1. 1. Explain file management in operating system

2. 1. Identify the basic function of file system 2. Describe the following file organization techniques 3. Identify which file organization technique is appropriate for as specific device 4. Describe the three types of file structure 5. Describe the various methods of file allocation 3. 6. State the benefits and weaknesses of using each of the file allocation method 7. Describe the two free space management techniques 8. Describe the various techniques for implementing file access control 9. Explain the techniques used to prevent data loss 10. Implement specific backup technique in a given situation 4. Files Named collection of data that is manipulated as a unit Reside on secondary storage devices Operating systems can create an interface that facilitates navigation of a users files File systems can protect such data from corruption or total loss from disasters Systems that manage large amounts of shared data can benefit from databases as an alternative to files 5. File: a named collection of data that may be manipulated as a unit by operations such as: Open Close Create Destroy Copy Rename List 6. Individual data items within a file may be manipulated by operations like: Read Write Update Insert Delete File characteristics include: Location Accessibility Type Volatility Activity Files can consist of one or more records 7. Directories is . : Files containing the names and locations of other files in the file system, to organize and quickly locate files Directory entry stores information such as: File name Location Size Type Accessed Modified and creation times 8. File systems Organize files and manages access to data Responsible for file management, auxiliary storage management, file integrity mechanisms and access methods Primarily are concerned with managing secondary storage space, particularly disk storage 9. FileSystem Function Organize files Manages access Responsible file management storage management file integrity access methods 10. It is about . How records are arranged & characteristics of medium? used to store it It is HOW the records of a file are arranged on secondary storage On magnetic disks, files can be organized as sequential direct indexed sequential Partitioned *refers to Notes below. 11. Sequential Easiest to implement because records are stored & retrieved serially, one after other. Optimization features - Built into system to speed process. E.g., select a key field from record & then sort records by that field before storing them. Aids search process. (filtering) Complicates maintenance algorithms because original order must be preserved every time records added or deleted. 12. Direct (a.k.a Random) Uses direct access files which can be implemented only on direct access storage devices (pronounced DAZ-dee magnetic storage / drum). Give users flexibility of accessing any record in any order without having to begin search from beginning of file. Records are identified by their relative addresses (their addresses relative to beginning of file). Logical addresses computed when records are stored & again when records are retrieved. 13. Direct Advantages Fast access to records. Can be accessed sequentially by starting at first relative address & incrementing it by one to get to next record. Can be updated more quickly than sequential files because records quickly rewritten to original addresses after modifications. No need to preserve order of the records, so adding or deleting them takes very little time. 14. Indexed Sequential Combines best of sequential & direct access. Records are arranged in a logical sequence according to a key contained in each record. The system maintains an index containing the physical address of contain records 15. Partitioned This refers to a file of sequential sub-files. Each sequential sub-file is called a member of the partitioned file. Improvement for sequential technique. 16. FILE STRUCTURE Unstructured BYTE Structured RECORD Complex TREE 17. OS considers a file to be unstructured OS does not know what is in the file. All it sees are bytes. Any meaning? must be imposed by user-level programs. Used by UNIX, Windows, most modern OS (FAT family filesystem) The byte sequences provides the maximum flexibility (easy to read/fast read). 18. Applications can impose their own structure 19. File is a sequence of fixed-length records, each with some internal structure (head-body- tail). Read operation : returns one record Write operation : overwrites or appends (tambah/topup) one record. OS can optimize operations on records 20. A file consists of a tree of records. Record not necessarily all the same length Each record containing a key field in a fixed position in the record. The tree is sorted on the key field why ? allow fast searching for a particular key. The basic operation here is to get the record with a specific key (NTFS family filesystem). 21. Furthermore, new records can be added to the file, with the operating system deciding where to place them. This type of file is clearly quite different from the unstructured byte sequence used in UNIX and Win. 98 (FAT filesystem) Widely used on the large mainframe computers. Still used in some commercial data processing. 