File Systems and Management Ppt

8/4/2019 File Systems and Management Ppt

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File System and managementFile systems: What is a file, user view of files

file types and file operations, file types in Unix/Linux and

Microsoft

file operation commands, file access rights,

file storage management,

Inode or FAT structure, file control blocks,

root file system ,directory

file paths, blocks ,

impact of block size selection

Allocation Techniques -: contiguous allocation, chained andindexed allocations,

, file related system services, disk access control

and scheduling.

Notes Compiled by BJ


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Role Of Operating System

The operating system has various roles:

Management of the processor:

Management of the random access memory

Management of input/output:

Management of execution of applications:

Management of authorizations:

File management:

Information management



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File systems: What is a file

A file is a collection of data stored in one unit, identified by a filename.It can be a document, picture, audio or video stream, data library,

application, or other collection of data.

A file system is a method of storing and organizing computer files andtheir data. Essentially, it organizes these files into a database for thestorage, organization, manipulation, and retrieval by the computer's

operating system.

File systems are used on data storage devices such as a hard disksor CD-ROMs to maintain the physical location of the files.

A named collection of related infoConsists of asequence of bits, bytes, lines, orrecords



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File Attributes

Name

only information kept in human-readable form Identifier unique tag (number) identifies file within file

system

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

Information about files are kept in the directory structure,which is maintained on the disk



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File Operations

File is an abstract data type Create

Write

Read

Reposition within file Delete

Truncate

Open(Fi) search the directory structure on disk

for entry Fi, and move the content of entry tomemory

Close (Fi) move the content of entry Fi inmemory to directory structure on disk



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Open Files

Several pieces of data are needed to manageopen files:

File pointer: pointer to last read/write location,per process that has the file open

File-open count: counter of number of times afile is open to allow removal of data fromopen-file table when last processes closes it

Disk location of the file: cache of data accessinformation

Access rights: per-process access modeinformation



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Linux/UNIX File Types

Regular file

Text or binary data (e.g. an image).

Directory Contains other files and directories.

Device special file

An interface to a piece of hardware, such as aprinter. You don't have to worry about this.



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Microsoft File Types

.doc Microsoft Word File

.xls Microsoft Excel File

.exe Executable File - one that starts up a software application,for instance

.txt A plain text file, can be read by Word, Notepad, Wordpadmany others

.bmpA full detail (ie large file size)

picture.jpg A compressed (ie smaller file size) picture.wav A full

detail (ie large file size) sound file.mp3 A compressed (ie smaller file size) sound file



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File Types Name, Extension



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Access Methods

Sequential Access

Direct Access

Indexed Access



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Sequential Access

The simplest access method is sequential access. Informationin the file is processed in order, one record after the other.This mode of access is by far the

editors and compilers usually access files in this fashion.

Reads and writes make up the bulk of the operations on a file.A read operationread nextreads the next portion of thefile and automatically advances a file pointer, which tracks theI/O location.

Similarly, the write operationwrite nextappends to theend of the file and advances to the end of the newly writtenmaterial



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Sequential-access File



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Simulation of Sequential Access on a

Direct-access File



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Direct Access

Another method is direct access (or relative access). A file ismade up of fixed length

logical records that allow programs to read and write recordsrapidly

in no particular order. The direct-access method is based on adisk model of a file, since disks allow random access to any fileblock.

For direct access, the file is viewed as a numbered sequenceof blocks or records. Thus, we may read

block 14, then read block 53, and then write block 7. Ther



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Access Methods

Sequential Accessread nextwrite nextresetno read after last write

(rewrite)

Direct Accessread nwrite n

position to nread nextwrite next

rewrite n

n= relative block numberNotes Compiled by BJ

I d d R l ti Fil


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Index and Relative FilesThe index, like an index in the back of a book, contains pointersto the various blocks. To find a record in the file, we first search

the index and then use the pointer to access the file directly andto find the desired record.



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File operation CommandsCatTouch

Vi

ChmodUmask

Rm

rmdir



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Root File System



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Allocation Methods

An allocation method refers to how disk blocks

are allocated for files:

Contiguous allocation

Linked allocation

Indexed allocation



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Contiguous Allocation Contiguous allocation requires that each file occupy a set of

contiguous blocks on the disk. Disk addresses define a linearordering on the disk. With this ordering, assuming that only onejob is accessing the disk, accessing block b + 1 after block b

normally requires no head movement. When head movement isneeded (from the last sector of one cylinder to the first sector ofthe next cylinder), the head need only move from one track tothe next. Thus, the number of disk seeks required for accessing

contiguously allocated files is minimal. The IBM VM/CMS operating system uses contiguous allocation

because it provides such good performance.

