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Overview of MS-DOS and NTFS File Systems: Structure, Naming, and Functionality

This document offers a comprehensive exploration of MS-DOS and NTFS file systems, highlighting key features such as naming conventions (8+3 format), hierarchical directory structures, and disk partitioning strategies. It compares FAT-12, FAT-16, and FAT-32, explaining their specifications and historical development. The NTFS section details its Master File Table (MFT) structure, metadata management, and journaling capabilities, emphasizing system robustness and file organization. It also touches on compatibility with earlier DOS versions and introduces concepts like long file names and B+ tree directory structures.

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Overview of MS-DOS and NTFS File Systems: Structure, Naming, and Functionality

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  1. File Systems Examples

  2. MS-DOS File System • Naming: 8+3 in upper case • Directories: Hierarchical directory structure • No soft or hard links • 32 byte directory entry • Max file size: 232 = 4 GB (not possible due to other reasons)

  3. FAT • MS-DOS uses a File Allocation Table (FAT) • FAT-12, FAT-16, FAT-32 based on # bits in disk address • Disk block is some multiple of 512 bytes • Block size also known as cluster size in Microsoft terminology • 1st MS-DOS version used FAT-12 with 512-byte blocks • Partition size: 212*512, actually 4086*512 ~ 2 MB • Memory space: 4096 entries of 2 bytes each • Later versions had variable disk block sizes  16 MB partitions • MS-DOS supported 4 disk partitions  64 MB disks • MS-DOS uses FAT to track free disk blocks • Mark free blocks with special code • Does not need free-list of bitmap

  4. FAT-16 and FAT-32 • FAT-16: 16-bit disk pointers, with varying block sizes • Block sizes from 512 bytes to 32 KB were supported • Memory requirement: 216*2 bytes = 128 KB • Largest disk partition: 216*32 KB ~ 2 GB • Total of 8 GB disk space • FAT-32: 28-bit disk pointers • Introduced with 2nd version of Windows 95 • Theoretical partition size: 228*32 KB • Internal representation using 32-bits and 512-byte sectors  2 TB • Pros over FAT-16: other than larger disks • 8 GB can be single partition • Smaller block size can be used for same disk partition

  5. FAT Comparison

  6. Windows 98 FS • Uses FAT-32 and long file names • Also used in Windows Me • New directory structure: • How to store long file names? • Challenge: compatibility with earlier DOS versions • Solution: 2 names: DOS compliant (8 + 3), and original name • SampleFile is also known as SAMPLE~1

  7. Storing Long File Names • Use long file name fields to store the longer file name • Attributes field ensures that MS-DOS ignores these entries Checksum

  8. NTFS • Partition also called a volume • Cluster size from 512 bytes to 64 KB, usually 4 KB used • Addressing: uses logical cluster numbers • File Structure: object with certain attributes • Opposed to stream of bytes in MS-DOS and UNIX • User data in data attributes • NTFS disk structure: • 12% allocated for MFT area

  9. Master File Table (MFT) • Every system component is a file, and MFT is most imp. • Has information of every other file on the disk • MFT divided into records of fixed size (1 to 4 KB) • Each record corresponds to a file • Each file described by one or more records • Small (resident) attributes are stored in MFT record • For small files, even data might be stored in MFT record • Large (nonresident) attributes stored on disk • Pointer stored in MFT • For files with large # of attributes, or high fragmentation • Base file record has info of other records with file info

  10. MFT Record Attributes

  11. MFT Record for 3-run, 9-block file

  12. Storing Large Files

  13. Other Details • Each file in NTFS has unique ID called File Reference • 64 bit in length with 48-bit file number, 16-bit sequence number • File number is array slot in MFT for that file • Sequence number incremented on every MFT reuse • Used for internal consistency checks • Directory Structure • As a B+ Tree: no tree reorganization, height of all leaves same • Index root of dir contains top level of B+ Tree • Might point to disk extents (seq. of contiguous blocks) for large dirs • Each dir entry has name, file ref., copy of update timestamp, size

  14. NTFS vs FAT directory structure

  15. NTFS Metadata • First 16 NTFS files are system files, called metafiles • Disk structure: • The first NTFS file is the MFT file • Second file contains copy of 1st 16 entries of MFT • Next few files are also special • $LogFile: metadata updates to FS • $Volume: housekeeping info • $Attrdef: list of attributes on volume • $. : root directory • $Bitmap: volume free space bitmap • $Boot: boot sector • $Quota: users rights on disk space (from NT5)

  16. NTFS Journaling • All FS data structure updates done within transactions • Before altering a data structure write redo and undo information • Write commit record to log after a successful update • Restore FS data structure after a crash by processing log • Redo committed transactions, and undo unsuccessful ones • Periodically write a checkpoint record to log • Log records before checkpoint not required • Note: Journaling does not ensure file data consistency

  17. WinFS • Bridge worlds of file systems, relation DBs, objects, XML • Eg. Store your data as object relations, which is also useful to other apps • Just too much work to be done! • Rewrite OS (apps), compatibility with NTFS, …

  18. Unix V7 FS • Structured as a tree, starting at the root • File names are 14 ASCII chars, other than / and NUL • Directories: one entry for each file • Each entry has 2 fields: file name (14 bytes) & i-node # (2 bytes) • Number of files is 216 = 64 K

  19. Unix I-nodes • I-node attributes: size, times (creation, modification, last access), owner, group, protection, num. of link pointers

  20. Steps for /usr/ast/mbox

  21. Linux ext2 File System • Very common on Linux systems • Extends on Unix FS • Generic code in /usr/src/linux/fs • ext2 specific code in /usr/src/linux/fs/ext2 • Disk partitioned into “block groups” • Ext2fs block structure: • Similar to FFS cylinder groups

  22. The Linux Ext2fs File System • File Allocation: first select a block group • For data blocks, tries to allocate block to group with file’s i-node • For i-node allocations, selects group of file’s parent’s dir • Directory files are dispersed across all block groups • Within block groups, tries to allocate contiguous blocks • Uses a bitmap for free blocks in the group • For first blocks for a file, searches free block from the beginning • For extending file, search for a free byte in bitmap from last alloc • If no free byte, then choose any free bit • After choosing block, back track to not leave holes!

  23. Ext2fs Block-Allocation Policies

  24. Linux Virtual File System • Operations differ on various OSes • Linux provides abstraction called virtual file system (VFS) • The Linux VFS is designed around OO principles • is composed of two components: • Set of definitions defining what a file object is allowed to look like • inode-object & file-object structures represent individual files • the file systemobject represents an entire file system • A layer of software to manipulate those objects

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