22. For the zoo file of Figure (c), one could ask the system to get the record whose key is pony, for example, without worrying about its exact position in the file. 23. Problem of allocating space and freeing space on secondary storage. Contiguous allocation systems have generally been replaced by more dynamic non-contiguous allocation systems. Why? Files tend to grow or shrink over time Users rarely know in advance how large their files will be 24. FILE ALLOCATION Contiguous Linked list Blocks Linked list Index 25. File Allocation will used THREE (3) types of algorithm to store files: Best fit (padanan TERBAIK sama padan) determine the best place to put the new data minimise the wasted space First fit (padanan PERTAMA cukup untuk padanan) scanning from the beginning of available memory to the end, which is at least big enough to accept the data is found Worst Fit (padanan TERBURUK ruang besar untuk data kecil) selects the largest possible free space that the information can be stored on File Allocation 26. Store each file as a contiguous run of disk blocks Wasteful of space (dynamic storage-allocation problem). Simple only starting location and length (number of blocks) are required. E.g: Assume a disk of 1KB = 1 blocks 50 KB file is allocated 50 consecutive blocks Types of allocation : best fit or first fit File Allocation 27. Advantages Easy to implement Two numbers needed for each file: 1 - disk address of the first block 2 - number of blocks in the file Read performance is excellent File Allocation 28. Disadvantages Fragmentation of blocks Will need periodic compaction (wasting time) Will need to manage free lists Have to know a files maximum possible size at the time it is created Good for CD-ROMs, DVDs All file sizes are known in advance Files are never deleted File Allocation 29. File Allocation File Start Length Dip2a 2 4 Dip2b 8 8 Dip3a 24 1 No click 30. The first word of each block is used as a pointer to the next one. The rest of the block is for data Unlike contiguous allocation, every empty disk block can be used in this method No space is lost to disk fragmentation (except for internal fragmentation in the last block of each file) FAT used by DOS is a variation of linked allocation, where all the links are stored in a separate table at the beginning of the disk. Benefits : cached in memory, improving random access speeds File Allocation 31. Advantage No fragmentation on disk (except internal fragmentation on block) No need to pre-specify file sizes (files can grow/shrink). Never necessary to defragment disk. For sequentially accessed files, performance is optimal. File Allocation 32. Disadvantage Random access is slow Can only be used effectively for sequentially accessed files. The pointers use additional disk space. Reliability. What if a pointer gets corrupted? File Allocation 33. File Allocation 34. Index allocation solves all of the problems of contiguous allocation. Support the random access of a file. With indexed allocation each file is like a set of linked blocks, except the pointers are all stored in an index (the index block unique to each file). The directory contains the location (disk block) of the index block for each file. File Allocation 35. Advantage No external fragmentation. No need to pre-specify file sizes (files can grow/shrink). Never necessary to defragment disk. Disadvantage The pointers use an additional disk block (wastes more space than linked allocation does). File Allocation 36. File Allocation 37. Since the amount of disk space is limited, it is necessary to reuse the space released by deleted files. In general, file systems keep a list of free disk blocks (initially, all the blocks are free) and manage this list by one of the following techniques : a. using free lists b. using bitmaps 38. Free List - Linked list of blocks containing the locations of free blocks Manage as LIFO or FIFO on disk and store it in main memory Blocks are allocated from the beginning of the free list Newly freed blocks are appended to the end of the list Files are likely to be allocated in noncontiguous blocks Increases file access time 39. Secondary Storage 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Addres s Free Blocks 0 4 1 5 2 6 3 11 4 12 5 21 6 22 7 23 9 28 10 29 : : n Block n 40. used to track allocated sector by some file systems If block is free : bit 0 If block is occupied : bit 1 A bitmap (bit-array) contains one bit for each block in memory Xth bit corresponds to the Xth block on the storage device May be too large to hold in main memory Hard to search; the bigger the storage more hard to search 41. Advantage: Simple: Each bit directly corresponds to a sector Efficient to find first free block Disadvantage : The file system may need to search the entire bitmap to find a free block 42. Files are often used to store sensitive data such as: Credit card numbers Passwords Social security numbers Therefore, they should include mechanisms to control user access to data. Access control matrix Access control by user classes 43. Two-dimensional access control matrix: In an installation with a large number of users and a large number of files, this matrix generally would be large. Inappropriate for most systems 44. Figure 13.12 Access control matrix. A B C D E F G H I J 0 Cannot Access 1 Can Access 45. A technique that requires considerably less space is to control access to various user classes User classes can include: The file owner A specified user Group Project Public 46. Access control data Can be stored as part of the file control block Often consumes an insignificant amount of space 47. Backup techniques Store redundant copies of information Recovery techniques Enable the system to restore data after a system failure Physical safeguards such as locks and fire alarms are the lowest level of data protection Performing periodic backups is the most common technique used to ensure the continued availability of data 48. Physical backups Duplicate a storage devices data at the bit level Logical backups Store file system data and its logical structure Inspect the directory structure to determine which files need to be backed up, then write these files to a backup device in a common, often compressed, archival format Incremental backups are logical backups that store only file system data that has changed since the previous backup