section 2 – storage systems architecture

Section 2 – Storage Systems ArchitectureSection 2 – Storage Systems Architecture

Introduction

本章目标及内容本章主要介绍存储架构的各个组成部分。首先介绍主机与存储的连接关系，然后从磁盘的基本概念出发，介绍了磁盘阵列和磁盘的 RAID数据保护等概念。最后，本章介绍了磁盘存储系统的系统结构并深入说明了磁盘存储系统是如何高效的应用于系统环境的。

本章内容包括 5个方面：2.1 主机环境（ The Host Environment）2.2 连接（ Connectivity）2.3 物理磁盘（ Physical Disks）2.4 磁盘阵列（ Disk Arrays）2.5 磁盘存储系统（ Disk Storage Systems）

Section Objectives

Upon completion of this Section, you will be able to:

Describe the host environment.

Describe common connectivity components and protocols.

Describe features of intelligent disk subsystems.

Describe data flow from the host to/from the disk.

In this Section …

This section contains the following modules:

1. The Host Environment

2. Connectivity

3. Physical Disks

4. RAID Arrays

5. Storage Systems

Storage System Environment

Parts of a Storage Environment: Host

Parts of a Storage Environment: Connectivity

Parts of a Storage Environment: Storage

The Host Environment The Host Environment

Module 2.1

The Host Environment

Upon completion of this module, you will be able to:

List the hardware and software components of the host environment

Describe key protocols and concepts used by each component

Examples of Hosts

Laptop

Server

Group of Servers

Mainframe

Host Physical Components

I/O Devices

CPU Storage

BusALU

Registers

L1 Cache

Storage

Data 0

Data n

Data 2

Data 3

Data 1

Address Content

Memory

Storage Hierarchy – Speed and Cost

CostHighLow

TapeOptical

Magnetic disk

L2 cache L1 cache

CPU registers

I/O Devices

Human interface– Keyboard

– Mouse

– Monitor

Computer-computer interface– Network Interface Card (NIC)

Computer-peripheral interface– USB (Universal Serial Bus) port

– Host Bus Adapter (HBA)

Host Environment: Logical Components

Volume Management

DBMS Mgmt Utilities

File System

Multi-pathing Software

Device Drivers

HBA HBA HBA

Operating System

Volume Management

DBMS Mgmt Utilities

File System

Device Drivers

HBA HBA HBA

Operating System

File Systems

File System: Metadata Examples

UNIX (UFS)

File type and permissions

Number of links

Owner and group IDs

Number of bytes in the file

Last file access

Last file modification

Windows (NTFS)

Time stamp and link count

File name

Access rights

File data

Index information

Volume information

File Systems: Journaling and Logging

Improves data integrity and system restart time over non-journaling file systems.

Uses a separate area called a log or journal.– May hold all data to be written

– May hold only metadata

Disadvantage - slower than other file systems.– Each file system update requires at least 1 extra write – to the

Volume Management

DBMS Mgmt Utilities

File System

Device Drivers

HBA HBA HBA

Operating System

HBAsHost

Volume Management

DBMS Mgmt Utilities

File System

Device Drivers

HBA HBA HBA

Operating System

Improving Data Availability at the Host

Redundancy:

Multiple HBAs

Multi-pathing software

Clustering

How Files are Moved to and from Storage

Teacher

Configures / Manages

File System Files

Mapped by file system

Course File(s)

Reside in

File System Blocks

Disk Physical Extents

Consisting of

LVM Logical Extents

Residing in

Mapped by LVM to

Disk Sectors

Managed by Disk Storage Subsystem

Module Summary

Key points covered in this module:

Hosts typically have:– Hardware: CPU, memory, buses, disks, ports, and interfaces.

– Software: applications, operating systems, file systems, device drivers, volume managers

HBAs connect hosts to storage devices.

Multi-pathing software uses redundant paths to ensure uninterrupted communication between the host and the storage

Clustering uses redundant host systems to improve data availability

Check Your Knowledge

What are some examples of hosts?

Describe the hardware components found in most hosts.

What is the function of the operating system?

What is the function of the file system?

