File Systems Examples

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File Systems Examples
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MS-DOS File System

• Naming 83 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 212512, actually 4086512 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 2162 bytes 128 KB
• Largest disk partition 21632 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 22832 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 SAMPLE1

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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
• 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 files i-node
• For i-node allocations, selects group of files
  parents 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