Contiguous allocation of a file is defined by the disk address andlength (in block units) of the first block. If the file is n blocks long

and starts at location b, then it occupies blocks b, b + 1, b + 2, ...,b + n1. The directory entry for each file indicates the addressof the starting block and the length of the area allocated for thisfile



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Contiguous Allocation of Disk Space



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DISADVANTAGES Contiguous allocation has some problems, One

difficulty is finding space for a new file. Wasteful of space (dynamic storage-Allocation

problem) Files cannot grow

All these algorithms suffer from the problem ofexternal fragmentation. As files are allocated and deleted, the free disk

space is broken into little pieces. Externalfragmentation exists whenever free space isbroken into chunks. It becomes a problem whenthe largest contiguous chunk is insufficient for arequest;



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Linked allocation

Linked allocation solves all problems of contiguous allocation.

With linked allocation, each file is a linked list of disk blocks; thedisk blocks may be scattered anywhere on the disk. Thedirectory contains a pointer to the first and last blocks of thefile.

To create a new file, we simply create a new entry in the

directory. With linked allocation, each directory entry has apointer to the first disk block of the file. This pointer is initializedto nil(the end-of-list pointer value) to signify an empty file. Thesize field is also set to 0.

A write to the file causes the free-space management system tofind a free block, and this new block is written to and is linked tothe end of the file.

To read a file, we simply read blocks by following the pointersfrom block to block. So there is no external fragmentation



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Linked Allocation

a file of five blocks might start at block 9 and continue at block16, then block 1, then block 10, and finally block 25. Each blockcontains a pointer to the next block.



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Disadvantages The major problem is that it can be used effectively only for

sequential-access files.

To find the ith block of a file, we must start at the beginning ofthat file and follow the pointers until we get to the ith block.

Each access to a pointer requires a disk read, and some require adisk seek.

Another disadvantage is the space required for the pointers. If apointer requires 4 bytes out of a 512-byte block, then 0.78 percentof the disk is being used for pointers, rather than for information.

another problem of linked allocation is reliability what wouldhappen if a pointer were lost or damaged

A bug in the operating-system software or a disk hardware failuremight result in picking up the wrong pointer. This error could inturn result in linking into the free-space list or into another file.



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Indexed allocation Linked allocation solves the external-fragmentation and size-

declaration problems of contiguous allocation but linked

allocation cannot support efficient direct access, since thepointers to the blocks are scattered with the blocks themselvesall over the disk and must be retrieved in order.

Indexed allocation solves this problem by bringing all thepointers together into one location: the index block.

Each file has its own index block, which is an array of disk-blockaddresses.

The /"' entry in the index block points to the /"' block of the file.The directory contains the address of the index block

To find and read the /th block, we use the pointer in the /"'

index-block entry When the file is created, all pointers in theindex block are set to nil.

Each file has its own index block, which is an array of disk-blockaddresses.



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Indexed Allocation

Brings all pointers together into theindex block.

Logical view.

index table



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Indexed Allocation



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Indexed allocation

Indexed allocation supports direct access, without suffering from

external fragmentation, because any free block on the disk can

satisfy a request for more space.

Indexed allocation does suffer from wasted space, however. The

pointer overhead of the index block is generally greater than the

pointer overhead of linked allocation



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Disadvantages

Indexed-allocation schemes suffer from some

of the same performance problems as does

linked allocation. Specifically, the index blocks

can be cached in memory, but the data blocksmay be spread all over a volume.



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Mode of access: read, write, execute

Three classes of users

RWXa) owner access 7 1 1 1

RWX

b) group access 6 1 1 0

RWX

c) public access 1 0 0 1 Ask manager to create a group (unique name), say G, and

add some users to the group.

For a particular file (say game) or subdirectory, define anappropriate access.

owner group public

chmod 761 game

Attach a group to a file

chgrp G game

Access Rights



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Directory Structure A collection of nodes containing information about all files.

F 1 F 2F 3

F 4

F n

Directory

Files

Both the directory structure and the files reside on disk.Backups of these two structures are kept on tapes.



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file paths

Absolute or relative path name

Creating a new file is done in current directory



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Information in a Device Directory Name

Type Address

Current length

Maximum length

Date last accessed (for archival) Date last updated (for dump)

Owner ID (who pays)

Protection information (discuss later)



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Operations Performed on Directory

Search for a file

Create a file

Delete a file

List a directory

Rename a file

Traverse the file system



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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,

)



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Single-Level Directory

A single directory for all users.

Naming problem

Grouping problem



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Two-Level Directory

Separate directory for each user.

