File System Interface and Implementations Fred Kuhns CS523 – Operating Systems Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 descriptors and different file systems Fred Kuhns () CS523S: Operating Systems

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 Fred Kuhns () CS523S: Operating Systems

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]; } Fred Kuhns () CS523S: Operating Systems

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

CS523S: Operating Systems 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()} Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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. Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

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,...}} 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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. Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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. Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems OO Style Interfaces Abstract base class Instance of derived 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, …} Fred Kuhns () CS523S: Operating Systems

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

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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) Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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) Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

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

CS523S: Operating Systems 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 the inode list (fixed size) Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems Disk Layout tract sector heads cylinder platters Rotational speed disk seek time Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 .. “” 123 myfile 2 byte 14byte directory On-disk inode Fred Kuhns () CS523S: Operating Systems

Locating file data blocks Assume 1024 Byte Blocks 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 Fred Kuhns () CS523S: Operating Systems

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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems Problems with s5fs Superblock on-disk inodes Disk block allocation file name size Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems Buffer Cache Free (LRU) Hash (device,inode) Fred Kuhns () CS523S: Operating Systems

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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems Sun-FFS (cluster) Sets rotational delay to 0 read clustering write clustering Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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 Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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? Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 4.4BSD Portal FS Portal daemon User process /p/<path> <path> fd fd Protal file system Sockets Fred Kuhns () CS523S: Operating Systems

CS523S: Operating Systems 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? Fred Kuhns () CS523S: Operating Systems

Stackable Filesystems application application /mylocal MyFS /local UFS For a given mount point, there is now possible many file systems Fred Kuhns () CS523S: Operating Systems