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Transcript of File Management Chapter #12. 파일 관리 n In most applications, the file is the central element u...
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File ManagementFile Management
Chapter #12Chapter #12
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파일 관리파일 관리
In most applications, the file is the central element Input to applications is by means of a file Output is saved in a file for long-term storage
파일시스템 (File management system) A part of the operating system consists of system utility programs that run as privileged
applications
2Operating System
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파일시스템 기능 파일시스템 기능 (1)(1)
파일시스템은 데이터 집합인 ‘ 파일’ 이라는 추상화 객체를 생성하여 관리하는 기능을 제공
‘ 파일’ 객체의 특성 장기 보존 (Long-term existence) 프로세스들 간의 공유 (Sharable between processes) 구조 (Structure)
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파일시스템 기능 파일시스템 기능 (2)(2)
파일시스템은 일반적으로 ‘ 파일’ 객체에 대해 다음의 접근 연산을 지원한다 생성 (Create) 삭제 (Delete) 열기 (Open) 닫기 (Close) 읽기 (Read) 쓰기 (Write)
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파일시스템 기능 파일시스템 기능 (3)(3)
파일시스템은 ‘ 파일’ 객체에 대한 일련의 속성을 유지 , 관리한다 소유자 생성시간 최근 변경 시간 접근 권한 등
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Terms Used with Files (1)Terms Used with Files (1)
Field Basic element of data Contains a single value Characterized by its length and data type
Record Collection of related fields Treated as a unit
Example: employee record
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Terms Used with Files (2)Terms Used with Files (2)
File Collection of similar records Treated as a single entity by users Have unique file names Access control applies at this level
In some systems, such controls are enforced at the record level
Database Collection of related data Consists of one or more types of files Relationships exist among elements
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Typical Operations onTypical Operations on FilesFiles
파일 사용과 관련하여 지원되는 전형적인 연산 Retrieve_All Retrieve_One Retrieve_Next Retrieve_Previous Insert_One Delete_One Update_One Retrieve_Few
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File Management SystemsFile Management Systems
사용자와 응용프로그램에게 파일을 사용하기 위해 서비스를 제공하는 일련의 시스템 프로그램 집합
응용 프로그램 사용자가 파일을 접근할 수 있는 유일한 방법
운영체제에서 데이터 저장 관리 기능을 제공함으로써 응용 프로그래머는 별도의 관리 소프트웨어를 개발할 필요가 없음
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Objectives for aObjectives for aFile Management SystemFile Management System
사용자의 데이터 관리 요구와 요건을 만족시키는 것 파일 데이터의 유효성을 최대한 보장해 주는 것 성능 최대화
시스템 관점에서의 작업 처리량 , 사용자 관점에서의 응답 시간
다양한 유형의 저장 장치들에 대한 입출력 지원 데이터의 손실 및 파괴 가능성을 최소화하거나 제거 사용자 프로세스에게 일련의 표준화된 입출력
인터페이스를 제공 다수의 사용자에 대한 입출력 지원
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Minimal Set of Requirements (1)Minimal Set of Requirements (1)
1. 각 사용자는 파일들을 생성 , 삭제하고 , 읽고 쓰고 수정할 수 있어야 한다
2. 각 사용자는 다른 사용자들의 파일을 접근할 대에 제어를 받아야 한다
3. 각 사용자는 자신의 파일들에 대해 어떤 유형의 접근을 허용할지 제어할 수 있어야 한다
4. 각 사용자는 발생한 문제점에 대해 적절한 형태로 자신의 파일들을 재구성할 수 있어야 한다
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Minimal Set of Requirements (2)Minimal Set of Requirements (2)
5. 각 사용자는 파일들 간에 데이터를 이동시킬 수 있어야 한다
6. 각 사용자는 자신의 파일을 백업시킬 수 있어야 하고 , 그 파일이 손상을 입은 경우 회복시킬 수 있어야 한다
7. 각 사용자는 이름을 사용하여 자신의 파일들에 접근할 수 있어야 한다
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File System ArchitectureFile System Architecture
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Device DriversDevice Drivers
Lowest level Communicates directly with peripheral devices Responsible for starting I/O operations on a
device Processes the completion of an I/O request
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Basic File SystemBasic File System
Physical I/O Deals with exchanging blocks of data Concerned with the placement of blocks Concerned with buffering blocks in main memory
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Basic I/O SupervisorBasic I/O Supervisor
Responsible for file I/O initiation and termination Control structures are maintained Concerned with selection of the device on which
file I/O is to be performed Concerned with scheduling access to optimize
performance Part of the operating system
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Logical I/OLogical I/O
Enables users and applications to access records Provides general-purpose record I/O capability Maintains basic data about file
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Access MethodAccess Method
Reflect different file structures Provides different ways to access and process
data
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File Management FunctionsFile Management Functions
Identify and locate a selected file Use a directory to describe the location of all files
plus their attributes On a shared system describe user access control Blocking for access to files Allocate files to free blocks Manage free storage for available blocks
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File OrganizationFile Organization
파일 구성 (File Organization) 레코드에 접근하는 방식에 의해 결정되는 레코드의 논리적인
구조 파일의 물리적인 구조는 저장장치의 블록킹 전략과 파일 할당
전략에 의해 결정 종류 :
Pile Sequential file Indexed sequential file Indexed file Direct, or hashed, file
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Criteria for File OrganizationCriteria for File Organization
Short access time Needed when accessing a single record Not needed for batch mode
Ease of update File on CD-ROM will not be updated, so this is not a
concern Economy of storage
Should be minimum redundancy in the data Redundancy can be used to speed access such as an
index Simple maintenance Reliability
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Pipe (1)Pipe (1)