What are some techniques that can be used to improve availability at the host?

What is volume management?

ConnectivityConnectivity

Module 2.2

Connectivity

Upon completion of this module, you will be able to:

Describe the physical components of a networked storage environment.

Describe the logical components (communication protocols) of a networked storage environment.

Physical Components – Host with Internal Storage

Bus Technology

Serial

Serial Bi-directional

Parallel

Bus Technology

System Bus – connects CPU to Memory

Local (I/O) Bus – carries data to/from peripheral devices.

Bus width measured in bits

Bus speed measured in MHz

Throughput measured in MB/S

Connectivity Protocols

Protocol = a defined format for communication – allows the sending and receiving devices to agree on what is being communicated.

Tightly ConnectedEntities

DirectlyAttachedEntities

Network Connected

Entities

Communication Protocols

Operating System

SCSI or IDE/ATA Device Drivers

Bus Technology - PCI

Peripheral Component Interconnect (PCI) defines the local bus system within a computer

It is an interconnection between microprocessor and attached devices, in which expansion slots are spaced closely for high-speed operation.

Has Plug and Play functionality.

PCI is 32/64 bit

Throughput is 133 MB/sec

IDE/ATA

Integrated Device Electronics (IDE) / Advanced Technology Attachment (ATA)

Most popular interface used with modern hard disks

Good performance at low cost

Desktop and laptop systems

Inexpensive storage interconnect

SCSI - Small Computer System Interface

Most popular hard disk interface for servers.

Higher cost than IDE/ATA.

Supports multiple simultaneous data access.

Currently both parallel and serial forms.

Used primarily in “higher end” environments.

SCSI Model

Target

Initiator

SCSI Model

Target ID

Initiator ID

SCSI Addressing

Initiator ID - a number from 0 to 15 with the most common value being 7.

Target ID - a number from 0 to 15 LUN - a number that specifies a device addressable

through a target.

Initiator ID Target ID LUN

Disk Identifier - Addressing

c0 - ControllerInitiator, HBA

PeripheralController

Target

d0 d1 d2

Host Addressing– Controller

– Target

– LUN

c0 t0 d0

SCSI - Pros and Cons

Pros:– Fast transfer speeds, up to 320

megabytes per second

– Reliable, durable components

– Can connect many devices with a single bus, more than just HDs

– SCSI host cards can be put in almost any system

– Full backwards compatibility

Cons:– Configuration and setup

specific to one computer

– Unlike IDE, few BIOS support the standard

– Overwhelming number of variations in the standard, hardware, and connectors

– No common software interfaces and protocol

Comparison IDE/ATA vs SCSI

Feature IDE/ATA SCSI

Connectivity Market Internal Storage Internal and External Storage

Speed (MB/sec) 100/133/150 320

Hot Pluggable No Yes

Expandability Easier to set up Very good but veryexpensive to set up

Cost/Performance Good High cost/Fasttransfer speed

Physical Components – Host with External Storage

Fibre Channel

Storage Arrays

AppsDBMS Mgmt Utils

File System

Multipathing Software

Device Drivers

HBA HBA HBA

External Storage Interfaces – A Comparison

SCSI– Limited distance

– Limited device count

– Usually limited to single initiator

– Single-ported drives

Fibre Channel– Greater distance

– High device count in SANs

– Multiple initiators

– Dual-ported drives

Fibre Channel Connectivity

Switches Storage

Module Summary

The physical components of a networked storage environment.

The logical components (communication protocols) of a networked storage environment.

What are the key physical connectivity components of a small systems environment?

What are the key physical connectivity components of networked storage computing environments?

What are the key logical connectivity protocols found in all computing environments?

