Goals for Today Learning Objective: Learn about directory - - PowerPoint PPT Presentation

CS 423: Operating Systems Design 1

Goals for Today

Reminder: Please put away devices at the start of class

• Learning Objective:
• Learn about directory structures
• Run through some disk performance exercises
• Announcements, etc:
• C4: I removed 2 papers from your reading list
• No longer need to read Multics Security Eval or SELinux
• MP3 is out! Due April 18th.

CS 423: Operating Systems Design

Professor Adam Bates Spring 2018

CS 423
 Operating System Design: Directory Structures & Disk Performance

CS 423: Operating Systems Design

NTFS

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■ Master File Table ■ Flexible 1KB storage for metadata and data ■ Extents ■ Block pointers cover runs of blocks ■ Similar approach in linux (ext4) ■ File create can provide hint as to size of file ■ Journalling for reliability

CS 423: Operating Systems Design 4

NTFS

• Std. Info.

File Name Data (resident) (free)

MFT Record (small fjle) Master File Table

CS 423: Operating Systems Design 5

NTFS

MFT MFT Record

Start Length Start Length

• Std. Info.

File Name (free) Data (nonresident) Data Extent Data Extent

CS 423: Operating Systems Design 6

NTFS Indirect Block

MFT MFT Record (part 2)

• Std. Info.

(free) Data (nonresident) MFT Record (part 1)

• Std. Info.

Attr.list Data (nonresident) File Name

Data Extent Data Extent Data Extent Data Extent Data Extent

CS 423: Operating Systems Design 7

NTFS

MFT Record (normal file) MFT

• Std. Info.

Data (nonresident)

MFT Record (small file)

• Std. Info.

Data (resident)

MFT Record (big/fragmented file)

• Std. Info.

Attr.list Data (nonresident) Data (nonresident) Data (nonresident) Data (nonresident)

MFT MFT Record (huge/badly-fragmented file)

• Std. Info.

Attr.list (nonresident) Data (nonresident) Data (nonresident) Data (nonresident) Data (nonresident) Extent with part of attribute list Extent with part of attribute list Data (nonresident)

CS 423: Operating Systems Design

Named Data

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fjle name

• fgset

directory fjle number

• fgset

storage block index structure

CS 423: Operating Systems Design

Directory Structures

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■ maps symbolic names into logical file

names

■ search ■ create file ■ list directory ■ backup, archival, file migration

■ Directories are also files!

music 320 work 219 foo.txt 871

CS 423: Operating Systems Design

Directory Internals

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■ Directories are also files! Special files

■ Denoted by “File Type” in inode ■ Hold access paths to the files in the file system

■ They have data blocks (inodes) on disk ■ Enables support for very large directories ■ The partition superblock points to the

inode of the root directory.

CS 423: Operating Systems Design

Single-level Directory

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CS 423: Operating Systems Design 12

Tree-Structured Directories

CS 423: Operating Systems Design

Directory Layout

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■ Represent directory as a list of files ■ Linear search to find filename ■ Suitable for small directories

File 830 /home/tom

End of File Name File Number Next

. 830 .. 158 music 320 work 219

Free Space

foo.txt 871

Free Space

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What can we do to improve efficient in large directories?

CS 423: Operating Systems Design

B Trees

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■ Logarithmic search to find filename ■ Suitable for large directories

Search for Hash (foo.txt) = 0x30

Before Child Pointer

240 510 730 980 Root

Hash Entry Pointer

15 30 44 58 Leaf . 830 .. 158 30 foo.txt 871 music 320 ... ... work 219 code 3 bin 014 ... ... test 324 Leaf

Hash Number Name File Number Before Child Pointer

58 121 180 240 Child 780 841 930 980 Child

CS 423: Operating Systems Design

B Trees

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■ Logarithmic search to find filename ■ Suitable for large directories

Directory Entries B+Tree Nodes

... work 219 ... music 320 ... Child Root Leaf Leaf ... Child

Name File Number

File Containing Directory

CS 423: Operating Systems Design

Recursive Filename Lookup

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music 320 work 219 foo.txt 871 File 830

• /home/tom

mike 682 ada 818 tom 830 File 158

• /home

File 871

• /home/tom/foo.txt

bin 737 usr 924 home 158 File 2

• /

The quick brown fox jumped

• ver the

lazy dog.

