CALectureWeek9.ppt

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CARC103 – Computer Architecture
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Prescribed Text
Bird, S. D. (2017), Systems Architecture, 7th ed, Cengage Learning
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Systems Architecture,

Seventh Edition
Chapter 12
Secondary Storage Management
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Systems Architecture, Seventh Edition

Chapter Objectives
In this chapter, you will learn to:
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

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Systems Architecture, Seventh Edition

Chapter Objectives (continued)
Describe file manipulation operations
List access controls that can be applied to files and directories
Describe file migration, backup, and recovery methods
Explain methods for ensuring fault tolerance
Compare storage consolidation methods, such as storage area networks, network-attached storage, and cloud-based storage services

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Systems Architecture, Seventh Edition

FIGURE 12.1 Topics covered in this chapter
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

File Management Overview
Users and programs don’t interact directly with secondary storage blocks and devices
They interact with files – groups of storage blocks that hold programs and data
File management functions such as creation, copying, reading, and writing are implemented within the service layer (a.k.a. file control layer)
Application programs interact directly with file management service layer routines
Users interact with the command layer which, in turn, uses service layer routines to perform file management tasks

FIGURE 12.2 File management system layers
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Logical and Physical Storage Views
Secondary storage devices hold bits, but access those bits in larger units generically called blocks (for example, one sector of a magnetic hard disk)
Blocks are to small for users and programs to directly interact with
Users and programs need larger and more useful logical abstractions, including:
File – a group of blocks that holds one related set of data or instructions
Folder (directory) – a container for a related set of files
Volume – a container for a hierarchically organized set of folders and files
For example, C: on Windows

FIGURE 12.3 Logical and physical secondary storage views for a typical laptop computer
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Systems Architecture, Seventh Edition

File Content and Type
Most FMSs directly support only a few file types:
Executable programs
Operating system commands
Textual or unformatted binary data
Other file types are indirectly supported by file association: a defined relationship between file types and the programs or OS utilities that manipulate them

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Systems Architecture, Seventh Edition

Volumes, Files, and Folders in Windows Explorer
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Systems Architecture, Seventh Edition

Volumes
A single disk can be divided into multiple volumes or a volume can span multiple disks
A volume is an independent FMS management unit for purposes of:
Driver letter (Windows) or mount point (UNIX/Linux) assignment
Error checking, backup/recovery, defragmentation, and recycling
Root directory access controls
Quotas, versioning, journaling, and other optional FMS features

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Systems Architecture, Seventh Edition

File Content and Organization
Files can contain many different kinds of data:
Machine or OS instructions (for example, a Windows .exe file or Linux shell script)
Textual data (for example, as created by Notepad)
Image data (for example, as capture by a camera)
Formatted documents (for example, a Web page)
Variations in file content lead to variations in internal file organization, for example:
Files containing machine language programs are organized to simplify loading the instructions into memory
Files containing web pages are organized to simplify transmission across a network and Web browser display
Image files are organized to simplify display and editing

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Systems Architecture, Seventh Edition

File Type and Association
Operating systems generally support two file-related:
File type – A limited set of file types that the OS keeps track of and uses to enable or restrict certain functions, for example:
Types – executable instructions, OS commands, other
Enable/restrict:
Executable files can be loaded into memory and executed by the CPU but can’t be printed or read/written by programs
OS command files (scripts) can be executed by an OS utility and can be printed and read/written by programs
Other files can’t be executed but can be printed and read/written by programs
File type association – Specific file types are associated with specific programs that perform specific actions on those files, for example:
Ordinary text file – Notepad used to print/edit
JPEG image file – Adobe Illustrator used to print/edit
HTML file – Edited by Word and displayed/printed by Internet Explorer
Windows provides an extensible set of file types
Filename extension determines file type, for example
Document.txt is a text file associated with Notepad
Document.doc is a document file associated with Microsoft Word
Double-clicking a file opens it within the program associated with that file’s type

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Systems Architecture, Seventh Edition

