pipe and FIFO files: • FIFO created using mknod(), lives in the file system name space • Pipe created using pipe(), persists as long as opened for reading or writing. CS523S: Operating Systems

13. OO Style Interfaces Instance of derived class Abstract base class Struct interface_t { // Common functions: open (), close () // Common data: type, count // Pure virtual functions *ops (Null pointer) // Private data *data (Null pointer) } Struct interface_t { open (), close () type, count *ops *data } {my_read() my_write() my_init() my_open() … } {device_no, free_list, lock, …} CS523S: Operating Systems

14. Overview System calls vnode interface /proc tmpfs UFS HSFS PCFS RFS NFS swapfs cdrom diskette disk Process address space Anonymous memory Example from Solaris CS523S: Operating Systems

15. Vfs/Vnode Framework • Concurrently support multiple file system types • transparent interoperation of different file systems within one file hierarchy • enable file sharing over network • abstract interface allowing easy integration of new file systems by vendors CS523S: Operating Systems

16. Objectives • Operation performed on behalf of current process • Support serialized access, I.e. locking • must be stateless • must be reentrant • encourage use of global resources (cache, buffer) • support client server architectures • use dynamic storage allocation CS523S: Operating Systems

17. Vnode/vfs interface • Define abstract interfaces • vfs: Fundamental abstraction representing a file system to the kernel • Contains pointerss to file system (vfs) dependent operations such as mount, unmount. • vnode: Fundamental abstraction representing a file in the kernel • defines interface to the file, pointer to file system specific routines. Reference counted. • accessed in two ways: • 1) I/O related system calls • 2) pathname traversal CS523S: Operating Systems

18. vfs Overview Struct vfsops { *vfs_mount, *vfs_root, …} Struct vfsops { *vfs_mount, *vfs_root, …} rootvfs private data private data Struct vfs { *vfs_next, *vfs_vnodecovered, *vfs_ops, *vfs_data, …} Struct vfs { *vfs_next, *vfs_vnodecovered, *vfs_ops, *vfs_data, …} Struct vnode { *v_vfsp, *v_vfsmountedhere,…} Struct vnode { *v_vfsp, *v_vfsmountedhere,…} Struct vnode { *v_vfsp, *v_vfsmountedhere,…} / (root) /usr / (mounted fs) CS523S: Operating Systems

19. Mounting a FS • mount(spec, dir, flags, type, dataptr, datalen); • SVR5 uses a global virtual file system switch table (vfssw) • allocate and initialize private data • initialize vfs struct • initialize root vnode in memory (VFS_ROOT) CS523S: Operating Systems

20. Pathname traversal • Verify vnode is dir or stop • invoke VOP_LOOKUP (ufs_lookup()) • if found, return pointer to vnode (locked) • else not found and last component, return success and vnode of parent directory (locked) • not found, release directory, repeat loop CS523S: Operating Systems

21. Local File Systems • S5fs - System V file system. Based on the original implementation. • FFS/UFS - BSD developed filesystem with optimized disk usage algorithms CS523S: Operating Systems

22. S5fs - Disk layout • Viewed as a linear array of blocks • Typical disk block size 512, 1024, 2048 bytes • Physical block number is the block’s index • disk uses cylinder, track and sector • first few blocks are the boot area, which is followed by theinode list (fixed size) CS523S: Operating Systems

23. Disk Layout tract sector heads cylinder platters Rotational speed disk seek time CS523S: Operating Systems

24. S5fs disk layout bootarea superblock inode list data Boot area - code to initialize bootstrap the system Superblock - metadata for filesystem. Size of FS, size of inode list, number of free blocks/inodes, free block/inode list inode list - linear array of 64byte inode structs CS523S: Operating Systems

25. s5fs - some details inode name Di_mode (2) di_nlinks (2) di_uid (2) di_gid (2) di_size (4) di_addr (39) di_gen (1) di_atime (4) di_mtime (4) di_ctime (4) . 8 .. 45 “” 0 myfile 123 2 byte 14byte On-disk inode directory CS523S: Operating Systems

26. Locating file data blocks Assume 1024 Byte Blocks 0 1 2 3 4 5 6 7 8 9 10 - indirect 11 - double indirect 12 - triple indirect 256 blocks 65,536 blocks 16,777,216 blocks CS523S: Operating Systems

27. S5fs Kernel Implementation • In-Core Inodes - also include vnode, device id, inode number, flags • Inode lookup uses a hash queue based on inode number (amy also use device number) • kernel locks inode for reading/writing • Read/Write use a buffer cache or VM CS523S: Operating Systems

28. Problems with s5fs • Superblock • on-disk inodes • Disk block allocation • file name size CS523S: Operating Systems

29. Fast File System - FFS • Disk partition divided into cylinder groups • superblocks restructured and replicated across partition • Constant information • cylinder group summary info such as free inodes and free block • support block fragments • Long file names • new disk block allocation strategy CS523S: Operating Systems

30. FFS Allocation strategy • Goal: Collocate similar data/info. • file inodes located in same cyl group as dir. • new dirs created in different cyl groups. • Place file data blocks/inode in same cyl group - for size < 48K • allocate sequential blocks at a rotationally optimal position. • Choose cyl group with “best” free count CS523S: Operating Systems

31. Is FFS/UFS Better? • Measurements have shown substantial performance benefits over s5fs • FFS however, is sub-optimal when the disk is nearly full. Thus 10% is always kept free. • Modern disks however, no longer match the underlying assumptions of FFS CS523S: Operating Systems

32. Buffer Cache Free (LRU) Hash (device,inode) CS523S: Operating Systems

33. Other Limitations of s5fs and FFS • Performance - hardware designs and modern architectures have redefined the computing environment • Crash Recovery do you like waiting for fsck()? • Security - do we need more than just 7 bits • File Size limitations CS523S: Operating Systems

34. Performance Issues • FFS has a target rotational delay • read/write entire track • many disks have built-in caches • Due to FS Caching, most I/O operations are writes. • Synchronous writes of metadata • Disk head seeks are expensive CS523S: Operating Systems

35. Sun-FFS (cluster) • Sets rotational delay to 0 • read clustering • write clustering CS523S: Operating Systems

36. Log-Structured FS • Entire disk dedicated to log • writes to tail of log file • garbage collection daemon • Dir and Inode structures retained • Issue is locating inodes • writes a segment at a time CS523S: Operating Systems

37. Log-structured FS • Requires a large cache for read efficiency • Write efficiency is obtained since the system is always writing to the end of the log file. Why does this help? • Why does performance compare to Sun-FFS? • What about crash recovery? CS523S: Operating Systems

38. 4.4BSD Portal FS Portal daemon User process fd <path> /p/<path> fd Sockets Protal file system CS523S: Operating Systems

39. Review of vnode/vfs • Provides a general purpose interface • allows multiple file systems to be used simultaneously in a system • OO Interface - • although limited, no inheritance, fixed interfaces • How can we improve on this? CS523S: Operating Systems

40. Stackable Filesystems application application • For a given mount point, there is now possible many file systems /mylocal MyFS /local UFS CS523S: Operating Systems