File System Design

1
File System Design

• Lecture notes for CEG 433
• Lectures by Dr. Prabhaker Mateti
• Notes by Patrick Flaherty
• January 2005

2
Disk Structure

• Typical disks have multiple platters
• Each platter organized into n tracks with a fixed
  number of sectors on each track
• Tracks sectors may be though of as logically
  arranged in a linear order

3
File System Design

• For any hard disk, there is some fixed number of
  tracks and sectors
• Sector size may vary from disk to disk -- but not
  within a given disk
• For our initial design, we define
  block sector
• For the class project we will design an
  i-Node based file system

4
MyFile.txt

• Let blockksize 1024 bytes (i.e., 1k)
• Hard disks work only with blocks
• Thus, MyFile.txt will be stored in a number of
  blocks determined by the size of the file
• Why block 250, 23, etc.?
• Because the file system used the first 5 free
  blocks available

Q If a File occupies 5 blocks, how big is its
size in bytes ? A 4k1 Filesize 5k
Block
5
File System Design - Directory

• Think of a directory as a table
• File Local Name
• ? How to read it
• Problem Lots of block numbers gt cumbersome
• Put the block s away somewhere
• Identify the place where you put them with
  another number (call it i-node)

First concept
Now File ? (11) ? i-node
Revised
6
File System Design i-Nodes

• Given i-number ? list of block s
• How?
• i-number is defined as an index for an array
  named i-node array where each i-node is an
  array of (for example) 7 elements
• Look at i-node 5

A single i-Node
Notice these do not point to any address
A pointer to block number 250
7
File System Design i-Nodes (cont.)

• The i-node array is not in memory but on the disk
• How many i-nodes should we have?
• Obviously 5 is too few and 5 zillion is too many
• Need to think about file size before choosing the
  number of i-nodes we will need
• More questions
• How much space is required for these i-nodes?
• Where do we put them?

8
File System Design i-Nodes (cont.)

• i-node size ht wd
• Assume Width 4 bytes
• Implication 232 1 possible block numbers
• For our design
• Let each i-node be an array of 7 elements
• 6 elements available to point to a block
• 1 element reserved for later use

Width (wd)
Height (ht)
9
File System Design i-Nodes (cont.)

• Suppose
• i-node size ht wd
• Width 4 bytes
• Total number of i-nodes ni
• Total space for all i-nodes ht 4 ni
• How many blocks will this use?

10
File System Design i-Nodes (cont.)

• All i-nodes are initialized with all values set
  equal to 0
• Questions
• Where do we put the blocks? (Well get to this
  shortly)
• Where does the root directory go?

Blocks containing the i-node array
Block 0 is reserved
11
File System Design Directories

• By convention, put root directory in i-node 1
• But first, we must create it
• By design choice
• A single dot . refers to the current directory
  as a name
• A double dot .. refers to the parent directory
• Suppose current directory is d2 with parent
  directory d1
• Given /d1/d2
• d2 will look like this
• What is the size of the root directory? (Ans. 13
  bytes)

Root directory
All other directories
12
File System Design Bit Vectors

• How do we keep track of free blocks and free
  i-nodes?
• One way is to use bit vectors
• Free bit vector of blocks fbvbx
• Where for each bit
• Assume nb the number of blocks in the hard disk
• Then size of fbvb nb bits
  blocks
• Similarly for the free bit vector of i-nodes
  fbvix
• Size of fbvi ni bits blocks

0 the xth block is in use 1 the xth block
is free
13
File System Design File Volume

• Now we have
• The action that does this should be called Make
  File Volume but for historical reasons it is
  called mkfs
• The File System includes algorithms and other
  required code

fbvi
i-node array
The root directory
fbvb
14
Create A New Directory

• What are the steps?
• Find a free i-node for the new directory gt 6
• Parent directory i-node will be the current
  working directorys (CWD) i-node gt cwdin
• Find the next free block number gt 17
• Write block (17, P) where P is a pointer to the
  directory data structure mark i- 6 and block
  17 as in use
• Link the newly crated directory to its parent

All other directories