File Type and Association – Exercise
Perform the following tasks on computer running Windows:
From the desktop, click Start and select Control Panel
Click Programs
Click Default Programs
Click Associate a file type or protocol with a specific program
View the list of file types and their default program associations
Select .txt and click Change Program
View the list of programs that could be assigned to .txt files (click Cancel when you’re finished)
Close all of the open control panel windows
Open a folder containing some of your own files, preferably of several different types – note the displayed file names
Click Organize in the upper left menu bar
Click Folder and search options
Click the View tab
Uncheck the box labeled Hide extensions for know file types
Click OK
Reexamine the displayed file names and note that all files with the same extension have the same icon displayed to the left of the filename.

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Systems Architecture, Seventh Edition

FIGURE 12.4 Registered Windows file types and associated programs
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

FIGURE 12.5 Context menus and commands for a Windows file type
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Folder Content and Structure
Contain information about files and other folders, typically:
Name
File type
Location
Size
Ownership
Access controls
Time stamps

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Systems Architecture, Seventh Edition

FIGURE 12.7 Windows folder listing with additional details
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Hierarchical Folder Structure
In most OSs, folders can contain other folders, but no folder can be contained within more than one folder
Result is a hierarchical (or tree) folder structure as shown in the left frame of the next slide
Terminology (refer to next slide for examples)
Current (working) directory – folder currently displayed or the folder location within which a program looks for files
For example, Chapter08 in the figure
Home folder – default current folder for a user or program
For example, C:UsersBurdMy Documents for the user Burd under Windows
Complete path or fully qualified reference – filename plus all folder names upward through the volume root
For example, T:Systems Architecture6eChaptersChapter08Solutions_08_Au.doc
Relative path – file name plus all folder names upward to the current (working) folder
For example, if the current folder is 6e then .ChaptersChapter08Solutions_08_Au.doc is a relative path to Solutions_08_Au.doc where . is a shorthand for the current folder

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Systems Architecture, Seventh Edition

Current (working) directory
FIGURE 12.8 A hierarchical directory structure
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Graph Folder Structure
Graph folder structures allow a folder to be contained within multiple other folders
That introduces the possibilities of loops, which complicate operations such as error checking and enumerating volume contents
Many OSs take an “in-between” approach based on links or shortcuts (see dotted line in figure)

FIGURE 12.9 A graph folder structure
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Storage Allocation Overview
The user doesn’t “see” hard disk drives, SSDs, and other secondary storage devices
The user does see volumes, folders, and files managed by the operating system
Storage allocation is the processes of managing or linking the relationship(s) between:
Storage objects the user sees (a logical view), and
Storage devices/locations in hardware (a physical view)

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Systems Architecture, Seventh Edition

Allocation Units
Recall from chapter 5 that most disk drives (and devices that simulate them) exchange data move data back and forth to memory in 512 or 4096 byte chunks called sectors
An operating system manages chunks of secondary storage called allocation units, which may be the same size as sectors or a multiple thereof
Allocation units are assigned (or allocated) to logical storage objects such as volumes, directories, and files
Allocation unit size is determined when a volume is initialized – size determines:
How efficiently storage space is allocated and managed
The size of data structures (tables) that track allocation units

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Systems Architecture, Seventh Edition

Allocation Units (continued)
Allocation unit size trade-offs
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 for smaller storage allocation data structures

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Systems Architecture, Seventh Edition

Storage Allocation Tables
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Systems Architecture, Seventh Edition

Storage Allocation Tables – Continued
Small allocation unit size (and large storage allocation tables) wastes less storage space when there are many small files (smaller than the allocation unit size)
For example, storing a 1 byte file wastes:
511 bytes if allocation unit size is 512 bytes
4095 bytes if allocation unit size is 4096 bytes
Larger allocation unit size (and smaller storage allocation tables) speeds tasks that need to search or update the table
For example, calculating total free space or copying a large file
The proper trade-off among the two factors depends on many factors including average file size, volume size, disk I/O speed, and available RAM for buffering and caching
Allocation unit size can be chosen by the system administrator or automatically determined by the OS when a volume is created