Physical DisksPhysical Disks

Module 2.3

Physical Disks

After completing this module, you will be able to:

Describe the major physical components of a disk drive and their function

Define the logical constructs of a physical disk

Describe the access characteristics for disk drives and their performance implications

Describe the logical partitioning of physical drives

Lesson: Disk Drive Components

Upon completion of this lesson, you will be able to:

Describe the physical components of a disk drive

Describe the physical structure of a disk drive platter

Discuss how the geometry of a disk impacts how data is recorded on a platter

Differentiate between the logical organization of data and the physical organization on a disk drive

Disk Drive Components: Platters

0011010011101010101000110100111010101010

10110101011010101010

01010100111010101010

Disk Drive Components: Spindle

Spindle

Platters

Disk Drive Components: Read/Write Heads

Disk Drive Components: Actuator

Actuator

Spindle

Physical Disk Structures: Actuator Arm Assembly

Actuator

R/W Head

Disk Drive Components: Controller

Bottom View of Disk Drive

Controller

Interface

Power Connector

Physical Disk Structures: Sectors and Tracks

Sector

Platter

Platter Geometry and Zoned-Bit Recording

Platter Without Zones

Sector

Platter With Zones

Physical Disk Structures: Cylinders

Cylinder

Tracks, Cylinders and Sectors

Logical Block Addressing

Physical Address = CHS Logical Block Address = Block #

Sector

CylinderHead

Block 0

Block 16

Block 32

Block 48

Block 8

(lower surface)

What the Host Sees

One Logical VolumeMultiple Logical Volumes

Lesson Summary

Key points covered in this lesson:

Physical drives are made up of:– HDA

Platters connected via a spindle Read/write heads which are positioned by an actuator

– Controller Controls power, communication, positioning, and optimization

Data is structured on a drive using tracks, sectors, and cylinders

The geometry of a disk impacts how data is recorded on a platter

Lesson: Disk Drive Performance

Upon completion of this lesson, you will be able to:

Describe the factors that impact the performance of a drive

Describe how drive reliability is measured

Disk Drive Performance: Positioning

Seek time is the time for read/write heads to move between tracks

Seek time specifications include:

– Full stroke

– Average

– Track-to-track

Disk Drive Performance: Rotational Speed/Latency

Disk Drive Performance: Command Queuing

Request 1

Request 2

Request 3

Request 4

Request 1

Request 2

Request 3

Request 4

Without Command Queuing

With Command Queuing

Disk Drive Performance: Data Transfer Rate

InterfaceInterface BufferBufferHBAHBA

Disk Drive

Internal transfer rate measured here

External transfer rate measured here

Drive Reliability: MTBF

Mean Time Between Failure

Amount of time that one can anticipate a device to work before an incapacitating malfunction occurs– Based on averages

– Measured in hours

Determined by artificially aging the product

Lesson Summary

Drive performance is impacted by a number of factors including:– Seek time

– Rotational latency

– Command queuing

– Data transfer rate

Drive reliability is measured using MTBF

Module Summary

Physical drives are made up of a number of components– HDA – houses the platters, spindles, actuator assemblies (which

include the actuator and the read/write heads)

– Controller - Controls power, communication, positioning, and optimization

Data is structured on a drive using tracks, sectors, and cylinders

Drive performance is impacted by seek time, rotational latency, command queuing, and data transfer rate

Describe the purpose of the actuator, the read/write head, and the controller on a drive.

What is the difference between a track, a sector, and a cylinder?

Why is zoned-bit recording used?

What is the difference between seek time and rotational latency?

What is the difference between internal and external data transfer rates?

What purpose does the MTBF specification serve?

RAID ArraysRAID Arrays

Module 2.4

RAID Arrays

Describe what RAID is and the needs it addresses

Describe the concepts upon which RAID is built

Compare and contrast common RAID levels

Recommend the use of the common RAID levels based on performance and availability considerations

RAID - Redundant Array of Independent Disks

RAIDController

RAID Array

RAID Components

RAIDController

Logical Array

Physical Array

RAID Array

Data Organization: Strips and Stripes

Stripe 1Stripe 2Stripe 3

Strips

RAID Performance: Striping

Logical Array

LUNRAIDController

RAIDController

RAID Redundancy: Mirroring

RAID Array

Mirrored Disk

RAIDController

RAID Redundancy: Parity

Parity Disk

0 1 2 3

8 9 10 114 5 6 7

RAIDController

Parity Calculation

Parity

5 + 3 + 4 + 2 = 14

The middle drive fails:

5 + 3 + ? + 2 = 14

? = 14 – 5 – 3 – 2

RAID Array

RAID Levels

0 Striped Array with No Fault Tolerance

1 Disk Mirroring

3 Parallel Access Array with Dedicated Parity Disk

4 Striped Array with Independent Disks and a Dedicated Parity Disk

5 Striped Array with Independent Disks and Distributed Parity

Combinations of levels (I.e., 1 + 0, 0 + 1, etc.)

