File Systems and Disk Management

Sarah Diesburg Operating Systems

CS 3430

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

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

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

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

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

Ways to Access a File

People use file systems Design of file systems involves understanding

how people use file systems Sequential access—bytes are accessed in

order Random access (direct access)—bytes are

accessed in any order Content-based access—bytes are accessed

according to constraints on bye contents e.g., return 100 bytes starting with “aye carumba”

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

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

Some Definitions

A file contains a file header, which associates the file with its disk sectors

File header

data block location

data block location

name

Some Definitions

A file system needs a disk allocation bitmap to represent free space on the disk, one bit per block

Disk Allocation Policies

Contiguous allocation Link-list allocation Segment-based allocation Indexed allocation Multi-level indexed allocation Hashed allocation

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 data block location

number of blocks

data block location

File header

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

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)

data block location

next block entry

data block location

next block entry

File header

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

Indexed Allocation

Uses a preallocated index to directly track the file block locations

data block location

data block location

data block location

data block location

File header

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

Segment-Based Allocation

Needs a segment table to allocate multiple, contiguous regions of blocks

begin, end blocks

begin, end blocks

begin, end blocks

begin, end blocks

File header

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

Multilevel Indexed Allocation

Certain index entries point to index blocks, as opposed to data blocks (e.g., Linux ext2)

data block location

index block location

index block location

index block location

data block location

index block location

index block location

data block location

data block location

File header

12 data block location

data block location data block location

data block location

index block location

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

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

Hashed Allocation

Allocates a disk block by hashing the block content to a disk location

data block location

data block location

data block location

data block location

Old file header

data block location

data block location

data block location

data block location

New file header

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