Disk & File System Management
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Disk Allocation
Free Space Management
Directory Structure
Naming
Disk Scheduling
Protection
CSE 331 Operating Systems Design
File System Abstraction
Physical Reality
File System Abstraction
Block oriented
Byte oriented
Phys. Sector #
Named files
No protection
User protection
Corruption
Robust to machine failures
File System Components
Disk Management –Scheduling, Allocation, Free Space
Man.
Naming- Directory Structure (Flat, Hierarchical)
Protection , Authentication, Reliability
CSE 331 Operating Systems Design
File Concept
• Contiguous logical address space
• Types:
– Data (numeric, character, binary)
– Program
CSE 331 Operating Systems Design
File Structure
• None - sequence of words, bytes
• Simple record structure
– Fixed length
– Variable length
• Complex Structures
– Formatted document
– Relocatable load file
• Can simulate last two with first method by inserting appropriate
control characters.
• Who decides:
– Operating system
– Program
UNIX approach – no structure
CSE 331 Operating Systems Design
File Attributes
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Name – only information kept in human-readable form.
Type – needed for systems that support different types.
Location – pointer to file location on device.
Size – current file size.
Protection – controls who can do reading, writing,
executing.
• Time, date, and user identification – data for protection,
security, and usage monitoring.
Where are these information kept?
File Header (PCB structure for Files)
CSE 331 Operating Systems Design
File Operations
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Create
Write
Read
Reposition within file – file seek
Delete
Truncate
Open(Fi) – search the directory structure on disk for
entry Fi, and move the content of entry to memory.
• Close (Fi) – move the content of entry Fi in memory to
directory structure on disk.
CSE 331 Operating Systems Design
User vs System View
• User view
– Collection of bytes (UNIX)
– Collection of records (IBM)
• System View (inside OS)
– Collection of blocks
• Block is the logical transfer unit
• Sector is the physical transfer unit
block size >= sector size
ie. UNIX block size= 4KB
How to translate user to system view? Give me bytes 2-12
CSE 331 Operating Systems Design
File Access Patterns
– Sequential Access: Give me next X bytes..
– Random Access: Give me bytes i-j
– Content-based Access: Find 100 bytes
starting with “xxx”
Many file systems do not provide #3 –
DBs are built on FS for this purpose
CSE 331 Operating Systems Design
Sequential-access File
CSE 331 Operating Systems Design
Simulation of Sequential Access on a Direct-access File
CSE 331 Operating Systems Design
How are files typically used?
• Most files are small (eg. .login, .c)
• Large files use most of the disk space
• File System needs to be efficient
– for small files as many of them
– for large files as most of the I/O due to them
CSE 331 Operating Systems Design
Disk Management
• Disk drives are addressed as large 1-dimensional
arrays of logical blocks, where the logical block is
the smallest unit of transfer.
• The 1-dimensional array of logical blocks is
mapped into the sectors of the disk sequentially.
– Sector 0 is the first sector of the first track on the
outermost cylinder.
– Mapping proceeds in order through that track, then
the rest of the tracks in that cylinder, and then
through the rest of the cylinders from outermost to
innermost.
CSE 331 Operating Systems Design
Common Data Structures
– File Header – one for each file
– Bitmap to represent free space on disk (one bit
per block)
• Blocks are numbered in Cylinder major order
7
0
1
2
8
9
10 ...
• Track 0 surface 0 sector 0,1,... Surface 1, sector 0....
• Track 1 surface 1, sector 0...
CSE 331 Operating Systems Design
Allocation Methods
How disk blocks are allocated for files?
• Contiguous allocation
• Linked allocation
• Indexed allocation
CSE 331 Operating Systems Design
Contiguous Allocation
Each file occupies a set of contiguous blocks
on the disk.
– Simple – only starting location (block #) and
length (number of blocks) are required for
access.
– Search bitmap to locate space for file
– Create (allocate max. Size)
11110000010101
CSE 331 Operating Systems Design
Contiguous Allocation of Disk Space
Fast Seq. Access
Easy Random access.
–External fragmentation
–Files cannot grow.
CSE 331 Operating Systems Design
Extent-Based Systems
• Some newer file systems (i.e. Veritas File System)
use a modified contiguous allocation scheme.
• Extent-based file systems allocate disk blocks in
extents.