RAID 0 – Striped Array with no Fault Tolerance

RAIDController

RAIDControllerBlock 4Block 4 Block 4Block 4Block 3Block 3 Block 3Block 3Block 2Block 2 Block 2Block 2Block 1Block 1 Block 1Block 1Block 0Block 0 Block 0Block 0

RAID 1 – Disk Mirroring

RAIDController

RAIDControllerBlock 1Block 1 Block 1Block 1Block 1Block 1Block 0Block 0 Block 0Block 0Block 0Block 0

RAID 0+1 – Striping and Mirroring

RAIDController

RAIDControllerBlock 3Block 3 Block 3Block 3Block 3Block 3Block 2Block 2 Block 2Block 2Block 2Block 2Block 1Block 1 Block 1Block 1Block 1Block 1Block 0Block 0 Block 0Block 0Block 0Block 0

RAID 1+0 – Mirroring and Striping

RAIDController

RAIDControllerBlock 3Block 3 Block 3Block 3Block 3Block 3Block 2Block 2 Block 2Block 2Block 2Block 2Block 1Block 1 Block 1Block 1Block 1Block 1Block 0Block 0 Block 0Block 0Block 0Block 0

RAID 0+1 vs. RAID 1+0

Benefits are identical under normal operations.

Rebuild operations are very different.– RAID 1+0 uses a mirrored pair – only 1 disk is rebuilt if a disk fails

– RAID 0+1 if a single drive fails, the entire stripe is faulted RAID is 0+1 is a poorer solution and is less common

RAID 3 - Parallel Transfer with Dedicated Parity Disk

RAIDController

Block 1Block 1

Block 2Block 2

Block 3Block 3

P 0 1 2 3

Block 0Block 0Block 3Block 3Block 2Block 2Block 1Block 1Block 0Block 0

ParityGenerated

RAID 4 - Striping with Dedicated Parity Disk

RAIDController

P 0 1 2 3

Block 0Block 0

Block 4Block 4

Block 1Block 1

Block 5Block 5

Block 2Block 2

Block 6Block 6

Block 3Block 3

Block 7Block 7

P 0 1 2 3P 0 1 2 3

P 4 5 6 7P 4 5 6 7

ParityGenerated

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Block 0Block 0

P 0 1 2 3P 0 1 2 3

RAID 5 - Independent Disks with Distributed Parity

Block 7Block 7

RAIDController

P 0 1 2 3

Block 0Block 4Block 0

Block 1Block 1

Block 5Block 5

Block 2Block 2

Block 6Block 6

Block 3Block 3

ParityGenerated

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Block 4Block 4

P 4 5 6 7P 4 5 6 7P 4 5 6 7P 4 5 6 7

Block 4Block 4

P 4 5 6 7

Block 4ParityGenerated

RAID Implementations

Hardware (usually a specialized disk controller card)– Controls all drives that are attached it

– Performs all RAID-related functions including volume management

– Array(s) appear to the host operating system as a regular disk drive

– Dedicated cache to improve performance

– Generally provides some type of administrative software

Software – Generally runs as part of the operating system

– Volume management and performed by the server

– Provides more flexibility for hardware, which can reduce the cost

– Performance is dependent on CPU load & server performance

– Has limited functionality

Hot Spares

RAIDController

Hot Swap

RAIDController

Module Summary

What RAID is and the needs it addresses

The concepts upon which RAID is built

The difference between RAID levels

What is a RAID array?

What benefits do RAID arrays provide?

What methods can be used to provide higher data availability in a RAID array?

What is the primary difference between RAID 3 and RAID 5?

Why might you use a combined RAID level, such as RAID 1+0 or 0+1?

