Operating Systems CSE 410, Spring File Management. Stephen Wagner Michigan State University

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1 Operating Systems CSE 410, Spring 2004 File Management Stephen Wagner Michigan State University

2 File Management File management system has traditionally been considered part of the operating system. Applications are stored as files Input for application is stored in files Output from application is stored in files File management has become increasingly distributed and removed from the operating system

3 Objectives for a File Management System Guarantee that the data in the file are valid Optimize performance Provide I/O support for a variety of storage device types Minimize or eliminate the potential for lost or destroyed data Provide a standardized set of I/O interface routines Provide I/O support for multiple users Security

4 User Program Pile Sequential Indexed Sequential Indexed Hashed Logical I/O Basic I/O Supervisor Basic File System Disk Device Driver Tape Device Driver Figure 12.1 File System Software Architecture [GROS86]

5 Device Drivers Lowest level Communicates directly with peripheral devices Responsible for starting I/O operations on a device Processes the completion of an I/O request

6 Basic File System Physical I/O Deals with exchanging blocks Concerned with the placement of blocks Concerned with buffering blocks in main memory

7 File Structure Directory management Access method Records Blocking Physical blocks in main memory buffers Disk scheduling Physical blocks in secondary storage (disk) User & program comands Operation, File name File manipulation functions I/O Free storage management File allocation User access control File management concerns Operating system concerns Figure 12.2 Elements of File Management

8 File Organization Character Files Piles Sequential Files Indexed Sequential Files Indexed File Direct, or Hashed, File

9 Character Files File is organized as a sequence of characters Basic Operations Seek. Each character has a position in the file Seek may allow random access or be more restricted Read. User is allowed to read any number of bytes At lower level, blocks are read Overwrite Append Truncate Very common file organization

10 File Organization with Records A more sophisticated way is to organize files as records A record consists of a fixed set of fields (a structure) Each field has a data type and length All records do not necessarily have the same fields Character Files can be viewed as a file composed of very simples Records

11 Piles Data are collected in the order they arrive Purpose is to accumulate a mass of data and save it Records may have different fields This requires records to be self describing At the very least the length of each record needs to be stored Record access is by exhaustive search

12 Piles Variable-length records Variable set of fields Chronological order (a) Pile File Fixed-length records Fixed set of fields in fix Sequential order based (b) Seq Exhaustive index Exhaustive index Index levels n Main File

13 Sequential Files Fixed format used for records All records are identical Same length, same fields in same order Field names and lengths are attributes of the file Can support random access One field is the key field Uniquely identifies the record Records often stored in key sequence

14 Sequential Files h records fields order a) Pile File Fixed-length records Fixed set of fields in fixed order Sequential order based on key field (b) Sequential File Exhaustive index Exhaustive index Partial index Main File

15 Sequential Files If files are stored in key sequence, adding a record becomes expensive Requires moving records in the file To reduce the cost, records are not immediately added to the file New records are placed in a log file or transaction file Batch update is performed to merge the log file with the master file A special case needs to be made to search the log file

16 Indexed Sequential File Index provides provides a lookup capability to quickly search the file by key Index contains a key field and a pointer to the main file Not every single entry need be indexed Index is used to get close to desired record Sequential search is used to find desired record

17 Variable-length records Variable set of fields Chronological order Fixed-length records Fixed set of fields in fix Sequential order based Indexed Sequential (a) Pile File File Exhaustive index (b) Seq Exhaustive index Index levels 2 1 n Index Main File Overflow File (c) Indexed Sequential File Primary F (variable-length (d) Indexe Figure 12.3 Common File Organizati

18 Indexed Sequential File New records are stored in overflow file Overflow file is periodically merged with main file Main file includes pointers to entries in overflow file

19 Advantage of Index Example: a file contains 1 million records On average 500,000 accesses are required to find a record in a sequential file If an index contains 1000 entries it will take 500 accesses on average to find nearest key in index, followed by 500 accesses on average in the main file. A binary search can be used to find the nearest key in index with 10 accesses, followed by 500 accesses to main file