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Systems Architecture, Seventh Edition

FIGURE 12.8 Storage blocks allocated to three files
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Free allocation units are assigned to a hidden system file called SysFree.
TABLE 12.1 Directory content for the files in Figure 12.8
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Systems Architecture, Seventh Edition

FIGURE 12.9 A storage allocation table matching the storage allocations in Figure 12.8
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Blocking
A logical record is the unit of data read from or written to a file by an application program (e.g., all of the data about one customer)
A physical record is the unit of data read from or written to a storage device in a single operation
Comparing the relative size of logical and physical records, there are three possible relationships:
Logical record size = physical record size – unblocked
Logical record size < physical record size Logical record size > physical record size

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Systems Architecture, Seventh Edition

Blocking – Continued
If logical and physical records are of different sizes then the term blocking factor describes the number of logical records per physical record
Burd, Systems Architecture, 7th edition, Figure 12.12 Copyright © 2016 Cengage
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Systems Architecture, Seventh Edition

Blocking and Buffering
The FMS or OS uses buffers to support data transfer between secondary storage and an application program
Each buffer is the same size as a physical record – 512 or 4096 bytes for most disk drives
Multiple buffers can be used to improve performance
A physical read operation moves one physical record from a secondary storage device to a buffer
A logical read operation moves one logical record from one or more buffers to the application program

FIGURE 12.11 Input from secondary storage to an application program using a buffer
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

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

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Systems Architecture, Seventh Edition

File Open and Close Operations
The OS service layer contains utility functions to open and close files:
File open operation

Locate the file in the directory structure and read its directory entry
Search an internal table of open files to see whether the file is already open
Ensure that the process has enough privileges to access the file
Allocate one or more buffers
Update an internal table of open files
File close operation

Flush the program’s file I/O buffers to secondary storage
Deallocate buffer memory
Update the file’s directory entry time stamps
Update the open file table
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Systems Architecture, Seventh Edition

File Delete and Undelete Operations
There are multiple ways for an OS/FMS to perform a file delete operation – the most common are:
Move the file’s directory entry to a directory of deleted files (e.g., a recycle bin)
A file undelete operation simply moved the directory information back to its original location
Mark the file as deleted in its directory entry and mark its allocation units as free in the storage allocation table
File undelete is possible if neither the directory entry nor the storage allocation table have been overwritten with new data
Overwrite the data content of the file’s allocation units with zeros, mark the allocation units as free in the storage allocation table, and overwrite the file’s directory entry with zeros
File undelete isn’t possible via the OS/FMS, though some advanced forensic techniques can examine individual disk bits and determine earlier values of those bits

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Systems Architecture, Seventh Edition

Access controls
Each file or folder includes an access control list (ACL) that specifies users or groups and what they’re allowed to do (sometimes described by the word privileges)
The ACL is checked against user ID when a file is opened and manipulated
FIGURE 12.14 Windows access control list content
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

File Migration (Version Control)
Versioning – As file contents are updated creating new versions, older versions are archived
Older versions “migrate” from:
Fast local storage
Slower local storage
Remote storage such as cloud-based backups
Older versions can still be “seen” in folder listings and are copies back if needed
Balances storage cost of each file version with anticipated user demand for that version

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Systems Architecture, Seventh Edition

File Backup
Protects against data loss (file content, folder content, and storage allocation tables)
Storing backup copies:
Different storage device within local computer
Removable storage device attached to local computer
A local network-attached backup device or server
A remote network-attached backup server

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Systems Architecture, Seventh Edition

Archive Attributes and Timestamps
Each file/directory entry includes data used by backup utilities to determine if or when a file was backed up.
Windows uses a Boolean archive attribute which is set if file has been modified since last backup.

