Chapter 11: File System Chapter 11: File System ImplementationImplementation

Hung Q. Ngo

KyungHee University

Spring 2009

http://uclab.khu.ac.kr/lectures/2009-1-os.html

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Chapter 11: File System ImplementationChapter 11: File System Implementation

File-System Structure

Allocation Methods

Free-Space Management

Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated from my lecture notes by Kubiatowicz.

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ObjectivesObjectives

To describe the details of implementing local file systems and directory structures

To discuss block allocation and free-block algorithms and trade-offs

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File-System StructureFile-System Structure

File structure

Logical storage unit

Collection of related information

File system resides on secondary storage (disks)

File system organized into layers

File control block – storage structure consisting of information about a file

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Layered File SystemLayered File System

Basic file system: each physical block is identified by its numeric disk address, e.g. drive 1, cylinder 73, track 2, sector 10

File-organization module: translation between logical blocks and physical blocks. Free space management

Logical file system: manages directory structure, maintains file structure via file-control blocks (FCB), responsible for protection and security

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File-System ImplementationFile-System Implementation

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On-Disk File System StructureOn-Disk File System Structure

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In-Memory File System StructureIn-Memory File System Structure

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Opening & Reading a FileOpening & Reading a File

Per process: store current file pointer, access rights, accounting info, etc.

system-wide: location of file on disk, access dates, file size...

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Allocation MethodsAllocation Methods

An allocation method refers to how disk blocks are allocated for files:

Contiguous allocation

Linked allocation

Indexed allocation

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Contiguous AllocationContiguous Allocation

Each file occupies a set of contiguous blocks on the disk

Simple – only starting location (block #) and length (number of blocks) are required

Random access

Wasteful of space (dynamic storage-allocation problem)

Files cannot grow

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Linked AllocationLinked Allocation

Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.

Directory table stores pointers to the start and end blocks

Each block stores pointer to next block (usually first/last 4bytes) and block data

pointer

Data

block =

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Linked Allocation (Cont.)Linked Allocation (Cont.)

Simple – need only starting address

Free-space management system – no waste of space

No random access

File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2.

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File-Allocation TableFile-Allocation Table

EOF

0300

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Indexed AllocationIndexed Allocation

Brings all pointers together into the index blocks.

Logical view.

Random access Dynamic access without external fragmentation, but have overhead of index block.

index table

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Example of Indexed AllocationExample of Indexed Allocation

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Quiz on Indexed AllocationQuiz on Indexed Allocation

HDD = 32GB

Block size = 512Bytes

Maximum file size =256KB

How many blocks used to store the index tables?

5 10 10 25 26 2632 2 2 2 2 2 2 block numbers

needs at least 26bits to encode each entry in the index table=4bytes

File size max = 256KB = 512 blocks entry table must store 512 entries

512 4 4

GB KB KB blocks

bytes

blocks

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Combined Scheme: UNIX iNode Table Combined Scheme: UNIX iNode Table (4K bytes per block)(4K bytes per block)

(12)

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Memory Management Using iNodesMemory Management Using iNodes

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Free-Space ManagementFree-Space Management

Free-space linked list

Bitmap

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Linked Free Space List on DiskLinked Free Space List on Disk

Block #2 Block #3

Quiz:

HDD = 20MB

Block size = 1KB

If disk is totally empty, how many blocks to store the free space linked list?

15 15

15

1515

20 2 2

2 block numbers need 2bytes to save the block numbers

1 block = 1024 bytes save up to 511 free blocks

22 free blocks 40511

MB KB blocks

blocks

Next block in the linked list

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Free-Space ManagementFree-Space Management

Bit vector (n blocks)

…

0 1 2 n-1

bit[i] = 1 block[i] free

0 block[i] occupied

First free block number calculation:

(number of bits per word) *(number of 0-value words) +offset of first 1 bit

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Free-Space Management (Cont.)Free-Space Management (Cont.)

Bit map requires extra space

Example:

block size = 212 bytes

disk size = 230 bytes (1 gigabyte)

n = 230/212 = 218 bits (or 32K bytes)

Easy to get contiguous files

Linked list (free list)

Cannot get contiguous space easily

End of Chapter 11End of Chapter 11