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File Systems and Disk Management

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... mp3 files Large files account for most of the bytes transferred between memory and disk Bad news for file system designers File System Design ... Linux ext2) data ... – PowerPoint PPT presentation

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Title: File Systems and Disk Management


1
File Systems and Disk Management
  • Sarah Diesburg
  • Operating Systems
  • COP 4610

2
Design Goals of File Systems
Physical reality File system abstraction
Block-oriented Byte-oriented
Physical sectors Named files
No protection Users protected from one another
Data might be corrupted if machine crashes Robust to machine failures
3
File System Components
  • Disk management organizes disk blocks into files
  • Naming provides file names and directories to
    users, instead of tracks and sector numbers (e.g.
    Diesburg)
  • Protection keeps information secure from other
    users
  • Reliability protects information loss due to
    system crashes

4
User vs. System View of a File
  • User level individual files
  • System call level collection of bytes
  • Operating system level
  • A block is a logical transfer unit
  • Even for getc() and putc()
  • 4 Kbytes under UNIX
  • A sector is a physical transfer unit
  • 512-byte sectors on disks
  • File a named collection of blocks

5
User vs. System View of a File
  • A process
  • Read bytes 2 to 12
  • OS
  • Fetch the block containing those bytes
  • Return those bytes to the process

6
User vs. System View of a File
  • A process
  • Write bytes 2 to 12
  • OS
  • Fetch the block containing those bytes
  • Modify those bytes
  • Write out the block

7
Ways to Access a File
  • People use file systems
  • Design of file systems involves understanding how
    people use file systems
  • Sequential accessbytes are accessed in order
  • Random access (direct access)bytes are accessed
    in any order
  • Content-based accessbytes are accessed according
    to constraints on bye contents
  • e.g., return 100 bytes starting with aye carumba

8
File Usage Patterns
  • Most files are small, and most references are to
    small files
  • e.g., .login and .c files
  • Large files use up most of the disk space
  • e.g., mp3 files
  • Large files account for most of the bytes
    transferred between memory and disk
  • Bad news for file system designers

9
File System Design Constraints
  • High performance
  • Efficient access of small files
  • Many small files
  • Used frequently
  • Efficient access of large files
  • Consume most disk space
  • Account for most of the data movement

10
Some Definitions
  • A file contains a file header, which associates
    the file with its disk sectors

File header
11
Some Definitions
  • A file system needs a disk allocation bitmap to
    represent free space on the disk, one bit per
    block

12
Disk Allocation Policies
  • Contiguous allocation
  • Link-list allocation
  • Segment-based allocation
  • Indexed allocation
  • Multi-level indexed allocation
  • Hashed allocation

13
Contiguous Allocation
  • File blocks are stored contiguously on disk
  • To allocate a file,
  • Specify the file size
  • Search the disk allocation bitmap for consecutive
    free blocks

File header
14
Pros and Cons of Contiguous Allocation
  • Fast sequential access
  • Ease of computing random file locations
  • Adding an offset to the first disk block location
  • - External fragmentation
  • - Difficulty in growing files

15
Linked-List Allocation
  • Each file block on a disk is associated with a
    pointer to the next block
  • A special marker to indicate the end of the file
  • e.g., MS-DOS file system
  • File attribute table (FAT)

File header
16
Pros and Cons of Linked-List Allocation
  • Files can grow dynamically with incremental
    allocation of blocks
  • - Sequential access may suffer
  • Blocks may not be contiguous
  • - Horrible random accesses
  • May involve multiple sequential searches
  • - Unreliable
  • A corrupted pointer can lead to loss of the
    remaining file

17
Indexed Allocation
  • Uses a preallocated index to directly track the
    file block locations

File header
18
Pros and Cons of Indexed Allocation
  • Fast lookups and random accesses
  • - File blocks may be scattered all over the disk
  • Poor sequential access
  • Needs defragmenter
  • - Needs to reallocate index as the file size
    increases

19
Segment-Based Allocation
  • Needs a segment table to allocate multiple,
    contiguous regions of blocks

File header
20
Pros and Cons of Segment-Based Allocation
  • Relax the requirements for large contiguous
    disk regions
  • - Fragmentation ? 100
  • Segment-based allocation ? Indexed allocation
  • - Random accesses not as fast as pure contiguous
    allocation

21
Multilevel Indexed Allocation
  • Certain index entries point to index blocks, as
    opposed to data blocks (e.g., Linux ext2)

File header
22
Multilevel Indexed Allocation
  • A single indirect block contains pointers to data
    blocks
  • A double indirect block contains pointers to
    single indirect blocks
  • A triple indirect block contains pointers to
    double indirect blocks

23
Pros and Cons of Multilevel Indexed Allocation
  • Optimized for small and large files
  • Small files accessed through the first 12
    pointers
  • Large files can grow incrementally
  • - Multiple disk accesses to fetch a data block
    under triple indirect block
  • - Largest file size capped by the number of
    pointers
  • - Arbitrary file size boundaries among levels

24
Hashed Allocation
  • Allocates a disk block by hashing the block
    content to a disk location

Old file header
New file header
25
Pros and Cons of Hashed Allocation
  • File blocks of the same content can share the
    same disk block to save storage
  • e.g., empty blocks
  • Good for backups and archival
  • Small modifications to a large file result in
    only additional storage of the changes
  • - Poor disk performance
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