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Systems Architecture, Seventh Edition

Backup Types
Full backup
Copies all selected files and directories
Clears archive attributes
Incremental backup
Copies only files and directories created or changed since the last incremental backup (files with the archive attribute set)
Clears archive attributes
Differential backup
Copies only files and directories created or changed since the last normal or incremental backup (files with the archive attribute set)
Doesn’t clear archive attributes

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Systems Architecture, Seventh Edition

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

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Systems Architecture, Seventh Edition

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
All storage locations appear in the storage allocation table and other data structures.
All files have correct folder entries.
All storage locations of a file can be accessed through the storage allocation table.
All storage locations can be read and/or written.

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Systems Architecture, Seventh Edition

Fault Tolerance
What is fault tolerance?
The term fault tolerance describes software, hardware, and operating procedure characteristics that ensure:
Minimal resource/service unavailability due to faults
Minimal data loss due to faults
Common fault causes:
Power interruption
Hardware failure (e.g., disk or network crash)
Operating system failure (BSOD – blue screen of death)
Service failure (e.g., web service crash, denial of service attack)

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Systems Architecture, Seventh Edition

Fault Prevention and Mitigation
Most faults can’t be completely prevented, but their probability of occurrence can be reduced and their negative consequences can be minimized
Common fault prevention strategies:
Adequate security
Power conditioning
Reliability testing and configuration
General fault mitigation strategies
Hardware redundancy (e.g., RAID, backup servers)
Service redundancy (e.g., load balancing)
Resource redundancy (e.g., file replication and periodic backup to redundant and/or remote storage devices)

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Systems Architecture, Seventh Edition

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

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Systems Architecture, Seventh Edition

RAID Overview
RAID stands for redundant array of inexpensive disks
RAID (most levels) achieves fault tolerance through various types and levels of redundant storage
Higher redundancy usually implies greater fault tolerance
RAID (most levels) achieves performance improvements through parallelism
There are multiple kinds of RAID (a.k.a. RAID levels)
Each level is a different combination or parallelism, redundancy amount, and redundancy type
“Standard” RAID levels include 0-5
RAID 2, 3, and 4 are antiques
RAID 10 is widely used but there is no “standard”

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Systems Architecture, Seventh Edition

RAID 0 (Striped Volumes)
Striped Volume (RAID-0)
Volumes on multiple disks are combined into one volume
Data is systematically distributed across all disks
For example, a 1 MB file is spread equally across four disks in rotating 64 KB blocks resulting in 256 KB stored on each disk
There is some associated CPU overhead.
Pros – data access is faster IF all disks can be read/written in parallel (disk controller must support parallelism)
Cons – if any disk fails the entire volume is trashed
Alternatives:
One big VERY fast disk
RAID-5 or higher
Striping by itself is primarily used to increase read/write performance (e.g., a gaming desktop)
Higher RAID levels combine striping with redundancy

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Systems Architecture, Seventh Edition

FIGURE 12.16 Data striping across four disks
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

RAID 1 (Mirrored Volumes)
Mirrored Volumes (RAID-1)
Every volume has a duplicate on a different disk
All writes go to both disks in parallel
Reads come from either disk
Pros:
High fault tolerance
Cons:
Double cost or half capacity
Write performance is always slower but not by much if using a smart disk controller
As disks have become cheaper, mirroring (alone or in combination with redundancy) has become much more common

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Systems Architecture, Seventh Edition

RAID-5
RAID-5 Volume
Volume spans multiple disks (at least three)
Data content is striped
Parity information is added within file in such a way that parity data alternates across disks per file/directory
If any drive fails, data can be recovered from the remaining drives
Lost parity information isn’t a problem (it’s redundant)
Lost data can be recomputed from remaining blocks
For example, assume 9 disks, 1 byte, even parity
1 1 0 0 1 1 0 0 + parity bit (0)
A “lost” bit is zero if there are an even number of remaining 1 bits, one otherwise

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Systems Architecture, Seventh Edition

