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File Systems

File Systems

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File Systems

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  1. File Systems Guide to Operating Systems Second Edition

  2. Understanding File System Functions • A file system is a design for storing and managing files on storage media • The file systems used by personal computer operating systems perform the following general functions: • Enable files to be organized through directories and folders • Partition and format disks to store and retrieve information • Establish file-naming conventions • Provide utilities for functions such as file compression and disk defragmentation • Provide for file and data integrity • Provide storage media management functions • Enable error recovery or prevention

  3. Understanding File System Functions • The overall purpose of a file system is to create a structure for filing data • A file is a set of data that is grouped in some logical manner, assigned a name, and stored on the disk • The data contained in files can be text, images, music and sounds, video, or Web pages for the Internet • But no matter what kind of data is stored in the file system, it must be converted into digital format—a series of 1s and 0s, or electrical signal “ons” and “offs” —that the computer understands

  4. Understanding File System Functions • There must be a way to write digital information onto disk, track it, update it when necessary, and call it back when the user, or a program under the user’s control, wants it • To achieve all this, the operating system typically groups disk sectors in some logical way, creates a record of this structure, and builds a directory to track the type of data stored in each file

  5. A File System

  6. Understanding File System Functions • In addition to the names of files and where to find them on the disk, directories also store the following information: • Date and time the directory or file was created (a timestamp for that directory or file) • Date and time the directory or file was last modified (another form of timestamp) • Directory or file size • Directory of file attributes, such as security information, or if the directory or file was backed up

  7. Designing a Directory Structure • For users, one of the most important features of a file system is the ability to store information according to a pattern of organization that is enabled by the use of directories • In the early Windows systems and UNIX systems, files are organized by directories, while in later Windows versions and the Mac OS, these are called folders

  8. Designing a Directory Structure • Directories and folders can be organized in a hierarchy that is similar to a tree structure

  9. Designing a Directory Structure • When you design a directory structure consider directories for the following: • Operating system files • Software applications • Work files, such as word processing, graphics, spreadsheets, and database files • Public files that you share over the network • Utilities files • Temporary files

  10. Disk Storage • When a hard disk is delivered from the manufacturer, it is low-level formatted • A low-level format is a software process that marks the location of disk tracks and sectors • Every disk is divided into tracks, which are like several circles around a disk • Each track is divided into sections of equal size called sectors • Figure 3-4 illustrates a hard disk divided into tracks and sectors

  11. Disk Tracks and Sectors

  12. Block Allocation • The operating systems discussed in this book use a method called block allocation to keep track of where specific files are stored on the disk • With block allocation the disk is divided into logical blocks (in MS-DOS and Windows these are called clusters), which are in turn mapped to sectors, heads, and tracks on the disk • The data regarding block allocation is stored on the disk itself, using two techniques

  13. Block Allocation • One technique uses a fixed portion of the disk to store this data, such as the FAT (file allocation table) file system used by MS-DOS and supported by all versions of Windows • The other technique uses various locations on the disk to store a special type of file that is used for directory and file allocation information, such as the New Technology File System (NTFS) and the UNIX file systems

  14. Partitions • Before a file system can be placed on a hard disk, the disk must be partitioned and formatted • Partitioning is the process of blocking a group of tracks and sectors to be used by a particular file system, such as FAT or NTFS • After a disk is partitioned, it must be high-level formatted (usually referred to as just formatted) so that the partition contains the disk divisions and patterns needed by a particular operating system to store files

  15. Partitions • Figure 3-5 illustrates a Windows 2000 system that has multiple partitions (FAT, NTFS, and compact disk file system (CDFS)) used for segmenting data on two hard disks and a CD-ROM drive

  16. Partitions • Partitioning can typically be done only on hard disks or large removable disks • Floppy disks and other low-capacity media do not support partitioning because there simply isn’t enough room on these low-capacity disks to make partitioning practical • On systems in the IBM/Intel PC hardware architecture, typically there is room to store information for up to four partitions on each disk

  17. Partitions • This area is known as the partition table in MS-DOS, Mac OS, and Windows, and the disk label in UNIX • In addition to the disk label and partition table, there is another piece of disk reserved, known as the boot block in UNIX, or the Master Boot Record (MBR) in MS-DOS and Windows • This area hold a tiny program used to begin booting an operating system from a disk

  18. The Basic MS-DOS File System • The first Microsoft file system, called the file allocation table (FAT) file system started with MS-DOS in the early eighties, and later versions of FAT are in many ways similar to the earliest version • FAT uses a file allocation table to store directory information about files, such as filenames, file attributes, and file location • MS-DOS versions prior to 4.0 use the FAT12 file system in which the maximum size of a file system is 32 MB

