chapter 12 file management systems. introduction to computer science 2 chapter goals describe the...

Chapter 12

File Management Systems

Introduction to Computer Science 2

Chapter Goals

Describe the components and functions of a file management system

Compare the logical and physical organization of files and directories

Explain how secondary storage locations are allocated to files and describe the data structures used to record those allocations


Chapter Goals (continued)

Describe file manipulation operations, including open, close, read, delete, and undelete operations

List access controls that can be applied to files and directories

Describe security, backup, recovery, and fault tolerance methods and procedures

Compare and contrast storage area networks and network-attached storage


File Management Systems

Collection of system software that manages all aspects of user and program access to secondary storage

Usually part of the operating system Translates operations into commands to

physical storage devices Implemented in four layers (command layer,

file control, storage I/O control, and secondary storage devices)


Bridges between logical and physical views of secondary storage

Allocates secondary storage locations to individual files and directories

Includes software modules for device drivers for each storage device or device controller, interrupt handlers, buffers and cache managers


Logical and PhysicalStorage Views Logical view

Collection of files organized within directories and storage volumes

Physical view Collection of physical storage locations organized

as a linear address space


The file is subdivided into multiple records and each record is composed of multiple fields.


File Content and Type

FMS supports limited number of file types: Executable programs Operating system commands Textual or unformatted binary data

Modern FMSs can define new file types and install utility programs to manipulate them (file association)


File Types

Normally declared when a file is created and: Stored within a directory, or Declared through a filename convention

Determine: Physical organization of data items and data

structures within secondary storage Operations that may be performed upon the file Filename restrictions


Directory Content and Structure

Contain information about files and other directories, typically name, file type, location, size, ownership, access controls, and time stamps


Hierarchical Directory Structure

Directories can contain other directories, creating a tree structure, but cannot be contained within more than one parent

Ways that names of access paths can be specified: Complete path (fully qualified reference) Relative path

Each storage device has a root directory


Active (working) directory


Graph Directory Structure

More flexible than hierarchical directory structure Files and subdirectories can be contained within

multiple directories Directory links can form a cycle


Storage Allocation

Secondary storage devices Large number of storage locations; low frequency

of allocation changes Divided into allocation units


Allocation Units

Smallest number of secondary storage bytes that can be allocated to a file; cannot be smaller than unit of data transfer between storage device and controller (block)

Assigned/reclaimed by FMS as files and directories are created or expanded/shrink or are deleted

Size difficult to change once set


Allocation Unit Size

Tradeoffs Efficient use of secondary storage space for files Size of storage allocation data structures Efficiency of storage allocation procedures

Smaller units: More efficient use of storage space

Larger units: Allow smaller storage allocation data structures


Storage Allocation Tables

Data structures that record which allocation units are free and which belong to files

Format and content vary across FMSs Can contain linked lists in simpler FMSs or

indices or other complex data structures in more complex FMSs


Free allocation units are assigned to a hidden system file called SysFree.


All of a file allocation’s units are “chained” together in sequential order by a series of pointers.


Blocking

Logical record grouping within physical records

Described by a numeric ratio of logical records to physical records (blocking factor)


Blocking factor = 4:3

Blocking factor = 2:3


Buffering

Temporary storage of data as it moves between programs and secondary storage devices Physical records are stored in the buffer as they

are read from secondary storage FMS extracts logical records from buffers and

copies them to data area of the application program

Each buffer is the size of one allocation unit Improves I/O performance if enough are used


File Manipulation

Exact set of service layer functions varies among FMSs, but typically includes create, copy, move, delete, read, and write

Application programs interact directly with FMS through OS service layer

Users interact indirectly with FMS through command layer


File Open and Close Operations

File open Causes FMS to find the file, verify access

privileges, allocate buffers, and update internal table of open files

File close Causes FMS to flush buffer content to the storage

device, release buffers, update file time stamps, and update table of open files


Delete and Undelete Operations Delete

Does not immediately remove files; some content remains on secondary storage unit all allocation units have been reassigned and overwritten

File content can be visible to intruders Undelete

Can be used to reconstruct directory and storage allocation table contents


Access Controls

Granted by file owners and system administrators for reading, writing, and executing files

Provide security at the expense of additional FMS overhead


File Migration, Backup, and Recovery Provided by most FMSs to protect files

against damage or loss


File Migration(Version Control) Automatic storage and backup of old file

versions Balances storage cost of each file version

with anticipated user demand for that version


Original

Copy that has been updated to reflect new data


File Backup

Protects against data loss (file content, directory content, and storage allocation tables)

Store backup copies on a different storage device in a different physical location

Manual or automatic Full or incremental


Transaction Logging

Automatically records all changes to file content and attributes in a separate storage area; also writes them to the file’s I/O buffer

Provides high degree of protection against data loss due to program or hardware failure

Imposes a performance penalty; used only when costs of data loss are high


File Recovery

Automated and manual components Can search backup logs for copies of lost or

damaged files Can perform consistency checking and repair

procedures for crashed system or physically damaged storage device


Fault Tolerance

Methods of securing file content against hardware failure File backup Recovery Transaction logging Mirroring RAID (Redundant Array of Inexpensive Disks)


Mirroring

All disk write operations are made concurrently to two different storage devices

Provides high degree of protection against data loss with no performance penalty if implemented in hardware

Disadvantages Cost of redundant disk drives Higher cost of disk controllers that implement

mirroring


RAID

Disk storage technique that improves performance and fault tolerance

All levels except RAID 1 use data striping Breaks a unit of data into smaller segments and

stores them on multiple disks Multiple levels can be layered to combine

their best features (e.g. RAID 10) Can be implemented in hardware or software


Data striping: Each segment is written in parallel to a separate disk.


If the parity disk fails, the other disks still retain their original data bits.


RAID 10: Mirrors individual disks (RAID 1), then stripes data (RAID 0) across multiple mirrored pairs.


Storage Consolidation

Overcomes inefficiencies of direct-attached storage (DAS) in multiple-server environments

Common approaches Storage area network (SAN) Network-attached storage (NAS)


Storage Consolidation

Storage Area Network (SAN)

Network-Attached Storage (NAS)

High-speed interconnection among general-purpose servers and one or more storage servers

Block-oriented access Common in multi-server

environments with mainframes or supercomputers and substantial overlap among server storage needs

Expensive to purchase and administer, but avoid costs of duplicate storage and storage administration

Dedicated to managing one or more file systems

Accessed by other servers and clients over a local or wide area network

File-oriented access Common when geographically

dispersed servers need access to a common file system

Cheaper to acquire than SAN, but at the price of lower performance


Summary

File management systems Directory content and structure Storage allocation File manipulation Access controls File migration, backup, and recovery Storage consolidation

chapter 12 file management systems. introduction to computer science 2 chapter goals describe the...

Documents

storage slide

computer science

file association slide

secondary storage operations

physical storage devices

secondary storage locations

file content

file control