topik5.0file management (2)

8/13/2019 Topik5.0file Management (2)

1/27

Pn Nin Hayati Binti Mohd Yusoff

Penyelaras kursus EC503


2/27

Data collections created byusers

The File System is one of themost important parts of the OSto a user


3/27

Filing systems provideo A stor ge service clients dont know about the physical

characteristics of the disks or where the files have been stored onthem

The filing systems must make sure that a file is not lost, even if thereare hardware failures or software crashes

o A directory serviceclients can give convenient text names to filesand group them in directories (establish some relationship betweenthem)

Clients should be able to access the sharing of their files with other byspecifying who can access a given file and in what way


4/27


5/27

USER PROCESS

LOGICAL FILE ORGANIZATION

LOGICAL I/O METHODS

I/O SUPERVISOR FILESYSTEM

DISK (TAPE, CD) SYSTEM

I/O


6/27

1

File directory service File storage service

Disk Handler

Client

Operating

System

Text-name

2

User-file id

3 4

Information

requested

from file

Disk

(1) client calls an operation such as open-file with the text name as an argument

o Another argument could be a field specifying the type of access (read-only or read-write)o The directory service will carry out an ccess checkto ensure that the client is uthorizedto

access the file

o The directory service will translate the text name into a form that enables the file storageservice to locate the file on disk (n me resolution)

o In order to do the name resolution, the directory service may call the file storage service, thefile storage service may call the disk handler to access the disk and find the requiredinformation


7/27

1


Disk Handler

Client

Operating

System

Text-name

2

User-file id

3 4

Information

requested

from file

Disk

(2) The filing systems stores a very large numbers of files, only a few beingused at any time.

The filing system is ready for the client to use the file (It will have set up theinformation about this file in its tables in main memory)

It will return a user-file-identifier (UFID) for the client to use in subsequentrequests to read or write the file


8/27

1


Disk Handler

Client

Operating

System

Text-name

2

User-file id

3 4

Information

requested

from file

Disk

(3) Request to read the file (4) The storage service returns the portion of the file that was requested

at (3)


9/27

Three common kinds of file structure:

1. Unstructured sequence of bytes2. Sequence of fixed-length records3. Tree of records, possible of different lengths

*Unix and windows both use unstructured byte sequence


10/27

1 byte 1 record

(a) Byte

Sequence

(b) Record

Sequence

(c) Tree


11/27

Most commonly used in modern operating systems is an unstructuredsequence of bytes. The operating system does not know nor care what is inthe file. It only sees bytes. This is used by UNIX and DOS/Windows. The user

can put anything it wants in the file and the OS does not help or get in the way

of the user.

In part b of fig , the file is made up of a series of fixed length records. The unitof access is a record which is may be many bytes. This roots back to early days

when a file represented a sequence of punch cards of 80 columns wide, or for

output a 132 byte record was sent to the printer. Read and write operations

affected a whole record. This organization is not used much anymore.

The 3rdtype of structure depicted in part (c) is a tree of variable size records,

each having a key field in a fixed position in the record for sorting. Can used

randomly by requesting a record having a certain key. Used in large

mainframes for commercial processing.


12/27

Issues in file allocation

o When a new file is created, do we specify the maximum size? Is thatspace allocated at once?

o

Space is allocated to a file as one or more contiguous units(portions). How big of a unit should we use when allocating spacefor a file?

o How do we keep trackof what space has been allocated to a givenfile (what kind of structure or table is used to keep track for a unit

allocated to a file)?


13/27

Pre-Allocation

o Declare max size in advance

o May be hard to guess space needed

o Tendency to overestimate space needed

o Ok if the file will never change

Dynamic allocation

o Get space as the file needs it

o Files are often no longer contiguous


14/27

Method of storing files:

Contiguous allocation

Linked list allocation disk blocks)Linked list allocation using index table in main memory)

Index-node


15/27

o Each file occupies a set of contiguous blocks on the disk. Simple - only starting location (block #) and length (number of

blocks) are required.

Suits sequential or direct access.

Fast (very little head movement) and easy to recover in the event

of system crash.

Problems

Wasteful of space (dynamic storage-allocation problem). Use firstfit or best fit. Leads to external fragmentation on disk.

Files cannot grow - expanding file requires copying

Users tend to overestimate space - internal fragmentation.

External fragmentation will occur

o Need to perform compaction


16/27

Contiguous File Allocation


17/27

Contiguous File Allocation After Compaction)


18/27

Each file is a linked list of disk blocks

Blocks may be scattered anywhere on the disk.

Each node in list can be a fixed size physical block or a contiguous

collection of blocks.

Allocate as needed and then link together via pointers. Disk space used to store pointers, if disk block is 512 bytes, and pointer

(disk address) requires 4 bytes, user sees 508 bytes of data.

Pointers in list not accessible to user.

pointer

Block =

Data


19/27


20/27

o Simple - need only starting address.

o Free-space management system - space efficient.

Can grow in middle and at ends. No estimation of size necessary.

o Suited for sequential access but not random access.

o Directory Table maps files into head of list for a file.


21/27

o Slow - defies principle of locality.

Need to read through linked list nodes sequentially to find the record of

interest.

o Not very reliable

System crashes can scramble files being updated.

o Important variation on linked allocation method

File-allocation table (FAT) - disk-space allocation used by MS-DOS and

OS/2.


22/27

Indexed allocation

o File allocation table contains a separate one-level index for each file

o The index has one entry for each portion allocated to the file

o The file allocation table contains block number for the index


23/27

Indexed Allocation with Block Portions

File allocation table contains onelevel index for each file; the index hasjust one entry for portion (unit)allocated to the file

Typically the file indexes are notstored as part of the allocation table

o The file index for a file is kept in a

separate block and the entry for thefile allocation table points to thatblock

Supports both sequential andrandom access to a file

In the figure above, a fixed sizeblocks allocation schema is

presentedo Eliminates external fragmentation


24/27

Indexed Allocation with Variable-Length Potrtions

Indexed allocation supportsalso a variable size portions(units) allocation schema

o Improves locality

o It is the most popular form of fileallocation

In both cases, from time to timeconsolidation may be done

o It reduces the size of the index for

the variable sized portionsschema


25/27

Control structure that contains key information for a particular file


26/27

Control structure that contains key information for a particular file


27/27

topik5.0file management (2)

Documents