  19. The Basic MS-DOS File System • Beginning with MS-DOS 4.0, the FAT16 file system is used • Another important characteristic of FAT is its use of “8.3” filenames, which can be up to eight characters long, followed by a period and an extension of three characters, such as Filename.ext

  20. Partitioning • The FAT file system supports two partitions per hard drive, a primary partition and a secondary partition • The secondary partition may then be divided further into a maximum of three logical drives • Each of these four possible logical drives can hold an individual MS-DOS file system • Although 26 drive definitions are technically possible, MS-DOS reserves drive A: and B: for floppy drives, practically limiting the number of hard drives (including logical drives) to 24

  21. Partitioning • Each MS-DOS file system is assigned a letter followed by a colon: A:, B:, C:, and so on through Z: • Typically, C: is reserved for the first hard disk or removable disk file system (and is normally the system that contains the boot partition)

  22. Partitioning • With a new system called large block allocation (LBA), it is now possible to make file systems much larger than 512 MB under MS-DOS • LBA translates larger logical blocks to smaller ones for support of larger physical disks • However, the disk controller and disk must support LBA, and they must be configured for LBA • The trick to LBA is that MS-DOS is told that the sector size of the hard disk is greater than 512 bytes per sector, which results in the ability to have much larger file systems

  23. Formatting • In MS-DOS (and in many Windows versions), the file system is placed on the partition using the format command • This command writes all of the file system structure to the disk • In the case of a floppy disk, it uses the first sector of the disk as the boot block • As with many system-level commands, format includes several additional switches that modify precise program operation

  24. format Command Switches

  25. Formatting • Command lines frequently use switches (extra code) to change the way a particular command operates • The boot block is placed in the first sector on the disk • Next comes the root directory, where the system stores file information, such as name, start cluster, file size, file modification date and time, and file attributes (file characteristics such a Hidden, Read-only, Archive, and so on)

  26. Formatting • The root directory on every partition is a fixed size that can contain a maximum of 512 entries in FAT16 (and unlimited entries in FAT32) • When a file is stored to disk, its data is written in the clusters on the disk • The filename is stored in the directory, along with the number of the first cluster in which the data is stored

  27. Formatting • When the operating system fills the first cluster, data is written to the next free cluster on the disk • When a file is completely written to the disk, the FAT entry for the final cluster is filled with all 1s, which means end of file • At this time, the directory entry for the file is updated with the total file size • This is commonly referred to as the “linked-list” method

  28. Formatting • Clusters are of a fixed length, and if a file does not exactly match the space available in the clusters it uses, you can end up with some unused space at the end of a cluster • Unusable or troubled spots on the disk are marked in the FAT as bad clusters, and these areas are never used for file storage • Formatting a disk removes all data that was on the disk because you lost the directory and FAT data needed to get to the data

  29. Formatting • On disks that have never been formatted, the format command writes new sector and track markers on the disk • The format process on a hard disk is the same as on a floppy disk, with two exceptions • The first is related to the size of each entry in the FAT table, which is 16 bits long on any disk larger than 16 MB • The second difference is in the cluster size, larger partitions use larger cluster sizes

  30. Formatting • The FAT tables and root directory are found at the beginning of each partition, and they are always at the same location • Each item in a directory consists of 32 bytes

  31. Formatting • In each entry, information about the file is stored, including the filename, the file change date and the file size and the file attributes • Apart from the filename, each directory entry also contains some status bits that identify the type of filename contained in each entry • The status bits in use are Volume, Directory, System, Hidden, Read-only, and Archive • The Volume bit indicates a file system volume label, or a nickname for the file system

  32. Formatting • The attrib command can be used to look at or set these attributes • Typing attrib in a directory shows all of the attribute settings for all the files, whereas typing attrib followed by a filename shows only attributes specific to that file

  33. The Windows 95/98/Me FAT File Systems • There are two versions of FAT that can be used with Windows 95, 98, and Me, depending on the version of Windows you are running, and the size of the disk you are using: • FAT16, similar to the system used in MS-DOS/Win 3.x • FAT32, a new system introduced in Windows 95, release 2 (OSR2)

  34. Windows 95/98/Me FAT16 • In Windows 95, there is support for long filenames (LFNs) • A filename in the Windows 95 file system: • Can contain as many as 255 characters • Is not case sensitive • Since LFN characters are stored in Unicode, a coding system that allows for representation of any character in any language, it is possible to use any character known to Windows in a filename