• An extent is a contiguous block of disks. Extents
are allocated for files. A file consists of one or
more extents.
CSE 331 Operating Systems Design
Linked Allocation
Each file is a linked list of disk blocks: blocks may be
scattered anywhere on the disk.
block
=
pointer
CSE 331 Operating Systems Design
Linked Allocation
CSE 331 Operating Systems Design
Linked Allocation (Cont.)
 Simple – need only starting address
 no waste of space for free space management
 Can grow files dynamically
– Slow sequential access (seek time between
blocks)
– No random access
– Unreliable
CSE 331 Operating Systems Design
File-Allocation Table
CSE 331 Operating Systems Design
Indexed Allocation
• Brings all pointers together into the index block.
• Logical view.
index table
CSE 331 Operating Systems Design
Example of Indexed Allocation
CSE 331 Operating Systems Design
Indexed Allocation (Cont.)
Random access is fast
Easy grow without external
fragmentation
– Need index table. Overhead of index
block.
– Hard to grow files bigger than table
size
– Still lots of seeks
CSE 331 Operating Systems Design
Indexed Allocation – Mapping (Cont.)

outer-index
index table
CSE 331 Operating Systems Design
file
Combined Scheme: UNIX (4K bytes per block)
10 direct blocks
3 indirect blocks
CSE 331 Operating Systems Design
Free-Space Management
• Bit vector (n blocks)
0 1
2
n-1
bit[i] =
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0  block[i] free
1  block[i] occupied
CSE 331 Operating Systems Design
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
– No waste of space
CSE 331 Operating Systems Design
Free-Space Management (Cont.)
• Need to protect:
– Pointer to free list
– Bit map
• Must be kept on disk
• Copy in memory and disk may differ.
• Cannot allow for block[i] to have a situation where
bit[i] = 1 in memory and bit[i] = 0 on disk.
– Solution:
• Set bit[i] = 1 in disk.
• Allocate block[i]
• Set bit[i] = 1 in memory
CSE 331 Operating Systems Design
Linked Free Space List on Disk
CSE 331 Operating Systems Design
Directory Structure
• A collection of nodes containing information about
all files. (Pointers to file headers)
File headers
Directory
Files
F1
F2
F3
F4
Fn
Both the directory structure and the files reside on disk.
Backups of these two structures are kept.
CSE 331 Operating Systems Design
A Typical File-system Organization
CSE 331 Operating Systems Design
Operations Performed on Device
Directory
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Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
CSE 331 Operating Systems Design
Organize the Directory (Logically) to Obtain
• Efficiency – locating a file quickly.
• Naming – convenient to users.
– Two users can have same name for different files.
– The same file can have several different names.
• Grouping – logical grouping of files by
properties, (e.g., all Java programs, all games,
…)
CSE 331 Operating Systems Design
Single-Level Directory
• A single directory for all users.
Naming problem
Grouping problem
CSE 331 Operating Systems Design
Two-Level Directory
• Separate directory for each user.
•Path name
•Can have the same file name for different user
•Efficient searching
•No grouping capability
CSE 331 Operating Systems Design
Tree-Structured Directories
CSE 331 Operating Systems Design
Tree-Structured Directories (Cont.)
• Efficient searching
• Grouping Capability
• Current directory (working directory)
– cd /spell/mail/prog
– type list
CSE 331 Operating Systems Design
Tree-Structured Directories (Cont.)
• Absolute or relative path name
• Creating a new file is done in current directory.
• Delete a file
rm <file-name>
• Creating a new subdirectory is done in current directory.
mkdir <dir-name>
Example: if in current directory /mail
mkdir count
mail
prog
copy prt exp count
Deleting “mail”  deleting the entire subtree rooted by “mail”.
CSE 331 Operating Systems Design
Where to put the Current “File
Position” Field
• Address of the next byte to be read or written
– i-node
– Process table
CSE 331 Operating Systems Design
CSE 331 Operating Systems Design
Efficiency and Performance
• Efficiency dependent on:
– disk allocation and directory algorithms
– types of data kept in file’s directory entry
• Performance
– disk cache – separate section of main memory for
frequently used blocks
– free-behind and read-ahead – techniques to optimize
sequential access
– improve PC performance by dedicating section of
memory as virtual disk, or RAM disk.
CSE 331 Operating Systems Design