FILE Implementation Introduction imp .pptx

File System
Implementation
File System Structure
File System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Efficiency and Performance
Recovery
Log-Structured File Systems
NFS
Operating System Concepts

File-System Structure
File structure
Logical storage unit
Collection of related information
File system resides on secondary storage (disks).
File system organized into layers.
File control block – storage structure consisting of
information about a file.

Layered File System

A Typical File Control Block

In-Memory File System Structures
open
Read/write

Virtual File Systems
Virtual File Systems (VFS) provide an object-
oriented way of implementing file systems.
VFS allows the same system call interface (the API)
to be used for different types of file systems.
The API is to the VFS interface, rather than any
specific type of file system.

User program & Kernel, VSF
interface
Note: This picture is excerpted from Write a Linux Hardware Device Driver, Andrew O’Shauqhnessy, Unix world

Schematic View of Virtual File System

Directory Implementation
Linear list of file names with pointer to the data
blocks.
simple to program
time-consuming to execute
Hash Table – linear list with hash data structure.
decreases directory search time
collisions – situations where two file names hash to the
same location
fixed size
An improved implementation is a hash table with a
linked list (entry)
Collision free
Can grow

Allocation Methods
An allocation method refers to how disk blocks are
allocated for files:
Contiguous allocation
Linked allocation
Indexed allocation

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.
Random access.
Wasteful of space (dynamic storage-
allocation problem).
Files cannot grow.

Contiguous Allocation of Disk
Space

Linked Allocation
Each file is a linked list of disk blocks: blocks
may be scattered anywhere on the disk.
pointer
block =

Linked Allocation

Linked Allocation (Cont.)
Simple – need only starting address
Free-space management system – no waste of
space
No random access
Space waste for pointer (e.g. 4byte of 512 B)
Reliability

File-Allocation Table
Improved linked method: File-allocation table (FAT) – disk-
space allocation used by MS-DOS and OS/2.

Indexed Allocation
Brings all pointers together into the index
block.
Logical view.
index table

Example of Indexed Allocation

Indexed Allocation (Cont.)
Need index table
Random access
Dynamic access without external
fragmentation, but have overhead of index
block.

Indexed Allocation – Mapping
(Cont.)

outer-index
index table file

Combined Scheme: UNIX (4K bytes per block)

Free-Space Management
• Bit vector (n blocks)
…
0 1 2 n-1
bit[i] =

0  block[i] free
1  block[i] occupied

Free-Space Management (Cont.)
Bit map requires extra space. Example:
 block size = 212
bytes (4 K)
 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
Grouping
Large free blocks can be quickly found
Counting

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.

Linked Free Space List on Disk

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.

Recovery
Consistency checking – compares data in directory
structure with data blocks on disk, and tries to fix
inconsistencies.
Use system programs to back up data from disk to
another storage device (floppy disk, magnetic
tape).
Recover lost file or disk by restoring data from
backup.

Log Structured File Systems
Log structured (or journaling) file systems record
each update to the file system as a transaction.
All transactions are written to a log. A transaction
is considered committed once it is written to the
log. However, the file system may not yet be
updated.
The transactions in the log are asynchronously
written to the file system. When the file system is
modified, the transaction is removed from the log.

The Sun Network File System
(NFS)
An implementation and a specification of a
software system for accessing remote files across
LANs (or WANs).
The implementation is part of the Solaris and
SunOS operating systems running on Sun
workstations using an unreliable datagram protocol
(UDP/IP protocol and Ethernet.
Interconnected workstations viewed as a set of
independent machines with independent file
systems, which allows sharing among these file
systems in a transparent manner.

NFS (Cont.)
NFS is designed to operate in a heterogeneous
environment of different machines, operating
systems, and network architectures; the NFS
specifications independent of these media.
This independence is achieved through the use of
RPC primitives built on top of an External Data
Representation (XDR) protocol used between two
implementation-independent interfaces.
The NFS specification distinguishes between the
services provided by a mount mechanism and the
actual remote-file-access services.

Three Major Layers of NFS Architecture
UNIX file-system interface (based on the open,
read, write, and close calls, and file descriptors).
Virtual File System (VFS) layer – distinguishes local
files from remote ones, and local files are further
distinguished according to their file-system types.
The VFS activates file-system-specific operations to
handle local requests according to their file-system
types.
Calls the NFS protocol procedures for remote requests.
NFS service layer – bottom layer of the
architecture; implements the NFS protocol.

Schematic View of NFS Architecture

NFS Protocol
Provides a set of remote procedure calls for remote file operations. The
procedures support the following operations:
 searching for a file within a directory
 reading a set of directory entries
 manipulating links and directories
 accessing file attributes
 reading and writing files
NFS servers are stateless; each request has to provide a full set of arguments.
Modified data must be committed to the server’s disk before results are
returned to the client (lose advantages of caching).
The NFS protocol does not provide concurrency-control mechanisms.

FILE Implementation Introduction imp .pptx

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