  35. Windows 95/98/Me FAT16 • The advantage of LFNs and Unicode is that LFNs can be read by Mac OS and UNIX systems • Formerly, letters and digits were represented by ASCII (American Standard Code for Information Interchange) values • The problem with this standard is that it uses an entire byte to represent each character, which limits the number of characters that can be represented to 255

  36. Windows 95/98/Me FAT16 • This is not enough to handle all the characters needed to represent world languages, including several different alphabets (Greek, Russian, Japanese, and Hindi, for example) • ASCII deals with this problem by employing different character sets, depending on the characters you’re trying to represent • Unicode is a 16-bit code that allots two bytes for each character, which allows 65,536 characters to be defined

  37. FAT32 • To accommodate the increasing capacities of hard disks, and avoid the problem of cluster size, the second release of Windows 95 (OSR2) introduced an improved FAT file system called FAT32 • FAT32 shares characteristics of FAT16 for Windows 95 • FAT32 allows partitions of up to 2 TB (theoretically); blocks can be allocated with clusters as small as 8 KB, and the maximum file size is raised to 4 GB

  38. FAT32 • FAT32 works by using many bits per FAT allocation unit, and is therefore incompatible with other operating systems such as MS-DOS and Windows NT • However, it is compatible with Windows 2000 and XP • You can choose the FAT32 file system when the Windows 95/98 fdisk utility is run, but unfortunately, the utility does not simply ask if you wish to use FAT32

  39. FAT32 • DO NOT use FAT32 if you plan to access your hard disk from MS-DOS or Windows NT because it is not compatible with these systems

  40. Windows 95/98/Me File System Utilities • The fdisk and format utilities in Windows 95, 98, and Me work identically to those in MS-DOS • There are now also graphical equivalents of these tools that you can launch from the Windows user environment, such as the Windows 95 format tool shown in Figure 3-11

  41. The Windows NT/2000/XP/.NET File Systems • Windows NT supports two types of file systems: the extended FAT16 system used by Windows 95, and the NTFS up through version 4 (for Windows NT 4.0)

  42. Extended FAT16 • The extended FAT16 file system under Windows NT/2000/XP/.NET has the same functionality as the FAT16 system under Windows 95, and it is possible to have multiple operating systems on one disk (called dual-boot or multiboot systems) with the FAT16 file system

  43. FAT32 • Windows 2000, XP, and .NET Server all support FAT32, which is the same file system that is used in Windows 95 OSR2, Windows 98, and Windows Me • Sometimes users choose to employ FAT16 or FAT32 because they are familiar with these file systems, or they have dual-boot systems, such as a system with Windows XP and Windows 98

  44. NTFS • Four important advantages of NTFS compared to FAT16 and FAT32 are: • Ability to compress file and directory contents on the fly • Better recoverability and stability • Less disk fragmentation • Local file and folder-level security

  45. NTFS • NTFS enables the use of LFNs that are compatible with LFN FAT filenames • If an LFN is used in NTFS, that file can be copied to a FAT16 or FAT32 volume, and the filename remains intact • As a full-featured network file system, NTFS is equipped with security features that meet the U.S. government’s C2 security specifications • C2 security refers to high-level, “top secret” standards for data protection, system auditing, and system access, which are required by some government agencies

  46. NTFS • NTFS can be scaled to accommodate very large files, particularly for database applications • A Microsoft SQL Server database file might be 20 GB or larger • Another NTFS feature is its ability to keep a log of file system activity

  47. NTFS • An important volume-handling feature of NTFS is the ability to create extensions on an existing volume, such as when new disk storage is added • Last, NTFS is less prone to file corruption than FAT, in part because it has a “hot fix” capability, which means that if a bad disk area is detected, NTFS automatically copies the information from the bad area to another disk area that is not damaged

  48. NTFS • The way NTFS keeps track of files and clusters is a little different from the FAT file system • Rather than using a structure of FAT tables and directories, NTFS uses a Master File Table (MFT) • Like the FAT tables and directories, this table is located at the beginning of the partition • When a file is made in NTFS, a record for that file is added to the MFT

  49. FAT16, FAT32, and NTFS Compared

  50. NTFS Utilities • Care should be taken to use only utilities designed to work with NTFS; serious damage can occur if other utilities are used • When you upgrade Windows NT Server 4.0 to Windows 2000 or Windows .NET, the current NTFS disk partitions are automatically converted to basic disks, which use traditional NTFS disk management techniques, with a limited number of volumes on one disk