Data are collected in the order they arrive Purpose is to accumulate a mass of data and save
it Records may have different fields
Length of a record must be specified either implicitly or explicitly
No structure Record access is by exhaustive search
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Pile (2)Pile (2)
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Sequential File (1)Sequential File (1)
Fixed format used for records Records are the same length All fields the same (order and length) Field names and lengths are attributes of the file One field is the key filed
Uniquely identifies the record Records are stored in key sequence
Typically used in batch applications Involve the processing of all the records
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Sequential File (2)Sequential File (2)
Poor performance for interactive applications Queries and/or updates of individual records
Additions to the sequential file New records are placed in a log file or transaction file Batch update is performed to merge the log file with the
master file An alternative is to organize the sequential file as
a linked list
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Sequential File (3)Sequential File (3)
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Indexed Sequential File (1)Indexed Sequential File (1)
An approach to overcome the disadvantages of the sequential file Records are organized in sequence based on a key field An index to the file to support random access overflow table - similar to the log file
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Indexed Sequential File (2)Indexed Sequential File (2)
Index provides a lookup capability to quickly reach the vicinity of the desired record Contains key field and a pointer to the main file Indexed is searched to find highest key value that is
equal to or precedes the desired key value Search continues in the main file at the location indicated
by the pointer
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Indexed Sequential File (3)Indexed Sequential File (3)
Comparison of sequential and indexed sequential Example: a file contains 1 million records On average 500,00 accesses are required to find a record
in a sequential file If an index contains 1000 entries, it will take on average
500 accesses to find the key, followed by 500 accesses in the main file. Now on average it is 1000 accesses
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Indexed Sequential File (4)Indexed Sequential File (4)
New records are added to an overflow file Record in main file that precedes it is updated to
contain a pointer to the new record The overflow is merged with the main file during a
batch update Multiple indexes for the same key field can be set
up to increase efficiency
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Indexed Sequential File (5)Indexed Sequential File (5)
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Indexed File (1)Indexed File (1)
Limitation of indexed sequential file Effective processing is limited to that which is based on
a single field of the file Uses multiple indexes for different key fields May contain an exhaustive index that contains one
entry for every record in the main file May contain a partial index Used in applications where timeliness of
information is critical airline reservation systems
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Indexed File (2)Indexed File (2)
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Direct(or Hashed) File (1)Direct(or Hashed) File (1)
Directly access a block at a known address Key field required for each record Hashing on the key value to get the location of the
record Used where rapid access is required or fixed
length records are used
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Direct(or Hashed) File (2)Direct(or Hashed) File (2)
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File DirectoriesFile Directories
Contains information about files Attributes Location Ownership
Directory itself is a file owned by the operating system
Provides mapping between file names and the files themselves
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Simple Structure for a DirectorySimple Structure for a Directory
List of entries, one for each file Sequential file with the name of the file serving as
the key Provides no help in organizing the files Forces user to be careful not to use the same
name for two different files The problem is much worse in shared system
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Two-level Scheme for a DirectoryTwo-level Scheme for a Directory
One directory for each user and a master directory
Master directory contains entry for each user Provides address and access control
information Each user directory is a simple list of files
for that user Still provides no help in structuring
collections of files
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Hierarchical, or Tree-Structured Hierarchical, or Tree-Structured Directory (1)Directory (1)
Master directory with user directories underneath it
Each user directory may have subdirectories and files as entries
Files can be located by following a path from the root, or master, directory down various branches This is the pathname for the file
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Hierarchical, or Tree-Structured Hierarchical, or Tree-Structured Directory (2)Directory (2)
Files can be located by following a path from the root, or master, directory down various branches This is the pathname for the file
Can have several files with the same file name as long as they have unique path names
