File Systems - Operating Systems - Lecture Slides, Slides of Computer Science

Download File Systems - Operating Systems - Lecture Slides and more Slides Computer Science in PDF only on Docsity! 10: File Systems 1 OPERATING SYSTEMS FILE SYSTEMS Docsity.com 10: File Systems 2 FILE SYSTEMS This material covers Silberschatz Chapters 10 and 11. File System Interface The user level (more visible) portion of the file system. • Access methods • Directory Structure • Protection File System Implementation The OS level (less visible) portion of the file system. • Allocation and Free Space Management • Directory Implementation Docsity.com 10: File Systems 5 FILE SYSTEMS INTERFACE Attributes of a File  Name – only information kept in human-readable form • Identifier – unique tag (number) identifies file within file system • Type – needed for systems that support different types • Location – pointer to file location on device • Size – current file size • Protection – controls who can do reading, writing, executing • Time, date, and user identification – data for protection, security, and usage monitoring • Information about files is kept in the directory structure, which is maintained on the disk. File Concept Docsity.com 10: File Systems 6 FILE SYSTEMS INTERFACE What can we find out about a Linux File? jbreecher@younger:~$ stat A_File File: `A_File' Size: 6491 Blocks: 16 IO Block: 4096 regular file Device: 14h/20d Inode: 20938754 Links: 1 Access: (0600/-rw-------) Uid: ( 1170/jbreecher) Gid: ( 100/ users) Access: 2006-11-15 15:38:17.000000000 -0500 Modify: 2006-09-27 17:44:10.000000000 -0400 Change: 2006-09-27 17:44:10.000000000 -0400 jbreecher@younger:~/public/os/Code$ stat protos.h File: `protos.h' Size: 2889 Blocks: 8 IO Block: 4096 regular file Device: 14h/20d Inode: 28442631 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 1170/jbreecher) Gid: ( 100/ users) Access: 2006-11-16 03:56:17.000000000 -0500 Modify: 2006-08-27 12:45:57.000000000 -0400 Change: 2006-08-27 13:25:24.000000000 -0400 File Concept Docsity.com 10: File Systems 7 FILE SYSTEMS INTERFACE Note: The command “LDE” – Linux Disk Editor – does amazing things but requires root privilege. -rw-rw-rw- 1 jbreecherusers 56243 Mon Dec 18 14:25:40 2006 TYPE: regular file LINKS: 1 DIRECT BLOCKS= 0x002462CA MODE: \0666 FLAGS: \10 0x002462CB UID: 01170(jbreecher)ID: 00100(users) 0x002462CC SIZE: 56243 SIZE(BLKS): 128 0x002462CD 0x002462CE ACCESS TIME: Mon Dec 18 14:35:35 2006 0x002462CF CREATION TIME: Mon Dec 18 14:25:40 2006 0x002462D0 MODIFICATION TIME: Mon Dec 18 14:25:40 2006 0x002462D1 DELETION TIME: Wed Dec 31 19:00:00 1969 0x002462D2 0x002462D3 0x002462D4 0x002462D5 INDIRECT BLOCK= 0x002462D6 2x INDIRECT BLOCK= 3x INDIRECT BLOCK= File Concept Expanded on next page Docsity.com 10: File Systems 10 If files had only one "chunk" of data, life would be simple. But for large files, the files themselves may contain structure, making access faster. SEQUENTIAL ACCESS • Implemented by the filesystem. • Data is accessed one record right after the last. • Reads cause a pointer to be moved ahead by one. • Writes allocate space for the record and move the pointer to the new End Of File. • Such a method is reasonable for tape FILE SYSTEMS INTERFACE Access Methods Docsity.com 10: File Systems 11 DIRECT ACCESS • Method useful for disks. • The file is viewed as a numbered sequence of blocks or records. • There are no restrictions on which blocks are read/written in any order. • User now says "read n" rather than "read next". • "n" is a number relative to the beginning of file, not relative to an absolute physical disk location. FILE SYSTEMS INTERFACE Access Methods Docsity.com 10: File Systems 12 OTHER ACCESS METHODS Built on top of direct access and often implemented by a user utility. Indexed ID plus pointer. An index block says what's in each remaining block or contains pointers to blocks containing particular items. Suppose a file contains many blocks of data arranged by name alphabetically. Example 1: Index contains the name appearing as the first record in each block. There are as many index entries as there are blocks. Example 2: Index contains the block number where "A" begins, where "B" begins, etc. Here there are only 26 index entries. FILE SYSTEMS INTERFACE Access Methods Docsity.com 10: File Systems 15 jbreecher@younger:~/public/os$ stat Code File: `Code' Size: 4096 Blocks: 8 IO Block: 4096 directory Device: 14h/20d Inode: 28606492 Links: 2 Access: (0755/drwxr-xr-x) Uid: ( 1170/jbreecher) Gid: ( 100/ users) Access: 2006-11-16 14:52:11.000000000 -0500 Modify: 2006-11-16 14:52:01.000000000 -0500 Change: 2006-11-16 14:52:01.000000000 -0500 FILE SYSTEMS INTERFACE Directory Structure Docsity.com 10: File Systems 16 Tree-Structured Directory FILE SYSTEMS INTERFACE Directory Structure Docsity.com 10: File Systems 17 Mounting:  Attaching portions of the file system into a directory