CS 423: Operating Systems Design

Tree-Structured Directories

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■ arbitrary depth of directories ■ leaf nodes are files ■ interior nodes are directories ■ path name lists nodes to traverse to find

node

■ use absolute paths from root ■ use relative paths from current working

directory pointer

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Tree-Structured Directories

■ We usually think of directories as trees…

CS 423: Operating Systems Design

Acyclic Graph Structured Dir.’s

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■ But in practice they’re actually acyclic graphs!

CS 423: Operating Systems Design

Symbolic Links

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■ Symbolic links are different than regular links (often

called hard links). Created with ln -s

■ Can be thought of as a directory entry that points to

the name of another file.

■ Does not change link count for file

■

When original deleted, symbolic link remains

■ They exist because:

■

Hard links don’t work across file systems

■

Hard links only work for regular files, not directories

Hard link Symbolic Link Contents of file Contents of file direct direct direct symlink

CS 423: Operating Systems Design

Changing Gears

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Some notes on disk performance…

CS 423: Operating Systems Design

Typical Modern Spec’s

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■ Disk rotation speed: 5,000-15,000 RPM ■ Number of sectors per track: 500-2000 ■ Number of tracks: 100,000-200,000 ■ Average seek latency: 2ms ■ Block size: 0.5K-4K ■ Multiple zones (layout is different for inner and

• uter tracks)

■ Multiple bays (stacked drives)

CS 423: Operating Systems Design

Estimated Sustained Average Transfer Rate

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■ Suppose that a disk drive spins at 7200 RPM

(revolutions per minute), has a sector size of 512 bytes, and holds 160 sectors per track.

■ What is sustained average transfer rate of this drive in

megabytes per second?

A: Track. B: Sector. C: Sector of Track. D: File

CS 423: Operating Systems Design

Estimated Sustained Average Transfer Rate

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■ Suppose that a disk drive spins at 7200 RPM

(revolutions per minute), has a sector size of 512 bytes, and holds 160 sectors per track.

■ What is sustained average transfer rate of this drive in

megabytes per second? Disk spins 120 times per second (7200 RPM/60) Each spin transfers a track of 80 KB (160 sectors x0.5K) Sustained average transfer rate is 120x80 = 9.6MB/s.

CS 423: Operating Systems Design

Average Performance of Random Access

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■ Suppose that a disk drive spins at 7200 RPM

(revolutions per minute), has a sector size of 512 bytes, and holds 160 sectors per track.

■ Average seek time for the drive is 8 milliseconds ■ Estimate # of random sector I/Os per second that can

be done and the effective average transfer rate for random-access of a sector?

Disk spins 120 times per second Average rotational cost is time to travel half track: 1/120 * 50%=4.167ms Transfer time is 8ms to seek + 4.167 ms rotational latency + 0.052 ms (reading one sector takes 0.0005MB/ 9.6MB). = 12.219 ms # of random sector access/second = 1/0.012219=81.8 Effective transferring rate: 0.5 KB * 81.8= 0.0409 MB/s.

CS 423: Operating Systems Design

Flash Memory

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■ Electronically Erasable Programmable Read Only Memory

(EEPROM)

■ Example specifications (NAND Flash):

■ Page size: 2KB (approx.) ■ Block size: 64 pages (128KB) ■ Device size: 16K blocks

■ Random READ: 25µs, Sequential READ: 25ns ■ WRITE performance

■ PROGRAM PAGE: 220µs, BLOCK ERASE: 1.5ms

■ Endurance: 100,000 PROGRAM/ERASE cycles