Current directory is the working directory Files can be referenced relative to the working
directory
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File SharingFile Sharing
In multiuser system, allow files to be shared among users
Two issues Access rights Management of simultaneous access
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Access Rights (1)Access Rights (1)
None User may not know of the existence of the file User is not allowed to read the user directory that
includes the file Knowledge
User can only determine that the file exists and who its owner is
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Access Rights (2)Access Rights (2)
Execution The user can load and execute a program but cannot
copy it Reading
The user can read the file for any purpose, including copying and execution
Appending The user can add data to the file but cannot modify or
delete any of the file’s contents useful in collecting data from different users
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Access Rights (3)Access Rights (3)
Updating The user can modify, deleted, and add to the file’s data.
This includes creating the file, rewriting it, and removing all or part of the data
Changing protection User can change access rights granted to other users
Deletion User can delete the file
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Access Rights (4)Access Rights (4)
Owners Has all rights previously listed May grant rights to others using the following classes of
users Specific user User groups All for public files
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Simultaneous AccessSimultaneous Access
User may lock entire file when it is to be updated User may lock the individual records during the
update Mutual exclusion and deadlock are issues for
shared access
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Record BlockingRecord Blocking
Records and Blocks Records are the logical unit of access of a file Blocks are the unit of I/O with secondary storage
Issues to consider Fixed or variable block Size of a block
If a file is processed sequentially, larger blocks can reduce number of I/O operations
If records are accessed randomly, larger blocks result in the unnecessary transfer of unused records
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Fixed Blocking (1)Fixed Blocking (1)
Fixed-length records are used Integral number of records are stored in a block There may be unused space at the end of each
block Internal fragmentation
Commonly used for sequential files
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Fixed Blocking (2)Fixed Blocking (2)
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Variable Blocking : Spanned (1)Variable Blocking : Spanned (1)
Variable-length records are used Records are packed into blocks with no unused
space Some records may span two blocks It is indicated by a pointer to the successor block
Efficient use of storage and no limit on the size of records
But difficult to implement
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Variable Blocking: Spanned (2)Variable Blocking: Spanned (2)
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Variable Blocking : Unspanned (1)Variable Blocking : Unspanned (1)
Variable-length records are used Spanning is not employed
There is a wasted space in most blocks Results in wasted space and limits record size
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Variable Blocking Unspanned (2)Variable Blocking Unspanned (2)
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Secondary Storage ManagementSecondary Storage Management
A file consists of a collection of blocks Management issues
File allocation space on secondary storage must be allocated to files
Free space management Must keep track of the space available for allocation
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File AllocationFile Allocation
Issues to consider Preallocation VS dynamic allocation Unit of allocation File allocation table(FAT) A data structure that is used to keep track of the space
assigned to a file
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PreallocationPreallocation
Need the maximum size for the file at the time of creation
Difficult to reliably estimate the maximum potential size of the file Tend to overestimated file size so as not to run out of
space So there are advantages to the use of dynamic
allocation
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Portion Size (1)Portion Size (1)
Tradeoff between user’s view efficiency vs overall system efficiency Contiguity of space increases performance Large number of small portions increases the size of
management tables Fixed-size simplifies the reallocation of space Variable-size minimizes waste of unused storage
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Portion Size (2)Portion Size (2)
Two major alternatives Variable, large contiguous portions
avoids wasted space file allocation tables are small space is hard to reuse
Blocks provides greater flexibility requires large allocation tables contiguity is abandoned
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Methods of File Allocation (1)Methods of File Allocation (1)
File allocation methods Contiguous allocation Chained allocation Indexed allocation
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Methods of File Allocation (2)Methods of File Allocation (2)
Contiguous allocation A single contiguous set of blocks is allocated to a file at
the time of creation Preallocation using variable-size portions
Only a single entry in the file allocation table Starting block and length of the file