structure. Sharing: • Sharing must be done through a protection scheme • May use networking to allow file system access between systems • Manually via programs like FTP or SSH • Automatically, seamlessly using distributed file systems • Semi automatically via the world wide web • Client-server model allows clients to mount remote file systems from servers • Server can serve multiple clients • Client and user-on-client identification is insecure or complicated • NFS is standard UNIX client-server file sharing protocol • CIFS is standard Windows protocol • Standard operating system file calls are translated into remote calls FILE SYSTEMS INTERFACE Other Issues Docsity.com 10: File Systems 20 FILE SYSTEM IMPLEMENTATION FILE SYSTEM STRUCTURE: When talking about “the file system”, you are making a statement about both the rules used for file access, and about the algorithms used to implement those rules. Here’s a breakdown of those algorithmic pieces. Application Programs The code that's making a file request. Logical File System This is the highest level in the OS; it does protection, and security. Uses the directory structure to do name resolution. File-organization Module Here we read the file control block maintained in the directory so we know about files and the logical blocks where information about that file is located. Basic File System Knowing specific blocks to access, we can now make generic requests to the appropriate device driver. IO Control These are device drivers and interrupt handlers. They cause the device to transfer information between that device and CPU memory. Devices The disks / tapes / etc. Docsity.com 10: File Systems 21 FILE SYSTEM IMPLEMENTATION Layered File System Handles the CONTENT of the file. Knows the file’s internal structure. Handles the OPEN, etc. system calls. Understands paths, directory structure, etc. Uses directory information to figure out blocks, etc. Implements the READ. POSITION calls. Determines where on the disk blocks are located. Interfaces with the devices – handles interrupts. Docsity.com 10: File Systems 22 FILE SYSTEM IMPLEMENTATION Example of Directory and File Structure Hash Hash Name Loc. Name Loc. Filename Filename Filename Disk Disk Disk Link bit other.. attributes File header Index Address Protection Address Creation Time Current Size Et. cetera Index Block Blk 0 Disk Address Blk 1 Disk Address ------------------- Blk N Disk Address Protection Data Name/Privileges Name/Privileges Data N Data 0 Data 1 Directory Hash Table Directory Brief Info. Docsity.com 10: File Systems 25 FILE SYSTEM IMPLEMENTATION LINKED ALLOCATION Each file is a linked list of disk blocks, scattered anywhere on the disk. At file creation time, simply tell the directory about the file. When writing, get a free block and write to it, enqueueing it to the file header. There's no external fragmentation since each request is for one block. Method can only be effectively used for sequential files. Allocation Methods Docsity.com 10: File Systems 26 FILE SYSTEM IMPLEMENTATION LINKED ALLOCATION Pointers use up space in each block. Reliability is not high because any loss of a pointer loses the rest of the file. A File Allocation Table is a variation of this. It uses a separate disk area to hold the links. This method doesn't use space in data blocks. Many pointers may remain in memory. A FAT file system is used by MS-DOS. Allocation Methods Docsity.com 10: File Systems 27 FILE SYSTEM IMPLEMENTATION INDEXED ALLOCATION • Each file uses an index block on disk to contain addresses of other disk blocks used by the file. • When the i th block is written, the address of a free block is placed at the i th position in the index block. • Method suffers from wasted space since, for small files, most of the index block is wasted. What is the optimum size of an index block? • If the index block is too small, we can: a) Link several together b) Use a multilevel index Allocation Methods UNIX keeps 12 pointers to blocks in its header. If a file is longer than this, then it uses pointers to single, double, and triple level index blocks. Docsity.com 10: File Systems 30 FILE SYSTEM IMPLEMENTATION We need a way to keep track of space currently free. This information is needed when we want to create or add (allocate) to a file. When a file is deleted, we need to show what space is freed up. BIT VECTOR METHOD • Each block is represented by a bit 1 1 0 0 1 1 0 means blocks 2, 3, 6 are free. • This method allows an easy way of finding contiguous free blocks. Requires the overhead of disk space to hold the bitmap. • A block is not REALLY allocated on the disk unless the bitmap is updated. • What operations (disk requests) are required to create and allocate a file using this implementation? Free Space Management Docsity.com 10: File Systems 31 FILE SYSTEM IMPLEMENTATION FREE LIST METHOD • Free blocks are chained together, each holding