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 New directory structure:
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 system object represents an entire file system
A layer of software to manipulate those objects