External fragmentation will occur Difficult to find contiguous blocks of sufficient length Compaction is needed from time to time
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Methods of File Allocation (3)Methods of File Allocation (3)
Chained allocation Allocation on basis of individual block Each block contains a pointer to the next block in the
chain Only single entry in the file allocation table
Starting block and length of file No external fragmentation Best for sequential files No accommodation of the principle of locality
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71Operating System
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Methods of File Allocation (4)Methods of File Allocation (4)
Indexed allocation : Unix file system File allocation table contains a separate one-level index
for each file The file allocation table contains block number for the
index The index has one entry for each portion allocated to the
file Allocation may be either fixed-size or variable-size
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Methods of File Allocation (5)Methods of File Allocation (5)
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Free Space ManagementFree Space Management
The space that is not currently allocated to any file must be managed
Disk allocation table Manages what blocks on the disk are free
Methods for free space management Bit tables Chained free portions Indexing Free block list
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Bit Tables Bit Tables
A vector containing one bit for each block on the disk is used Entry of 0 corresponds to a free block An example
00111000011111000011111111111011000 Easy to find free blocks It is as small as possible
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Chained Free PortionsChained Free Portions
Free portions are chained by using a pointer No need for a disk allocation table
Every time a block is allocated, pointer needs to be adjusted If many individual blocks need to be allocated at one
time, this greatly slows down the process
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IndexingIndexing
Index table is used One entry in the table for every free portion on the disk
Provides efficient support for all of the file allocation methods
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Free Block ListFree Block List
Each block is assigned a number Numbers of all free blocks are maintained
Assuming 32 bits for a block number, size ofthe free block list is 32 times the size of the bit table
Only a small part of the list may reside in main memory Stack or FIFO queue can be used for this purpose
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ReliabilityReliability
Consistency problem of disk allocation and file allocation table between main memory and disk Due to the fact that the system maintained a copy of the
disk allocation table and file allocation table in main memory for efficiency
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UNIX File ManagementUNIX File Management
Files are streams of bytes Types of files
Ordinary - contents entered by user or program Directory - contains list of file names and pointers to
inodes (index nodes) Special - used to access peripheral devices Named - named pipes
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UNIX File SystemUNIX File System
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Inodes(Index Nodes)Inodes(Index Nodes)
Control structure that contains key information for a particular file File owner, group owner identifier File type File access permission Access, modified time Number of links to the file File size Table of contents for the disk addresses of data
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Inodes(Index Nodes)Inodes(Index Nodes)
Control structure that contains key information for a particular file File owner, group owner identifier File type File access permission Access, modified time Number of links to the file File size Table of contents for the disk addresses of data
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86Operating System
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UNIX File System in More DetailUNIX File System in More Detail
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Linux Virtual File SystemLinux Virtual File System
Uniform file system interface to user processes Represents any conceivable file system’s general
feature and behavior Assumes files are objects that share basic
properties regardless of the target file system
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Primary Objects in VFSPrimary Objects in VFS
Superblock object Represents a specific mounted file system
Inode object Represents a specific file
Dentry object Represents a specific directory entry
File object Represents an open file associated with a process
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Windows File SystemWindows File System
Key features of NTFS Recoverability Security Large disks and large files Multiple data streams General indexing facility
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NTFS Volume and File Structure (1)NTFS Volume and File Structure (1)
Sector The smallest physical storage unit on the disk
Cluster One or more contiguous sectors
Volume Logical partition on a disk
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NTFS Volume and File Structure (2)NTFS Volume and File Structure (2)
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NTFS Volume and File Structure (3)NTFS Volume and File Structure (3)
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98Operating System