a pointer to the next one free. • This is very inefficient since a disk access is required to look at each sector. GROUPING METHOD • In one free block, put lots of pointers to other free blocks. Include a pointer to the next block of pointers. COUNTING METHOD • Since many free blocks are contiguous, keep a list of dyads holding the starting address of a "chunk", and the number of blocks in that chunk. • Format < disk address, number of free blocks > Free Space Management Docsity.com 10: File Systems 32 FILE SYSTEM IMPLEMENTATION • The issue here is how to be able to search for information about a file in a directory given its name. • Could have linear list of file names with pointers to the data blocks. This is: simple to program BUT time consuming to search. • Could use hash table - a linear list with hash data structure. a) Use the filename to produce a value that's used as entry to hash table. b) Hash table contains where in the list the file data is located. c) This decreases the directory search time (file creation and deletion are faster.) d) Must contend with collisions - where two names hash to the same location. e) The number of hashes generally can't be expanded on the fly. Directory Management Docsity.com 10: File Systems 35 FILE SYSTEM IMPLEMENTATION THE DISK CACHE MECHANISM • This is an essential part of any well- performing Operating System. • The goal is to ensure that the disk is accessed as seldom as possible. • Keep previously read data in memory so that it might be read again. • They also hold on to written data, hoping to aggregate several writes from a process. • Can also be “smart” and do things like read-ahead. Anticipate what will be needed. Efficiency and Performance Docsity.com 10: File Systems 36 OVERVIEW: • Runs on SUNOS - NFS is both an implementation and a specification of how to access remote files. It's both a definition and a specific instance. • The goal: to share a file system in a transparent way. • Uses client-server model ( for NFS, a node can be both simultaneously.) Can act between any two nodes ( no dedicated server. ) Mount makes a server file- system visible from a client. mount server:/usr/shared client:/usr/local • Then, transparently, a request for /usr/local/dir-server accesses a file that is on the server. • Can use heterogeneous machines - different hardware, operating systems, network protocols. • Uses RPC for isolation - thus all implementations must have the same RPC calls. These RPC's implement the mount protocol and the NFS protocol. DISTRIBUTED FILE SYSTEMS SUN Network File System Docsity.com 10: File Systems 37 THE MOUNT PROTOCOL: The following operations occur: 1. The client's request is sent via RPC to the mount server ( on server machine.) 2. Mount server checks export list containing a) file systems that can be exported, b) legal requesting clients. c) It's legitimate to mount any directory within the legal filesystem. 3. Server returns "file handle" to client. 4. Server maintains list of clients and mounted directories -- this is state information! But this data is only a "hint" and isn't treated as essential. 5. Mounting often occurs automatically when client or server boots. DISTRIBUTED FILE SYSTEMS SUN Network File System Docsity.com 10: File Systems 40 NFS ARCHITECTURE: 1. UNIX filesystem layer - does normal open / read / etc. commands. 2. Virtual file system ( VFS ) layer - a) Gives clean layer between user and filesystem. b) Acts as deflection point by using global vnodes. c) Understands the difference between local and remote names. d) Keeps in memory information about what should be deflected (mounted directories) and how to get to these remote directories. 3. System call interface layer - a) Presents sanitized validated requests in a uniform way to the VFS. DISTRIBUTED FILE SYSTEMS SUN Network File System Docsity.com 10: File Systems 41 PATH-NAME TRANSLATION: • Break the complete pathname into components. • For each component, do an NFS lookup using the component name + directory vnode. • After a mount point is reached, each component piece will cause a server access. • Can't hand the whole operation to server since the client may have a second mount on a subsidiary directory (a mount on a mount ). • A directory name cache on the client speeds up lookups. DISTRIBUTED FILE SYSTEMS SUN Network File System Docsity.com 10: File Systems 42 CACHES OF REMOTE DATA: • The client keeps: File block cache - ( the contents of a file ) File attribute cache - ( file header info (inode in UNIX) ). • The local kernel hangs on to the data after getting it the first time. • On an open, local kernel, it checks with server that cached data is still OK. • Cached attributes are thrown away after a few seconds. • Data blocks use read ahead and delayed write. • Mechanism has: Server consistency problems. Good performance. DISTRIBUTED FILE SYSTEMS SUN Network File System Docsity.com