Disk Storage SystemsDisk Storage Systems

Module 2.5

Disk Storage Systems

Describe the components of an intelligent storage system

Describe the configuration of a logical disk

Discuss the methods employed to ensure that a host can access a storage volume

Discuss back end volume protection

Discuss front end host configuration

Describe the I/O flow from the back end to the physical disks

Lesson: Intelligent Storage System Overview

After completing this lesson, you will be able to:

List the benefits of intelligent storage systems

Compare and contrast integrated and modular approaches to intelligent storage systems

Describe the I/O flow through the storage system

Describe the logical elements of an intelligent storage system

What is an Intelligent Storage System?

A disk storage system which distributes data over several devices and manages access to that data.

When implemented properly, it provides the following benefits over individual storage devices:– Increased capacity

– Improved performance

– Easier data management

– Better data availability

– More robust backup/restore capabilities

– Improved flexibility and scalability

Monolithic (Integrated) Storage Systems

Monolithic

FC Ports

Port Processors

RAID Controllers

Modular Storage Systems

Servers

Disk Modules

Control Modulewith Disks

FC Switches

Modular

Host Interface

Controller AController A

Host Interface

Controller BController B

Elements in an Intelligent Storage System

Intelligent Storage System

CacheCache

Front End Back End

Physical Disks

Host Connectivity

Intelligent Storage System: Front End

Note: Include redundancy in the channels to and from the ports.

Host Connectivity

Controllers

Front End Back End

Physical Disks

Front End Command Queuing

Request 1

Request 2

Request 3

Request 4

Request 1

Request 2

Request 3

Request 4

Without Command Queuing

With Command Queuing

Intelligent Storage System: Cache

Host Connectivity

Front End Back End

Physical Disks

Intelligent Storage System: Back End

Host Connectivity

PortsControllers

Front End Back End

Physical Disks

Intelligent Storage System: Physical Disks

Host Connectivity

Front End Back End

Physical Disks

I/O Example: Read RequestsI/O Example: Read Requests

Host Connectivity

Front End Back End

Physical Disks

I/O Example: Write Requests

Host Connectivity

Front End Back End

Physical Disks

What the Host Sees

LUN 0LUN 1LUN 2

Back EndPhysical Disks

The Host and Logical Device Names

HostVolumeManager

/dev/rdsk/c1t1d0

/dev/rdsk/c1t1d1

\\.\PhysicalDrive0

VolumeManager

LUN 0LUN 1LUN 2

Back EndPhysical Disks

Disk Organization in a Storage System

Back End Physical Disks

Lesson Summary

An intelligent disk storage system:– Distributes data over several devices and manages access to that

– Has a front end, cache, a back end, and physical disks.

– Use the virtual disks to provide optimal performance and capacity.

– Individual disks within a RAID set can be divided into logical units.

– The same concept can be applied to entire RAID sets.

Lesson: Cache – A Closer Look

After completing this lesson, you will be able to:

Define cache

Distinguish between multipurpose cache and configurable cache

Describe cache hits and misses

Describe algorithms to manage cache

Trace the I/O flow from the cache to the back end to the physical disks

What is Cache in a Storage System

A memory space used by a disk storage system to reduce the time required to read data/write data. It is usually made from very fast memory

CacheRead

RequestWrite

Request

Acknowledgment

How Cache is Structured

Data Store

Tag RAM

ReadRequest

Read Cache ‘Hits’ and ‘Misses’

CacheRead

Request

Data found in cache = ‘Hit’

No data found = ‘Miss’

Algorithms Used to Manage Read Cache

Least Recently Used (LRU) – Determines which items are

accessed frequently/infrequently

– Discards least recently used data

Read Ahead (pre-fetch)– Accesses data sequentially and puts

it into cache before it is requested

– May assume that data recently accessed will not be needed again.

New Data

Oldest Data

WriteRequest

Write Algorithms

WriteRequest

Write-through Cache

Write-back

Acknowledgement

Write Cache: Performance

Manage peak I/O requests “bursts” through flushing– Least-recently used pages are flushed from cache to the drives

For maximum performance:– Provide headroom in write cache for I/O bursts

Coalesce small host writes into larger disk writes– Improve sequentiality at the disk

Lesson Summary

Cache is a memory space used by a disk storage system to reduce the time required to read data/write data.

