File Systems Examples
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)
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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
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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
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FAT Comparison
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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
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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
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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
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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
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MFT Record Attributes
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MFT Record for 3-run, 9-block file
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Storing Large Files
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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
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NTFS vs FAT directory structure
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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
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$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)
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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
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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, …
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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
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Unix I-nodes
• I-node attributes: size, times (creation, modification, last
access), owner, group, protection, num. of link pointers
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Steps for /usr/ast/mbox
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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
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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!
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Ext2fs Block-Allocation Policies
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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
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