20 Indexed File Uses multiple indices for different key fields May contain an exhaustive index that contains one entry for every record in the main file May also contain a partial index

21 h records fields order Fixed-length records Fixed set of fields in fixed order Sequential order based on key field a) Pile File Exhaustive index Indexed (b) Sequential File Exhaustive index Partial index Main File Overflow File ed Sequential File Primary File (variable-length records) (d) Indexed File re 12.3 Common File Organizations

22 Direct File Similar to a sequential file, but records are not ordered Records are accessed directly by key Similar to an STL map Implemented using a hash table Hash function is used to find location of record in file Some sort of chaining is used to handle collisions

23 Implementation of Different File Types Most OS s do not provide direct support for the above file organizations All of the above organizations can be built on top of character files Piles can be used for web logs Relational databases can be built out of indexed sequential files UNIX dmb files are direct files

24 Files have several attributes Name Access Privileges Location File Directories To make locating files easy, files are organized into directories The Directory is a file Provides a mapping between file names and files A Directory is a sequential file sorted by the file name key.

25 Hierarchical Directories Directories can contain files an other directories The directories form a tree Their is some master director The root directory in UNIX Logical Organization Allows users to organize files and directories Physical Organization Allows system to quickly access traverse hierarchy

26 Master Directory Subirectory Subirectory Subirectory Subirectory Subirectory File File File File Figure 12.4 Tree-Structured Directory

27 Master Directory System User A User B User C Directory "User C" Directory "Word" Unit A Directory "User B" Draw Word Directory "User A" Directory "Draw" ABC Directory "Unit A" File "ABC" ABC File "ABC" Pathname: /User B/Word/UnitA/ABC Figure 12.5 Example of Tree-Structured Directory

28 Naming Conventions Files are located by following a path from root directory Several files can have the same file name as long they have unique path names Files can be referenced relative to the current directory

29 File Sharing In multiuser system, files may be shared among users Two issues Access rights Management of simultaneous access

30 None Access Rights User may not know of the existence of the file User is not allowed to read the user directory that includes the file Knowledge User can only determine that the file exists and who its owner is

31 Access Rights Execution The user can load and execute a program but not copy it Reading The user can read the file for any purpose, including copying and execution Appending The user can add data to the file but cannot modify or delete any of the file s contents

32 Access Rights Updating The user can modify, delete and add data. Changing Protection User can change access rights granted to other users Deletion User can delete file

33 Files have an owner Access Rights Owner has all rights previously listed Owner may grant rights to others users Specific users User groups Everybody

34 Simultaneous Access System may provide ability to lock a file locks may be at the file level user may be able to lock individual records Mutual exclusion and deadlock are issues for shared access

35 Record Blocking Records must be organized as blocks for I/O to be performed Records are the logical unit of access of a file Blocks are the unit of I/O with secondary storage Two issues: Should blocks be fixed or variable length? What should be relative size of a block compared to the average record size? Tradeoffs: Smaller block, more I/O operations needed Larger block, more records in one I/O operation

36 R1 R2 R3 R4 R4 R5 R6 Track 1 R6 R7 Fixed Blocking R9 R9 R10 R8 R11 R12 R13 Track 2 Variable Blocking: Spanned R1 R2 R3 R4 R5 R1 R2 R3 R4 Track 1 Track 1 R6 R5 R7 R6 R8 R7 R9 R10 R8 Track 2 Track 2 Variable Blocking: Unspanned Fixed Blocking Data Waste due to record fit to block size Gaps R1 due to hardware R2 design R3 R4 Waste R4 due to block R5 size constraint R6 from fixed record size Waste due to block fit to track size Track 1 R6 R7 R8 R9 R9 R10 R11 R12 R13 Figure 12.6 Record Blocking Methods [WIED87] Track 2 Variable Blocking: Spanned

