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Outline

File Systems Implementation How disks work How to organize data (files) on

disks Data structures Placement of files on disk

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Disk Surface Layout

Tracks: concentric rings on disk bits laid out serially on tracks

Tracks split into sectors or blocks Minimum unit of transfer from disk

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Disk Pack: Multiple Disks

Disks organized in disk pack = stack of platters

Use both sides of platters

Two read-write heads at end of each arm

Cylinders:

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Cost of Disk Operations

Latency: time to initiate disk transfer Seek time: time to position head

over correct cylinder Rotational time: time for correct

sector to rotate under disk head Bandwidth: rate of I/O transfer of

sectors once initiated

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Outline

File Systems Implementation How disks work How to organize data (files) on disks

Data structures Placement of files on disk

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File Organization on Disk

File system maps file blocks to disk location (file 0, block 0) = (platter 0, cylinder 0,

sector 0) Key performance issues:

Support sequential and random access What data structure is best for

maintaining file location information? How do we lay out files on the disk?

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File System Design Goals

Most systems fit following profile: Most files are small Most disk space taken up by large

files I/O operations target both small &

large Per-file cost must be low, but large

files must also have good performance

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On-Disk Data Structures

File descriptor/handle: (name, location on disk, attributes) Must be stored on disks just like files

Create file: Adds file descriptor to directory

that contains file Delete file

Find directory containing file Remove file descriptor from

directory

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Types of File Organization on Disk

Contiguous files Linked files Indexed files

Multilevel indexed files

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Contiguous Allocation

Operating system maintains ordered list of free disk blocks

OS allocates contiguous chunk of free blocks when it creates a file File descriptor: store start location

& size

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Contiguous Allocation:Pros & Cons

Advantages Simple Efficient to support which type of

access? Disadvantages

File size change? Fragmentation? Inefficient to support which type of

access? Examples: IBM OS/360, write-only

disks, early PCs

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Linked Files Maintain list of all free sectors/blocks File descriptor: keep pointer to first sector/block In each sector, keep pointer to next sector

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Linked Files: Pros & Cons

Advantages Fragmentation? File size changes? Efficient to support which type of access?

Disadvantages: Inefficient to support which type of

access? Number of seeks? Waste space than contiguous allocation?

Examples: MS-DOS

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Indexed Files

User or OS declares maximum length of file created

OS allocates array of block pointers for all blocks when it creates file But allocates blocks only

on demand OS fills pointers as it

allocates blocks

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Indexed Files: Pros & Cons

Advantages Sequential & random access: easy

Disadvantages Sets maximum file size Waste more space than linked list Lots of seeks (data not contiguous)

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Multilevel Indexed Files in UNIX Each file descriptor

contains 14 block pointers First 12 pointers point to

data blocks 13th pointer:

2nd-level/indirect blocks Points to block of 1024

pointers to 1024 more data blocks

14th pointer: 3rd-level blocks

Points to block of 1024 pointers to indirect blocks

Used in BSD UNIX 4.3

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Multilevel Indexed Files:Pros & Cons

Advantages Simple to implement Supports incremental file growth Small files?

Disadvantages Indirect access: inefficient for random

access to very large files Lots of seeks (data not contiguous)

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Free-Space Management: Bitmaps

Need free-space list to keep track of which disk blocks are free Akin to free-list for main memory

One approach: bitmap/bitvector One bit per block on disk Bit = 1 , block is free Special hardware to find 1st bit with

value of 1 in a word Marking block free very simple

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Free-Space Management

Problems: Space: Bitmap has big RAM footprint

80GB disk, 512 byte sectors = ? Mb Performance: Slow if most of disk is in

use Alternative: linked list

Link together free blocks Head of list – cached in kernel memory Each block contains next pointer Efficiency?

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Summary

Many of concerns & implementations of file systems similar to virtual memory implementations Contiguous allocation: simple but...

External fragmentation, compaction, relocation...

Linked file Indexed file ~ page tables Free space: managed with bitmap or

linked lists