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Operating Systems Lecture 34: File Systems and Directories - Prof. Godmar Volker Back, Study notes of Operating Systems

Virginia Polytechnic Institute and State University (Virginia Tech)Operating Systems
Prof. Godmar Volker Back

A portion of lecture notes from cs 3204 operating systems course at spring 2006. Topics such as multi-level indices, storing and positioning inodes, directories, using linear lists and b-trees, absolute paths, and some issues in name resolution. The lecture also discusses hard and soft links.

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CS 3204
Operating Systems
Godmar Back
Lecture 34
4/14/2006CS 3204 Spring 2006 2
Announcements
• Project 4 due Wed, May 3, 11:59pm
• Recommended reading
– Chapter 11.1-11.5, 11A
– Chapter 12, in particular 12.7
• Project 4 help sessions next Monday &
Tuesday
Filesystems
4/14/2006CS 3204 Spring 2006 4
Multi-Level Indices
• Used in Unix &
Pintos (P4)
1
2
3
..
N
FLI
SLI
TLI
1
2
index
N
index2
index
index
N+IN+1
N+I+1
index3index2
Direct
Blocks
Indirect
Block
Double
Indirect
Block
Triple
Indirect
Block index N+I+I2
4/14/2006CS 3204 Spring 2006 5
Storing Inodes
• Unix v7, BSD 4.3
• FFS (BSD 4.4)
• Cylindergroups have
superblock+bitmap+inode list+file space
• Try to allocate file & inode in same
cylinder group to improve access locality
I0I1I2 I3I4 …..
Superblock Rest of disk for files & directories
I0I1…
SB1 Files … I3I4 ….. Files … I8I9 ….. Files …SB2 SB3
CGi
4/14/2006CS 3204 Spring 2006 6
Positioning Inodes
• Putting inodes in fixed place makes finding
inodes easier
– Can refer to them simply by inode number
– After crash, there is no ambiguity as to what
are inodes vs. what are regular files
• Disadvantage: limits the number of files
per filesystem at creation time
– Use “df –ih” on Linux to see how many inodes
are used/free
pf3

Partial preview of the text

Download Operating Systems Lecture 34: File Systems and Directories - Prof. Godmar Volker Back and more Study notes Operating Systems in PDF only on Docsity!

CS 3204

Operating Systems

Godmar Back

Lecture 34

CS 3204 Spring 2006 4/14/2006 2

Announcements

• Project 4 due Wed, May 3, 11:59pm

• Recommended reading

– Chapter 11.1-11.5, 11A

– Chapter 12, in particular 12.

• Project 4 help sessions next Monday &

Tuesday

Filesystems

CS 3204 Spring 2006 4/14/2006 4

Multi-Level Indices

• Used in Unix &

1 Pintos (P4)

N

FLI

SLI

TLI

index

N

index 2

index

index

N+1 N+I

N+I+

index 3 index 2

Direct Blocks

Indirect Block

Double Indirect Block Triple Indirect Block index

N+I+I^2

CS 3204 Spring 2006 4/14/2006 5

Storing Inodes

• Unix v7, BSD 4.

• FFS (BSD 4.4)

• Cylindergroups have

superblock+bitmap+inode list+file space

• Try to allocate file & inode in same

cylinder group to improve access locality

Superblock I 0 I 1 I 2 I 3 I4 ….. Rest of disk for files & directories

SB1 I 0 I 1 …Files … SB2 I 3 I4 ….. Files … SB3I 8 I9 ….. Files …

CGi

CS 3204 Spring 2006 4/14/2006 6

Positioning Inodes

• Putting inodes in fixed place makes finding

inodes easier

– Can refer to them simply by inode number

– After crash, there is no ambiguity as to what

are inodes vs. what are regular files

• Disadvantage: limits the number of files

per filesystem at creation time

– Use “df –ih” on Linux to see how many inodes

are used/free

CS 3204 Spring 2006 4/14/2006 7

Directories

• Need to find file descriptor (inode), given a name

• Approaches:

  • Single directory (old PCs), Two-level approaches with 1 directory per user

• Now exclusively hierarchical approaches:

  • File system forms a tree (or DAG)

• How to tell regular file from directory?

  • Set a bit in the inode

• Data Structures

  • Linear list of (inode, name) pairs
  • B-Trees that map name -> inode

CS 3204 Spring 2006 4/14/2006 8

Using Linear Lists

• Advantage: (relatively) simple to

implement

• Disadvantages:

– Scan makes lookup (& delete!) really slow for

large directories

– Can cause fragmentation (though not a

problem in practice)

23 multi-oom 15 sample.txt

offset 0

inode #

CS 3204 Spring 2006 4/14/2006 9

Using B-Trees

• Advantages:

– Scalable to large number of files: in growth, in

lookup time

• Disadvantage:

– Complex

– Overhead for small directories

CS 3204 Spring 2006 4/14/2006 10

Absolute Paths

• How to resolve a path name such as

“/usr/bin/ls”?

– Split into tokens using “/” separator

– Find inode corresponding to root directory

  • (how? Use fixed inode # for root)

– (*) Look up “usr” in root directory, find inode

– If not last component in path, check that inode

is a directory. Go to (*), looking for next comp

– If last component in path, check inode is of

desired type, return

CS 3204 Spring 2006 4/14/2006 11

Some Issues in Name Resolution

• Must have a way to scan an entire directory

without other processes interfering -> need a

“lock” function

• But don’t need to hold lock on /usr when

scanning /usr/bin

  • Directories can only be removed if they’re empty

• Most OS cache translations in “namei” cache –

maps absolute pathnames to inode

CS 3204 Spring 2006 4/14/2006 12

Current Directory

• Relative pathnames are resolved relative

to current directory

– Provides default context

– Every process has one in Unix/Pintos

• chdir(2) changes current directory

• lookup algorithm the same, except starts

from current dir

– process should keep current directory open

– current directory inherited from parent