37 R6 R7 R8 R9 R9 R10 R11 R12 R13 R5 R6 R7 R8 Variable Variable Blocking: Spanned Spanned Fixed Blocking Track 2 Track 2 R1 R2 R3 R4 R5 R1 R2 R3 R4 R4 R5 R6 Track 1 Track 1 R6 R6 R7 R7 R8 R9 R8 R9 R10 R9 R10 R11 R12 R13 Track 2 Track 2 Variable Blocking: Unspanned Variable Blocking: Spanned Data Waste due to record fit to block size Gaps R1 due to hardware R2 design R3 Waste R4 due to block size R5 constraint from fixed record size Waste due to block fit to track size Track 1 R6 R7 Figure 12.6 Record Blocking Methods [WIED87] R8 Variable Blocking: Unspanned R9 R10 Track 2 Data Waste due to record fit to block size

38 R1 R2 R3 R4 R4 R5 R6 Track 1 Variable R6 R7 Blocking: R8 R9 R9Unspanned R10 R11 R12 R13 Track 2 Variable Blocking: Spanned R1 R2 R3 R4 R5 Track 1 R6 R7 R8 R9 R10 Track 2 Variable Blocking: Unspanned Data Gaps due to hardware design Waste due to block fit to track size Waste due to record fit to block size Waste due to block size constraint from fixed record size Figure 12.6 Record Blocking Methods [WIED87]

39 Secondary Storage Management Space must be allocated to files Must keep track of the space available for allocation

40 Preallocation VS Dynamic Allocation Preallocation Need maximum size for the file at creation time Difficult to reliably estimate the maximum potential size of the file Tend to overestimate file size so we do not run out of space Dynamic Allocation Allocate space to a file in portions as needed

41 Portion Size Portion Size: the size of the portion allocated to a file Variable, large contiguous: Advantages: better performance, avoids waste, file allocation tables are small Disadvantage: space is hard to reuse Blocks: Advantages: greater flexibility Disadvantage: requires large tables or complex structures for their allocation

42 Free Space Fragmentation Similar problem as allocating dynamic partitions First Fit Best Fit Nearest Fit Choose the unused group of sufficient size that is closest to the previous allocation for the file to increase locality

43 Methods of File Allocation Contiguous allocation Single set of blocks is allocated to a file at the time of creation Only a single entry in the file allocation table: starting block and length of file External fragmentation will occur, compaction is needed Need to declare the size of the file at the time of creation

44 File A File B File Name File A File B File C File D File E File Allocation Table Start Block Length File C File E File D Figure 12.7 Contiguous File Allocation

45 File A File B File C File E File D File Name File A File B File C File D File E File Allocation Table Start Block Length Figure 12.8 Contiguous File Allocation (After Compaction)

46 Chained allocation Methods of File Allocation Allocation on basis of individual block Each block contains a pointer to the next block in the chain Only single entry in the file allocation table: starting block and length of file No external fragmentation Best for sequential files Does not take advantage of locality

47 File B File Name File B File Allocation Table Start Block 1 Length Figure 12.9 Chained Allocation

48 File B File Name File B File Allocation Table Start Block 0 Length Figure Chained Allocation (After Consolidation)

49 Indexed allocation Methods of File Allocation File allocation table contains a separate on-level index for each file The index has one entry for each portion to the file The file allocation table contains block number for the index

50 File B File Allocation Table File Name File B Index Block Figure Indexed Allocation with Block Portions

51 File B File Allocation Table File Name File B Index Block Start Block Length Figure Indexed Allocation with Variable-Length Portions

52 Bit Tables Chained Free Portions Indexing Free Block List Free Space Management

53 Types of files Ordinary Directory Special Named I-nodes UNIX File Management A control structure that contains the key information needed by the OS for a particular file.

54 Direct(0) Direct(1) Direct(2) Direct(3) Direct(4) Direct(5) Direct(6) Direct(7) Direct(8) Direct(9) single indirect double indirect triple indirect Inode address fields Blocks on disk Figure UNIX Block Addressing Scheme

55 Flush the log file Log File Service Write the cache Log the transaction Read/write the file I/O Manager NTFS Driver Fault Tolerant Driver Disk Driver Read/write a mirrored or striped volume Read/write the disk Cache Manager Access the mapped file or flush the cache Load data from disk into memory Virtual Memory Manager Figure Windows NTFS Components [CUST94]

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