Path nameCan have the same file name for different user

Efficient searching

No grouping capability



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Tree-Structured Directories

Efficient searching

Grouping Capability Notes Compiled by BJ


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File Storage Management

OS need to maintain several piece of info that

can assist management of files

Like when file was last updated and whom

Unix have audit trail which maintain record of

who has accessed when and did what

audit trail useful from recovering from crash

also detect unauthorized access



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A Typical File-system Organization



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Storage Structure

Main memory only large storage media that the CPU can

access directly

Secondary storage extension of main memory that provides

large nonvolatile storage capacity

Magnetic disks rigid metal or glass platters covered with

magnetic recording material

Disk surface is logically divided into tracks, which are

subdivided into sectors

The disk controller determines the logical interaction

between the device and the computer



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Storage Hierarchy

Storage systems organized in hierarchy

Speed

Cost

Volatility

Caching copying information into faster

storage system; main memory can be viewed

as a ast cache for secondary storage



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Access latency

1 ns

25 ns

50 ns

5 ms

50 sec

< 1 KB

1 MB

256 MB40 GB

> 1 TB

Capacity

Storage pyramid

Registers

Cache (SRAM)

Main memory (DRAM)

Magnetic disk

Magnetic tape

Goal: really large memory with very low latency Latencies are smaller at the top of the hierarchy

Capacities are larger at the bottom of the hierarchy

Solution: move data between levels to create illusion of largememory with low latency

Better

Better



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Overview of Storage Structure Magnetic disks provide bulk of secondary storage of modern computers

Drives rotate at 60 to 200 times per second

Transfer rate is rate at which data flow between drive and computer

Positioning time (random-access time) is time to move disk arm todesired cylinder (seek time) and time for desired sector to rotate underthe disk head (rotational latency)

Head crash results from disk head making contact with the disk surface

Thats bad Disks can be removable

Drive attached to computer via I/O bus

Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI

Host controller in computer uses bus to talk to disk controller built into

drive or storage array



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Disk drive structure

sector

cylinder

platter

spindle

track

head

actuator

surfaces

Data stored on surfaces Up to two surfaces per

platter

One or more platters per disk

Data in concentric tracks Tracks broken into sectors

256B-1KB per sector

Cylinder: corresponding

tracks on all surfaces

Data read and written byheads

Actuator moves heads

Heads move in unison



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Moving-head Disk Machanism



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To read a given sector we first move the heads

to that sector's cylinder (seek time), then wait

for the sector to rotate under the head

(latency time), then copy data off of disk intomemory (transfer time).



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Booting from a Disk


d d


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Index Node


Indexed Allocation Mapping (Cont )


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Indexed Allocation Mapping (Cont.)

outer-index

index table file


INODE


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INODE

The inode pointer structure is a structure adopted by the inode of a file inthe Unix File System (UFS) or other related file systems to list the

addresses of a file's data blocks. In the past, the structure may have consisted of eleven or thirteen

pointers, but most modern file systems use fifteen pointers. Thesepointers consist of (assuming 12 pointers in the inode)

Twelve pointers that directly point to blocks of the file's data (directpointers)

One singly indirect pointer (a pointer that points to a block of pointers thatthen point to blocks of the file's data)

One doubly indirect pointer (a pointer that points to a block of pointersthat point to other blocks of pointers that then point to blocks of the file'sdata)

One triply indirect pointer (a pointer that points to a block of pointers that

point to other blocks of pointers that point to other blocks of pointers thatthen point to blocks of the file's data)

The structure allows for inodes to describe very large files in a file systemswith a fixed logical block size



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INODE

The inode pointer structure not only allows for files to easilybe allocated to non-contiguous blocks, it also allows the dataat a particular location inside a file to be easily located. This ispossible because the logical block size is fixed. For example, ifeach block is 8 kB, file data at 120 to 128 kB would be pointedto by the fourth pointer of the first indirect block

The number of pointers in the indirect blocks are dependenton the block size and size of block pointers. Example: with a512 byte block size, and 4 byte block pointers, each indirect

block can consist of 128 (512 / 4) pointers



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File Control Block

In Microsoft environment counterpart of

inode is File Control Block It stores Filename, Location of secondary

Storage, length of file in bytes, date and time

of creation, last access etc. Advantage of fcb is that it store file noting

which application created it

Uses extension name like .txt ,.doc to identifyhow file was created

Uses Simple cluster to store files



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File Control Block


File Allocation Table


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File-Allocation Table



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Disk Scheduling Policies

First-in, first-out (FIFO)

Process request sequentially

Fair to all processes

Approaches random scheduling in performance if

there are many processes



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we assume that the disk head is initially located attrack 100. we assume a disk with 200 tracks and that thedisk request queue has random requests in it. Therequested tracks, in the order receivedby the disk scheduler, are 55, 58, 39, 18, 90, 160, 150,38, 184.


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Shortest Service Time First

Select the disk I/O request that requires the least

movement of the disk arm from its current

position Always choose the minimum seek time



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SCAN

Arm moves in one direction only, satisfying all

outstanding requests until it reaches the last track

in that direction Direction is reversed



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C-SCAN

Restricts scanning to one direction only

When the last track has been visited in one

direction, the arm is returned to the opposite endof the disk and the scan begins again



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File Systems and Management Ppt

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