RAID-5 ― Continued
Pros:
High fault tolerance – access continues in the event of any single disk failure
High read performance – same as striped volume (parity data is ignored when reading)
Cons:
Slower write performance than striped volume due to computation and storage of parity information
Storage capacity is less than striped – one disk worth of data is redundant parity information

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Systems Architecture, Seventh Edition

RAID-5 Failure Recovery
If one disk dies the system continues to function but without protection against additional failures
To restore failure protection:
Install new disk
Format new disk
Recreate lost data on new disk by doing “parity math”
Recovery operation is slow because it’s both
CPU intensive
I/O intensive
System throughput for “real” work typically declines – by how much?

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Systems Architecture, Seventh Edition

FIGURE 12.18 16 KB stored in a four-disk RAID 10 array
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Storage Consolidation
Until at least the 1990s:
File systems were always integral parts of an operating system
Each operating system controlled one set of hardware, including secondary storage devices
The term direct-attached storage (DAS) describes any architecture in which software “talks” to secondary storage devices that are directly connected to the computer system that executes the software

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Systems Architecture, Seventh Edition

Direct-Attached Storage Limitations
DAS made a lot of sense in the mainframe era
Most organizations owned one mainframe
All software ran on the mainframe
All data was stored on directly-attached disks
By the late 1980s it started to be a limitation:
Organizations use multiple computer systems
Could a file or database stored on disks attached to one computer system be accessed by software running on another computer system?
Could two computer systems share a pool of disks or a common file system or database stored on that pool of disks?
After multiple evolutionary steps – three main approaches were developed to “answer” the questions above:
Storage-area network
Network-attached storage
Cloud-based storage

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Systems Architecture, Seventh Edition

Storage Area Network
A storage area network (SAN) consists of:
A “back-end” network connecting a family of servers
One server is a storage server and accepts I/O requests from the other servers via the network
A pool of secondary storage devices attached to a storage server and managed as one large set of allocation units (disk sectors)

FIGURE 12.19 A server cluster with a storage area network
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Network-Attached Storage
Network-attached storage (NAS) uses:
A pool of secondary storage devices attached to a storage
A storage server manages the storage pool as one or more file systems and responds to read/write requests from other servers
An ordinary WAN or LAN connects all servers

FIGURE 12.20 Network-attached storage
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

SAN and NAS Compared
Access types
SAN: Client servers perform direct disk I/O at the sector level via the storage server
NAS: Client servers transfer service layer file I/O requests to the storage server for processing
Performance
SAN: Supports high-performance disk I/O for a small number of servers in close geographic proximity
NAS: Supports lower-performance file I/O but can potentially support a larger number of servers over a wide geographic area
Cost
Relatively high – High-capacity back-end switching network
Relatively low – storage server is an “appliance” and existing network connections are used

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Systems Architecture, Seventh Edition

Cloud-Based Storage
DAS, SAN, and NAS are holdovers from an era when files were assumed to be stored and accessed from a single device
Data stored on a server and users accessed via an application
Data stored on a personal computing device
In the modern world, users have access to many computing devices and they want to access their files from any of them
Examples of modern tools that enable user-centric storage:
GoogleDrive
DropBox
Microsoft OneDrive

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Systems Architecture, Seventh Edition

Cloud-Based Storage – Continued
Common features of cloud-based storage services:
Each user has a unique account and user ID
A user can associated their cloud storage user ID with one or more computing devices (e.g., their smartphone, tablet, and laptop)
User files stored on any associated device are automatically synchronized to and from the cloud and then to and from all other associated devices
A Web-based application provides a way for users to access files from devices that haven’t been associated with their user ID
The underlying technology is similar to caching
A server or server group is the main storage device
A volume or folder points to cloud-based storage
As files area access they’re cached on local storage of each computing device from which the user accesses files
Changes to cached copies are replicated back to the server(s)
When a user access a cached file copy on any device, it’s compared to the server copy and a refresh is performed if needed

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Systems Architecture, Seventh Edition

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

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