It can speed up both read and write operations.

Cache read algorithms include:– Least Recently Used (LRU)

– Read Ahead (pre-fetch)

Cache write algorithms include:– Write-through

– Write-back

Module Summary

An intelligent disk storage system distributes data over several devices and manages access to that data.

Monolithic storage systems are generally aimed at the enterprise level, centralizing data in a powerful system with hundreds of drives.

Modular storage systems provide storage to a smaller number of (typically) Windows or Unix servers than larger integrated storage systems.

Cache is an important part of intelligent disk storage systems as it can be used to improve performance.

What are the parts of an Intelligent Disk Subsystem?

What is the difference between a monolithic and a modular array?

What is the difference between cache hit and a cache miss?

What is the difference between Least Recently Used and Read Ahead cache?

What is the difference between Write-through and Write-back cache?

Apply Your Knowledge

Upon completion of this case study, you will be able to:

Describe the basic architecture of the CLARiiON modular storage array.

Describe the basic architecture of the Symmetrix integrated storage array.

CLARiiON CX3-80 Architecture

Power supply

Fan FanFan

Up to 480 drives max per storage system (CX3-80)

4Gb/s LCC

UltraScaleStorage Processor

Fibre Channel

Mirrored cache

Fibre Channel

Mirrored cache

FCFC FC FCFC

2/4 Gb/s Fibre Channel Back End

1/2/4 Gb/s Fibre Channel Front End

CLARiiON Messaging Interface (CMI)

Multi-Lane PCI-Express bridge link

Assigning CLARiiON LUNs to Hosts

CLARiiON disks are grouped into RAID Groups– Disks from any enclosure may be used in a RAID Group

– All disks in a RAID Group must be either Fibre Channel or ATA

– A RAID Group is the ‘RAID set’ discussed earlier

– A RAID Group may be a single disk, or RAID Level 0, 1, 1/0, 3 or 5

The RAID Group is then partitioned into LUNs– All LUNs in a RAID Group will be the same RAID Level

The LUNs are then made accessible to hosts– CLARiiON-resident software ensures that LUNs are seen only by the

hosts that own them

EMC Symmetrix DMX Array

Direct Matrix Interconnect

Dynamic Global Memory

Enginuity Operating Environment

Processing Power

Flexible Back-End Configurations

Fault-tolerant Design

Symmetrix DMX Series Direct Matrix Architecture

Symmetrix DMX: Dual-ported Disk and Redundant Directors

Disk Director 1 Disk Director 16

P = Primary Connection to DriveS= Secondary Connection for Redundancy

Configuring Symmetrix Logical Volumes (SLV)

Initial configuration of Symmetrix Logical Volumes is done via the Symmetrix Service Processor and the SymmWin interface/application– A configuration file (IMPL.BIN) is created and loaded on to the array

Subsequent configuration changes can be performed online using EMC ControlCenter (GUI) or by using Solutions Enabler (CLI)

Physical Disk

Physical Disk Symmetrix Service Processor

Running SymmWin Application

RAID1 – Symmetrix Logical Volume

RAID1 SLV– Data is written to two hyper volumes on two different physical disks

which are accessed via two different disk directors

Host is unaware of data protection being applied

Physical Drive

LV 04B M2

Different Disk Director

Physical Drive

LV 04B M1

Disk Director

Logical Volume 04B

Host AddressTarget = 1LUN = 0

Volumes

Data Protection

Mirroring (RAID 1) – Highest performance, availability and functionality

– Two hyper mirrors form one Symmetrix Logical Volume located on separate physical drives

Parity RAID (Not available on DMX3)– 3 +1 (3 data and 1 parity volume) or 7 +1 (7 data and 1 parity volume)

Raid 5 Striped RAID volumes– Data blocks are striped horizontally across the members of the RAID group

( 4 or 8 member group); parity blocks rotate among the group members

RAID 10 Mirrored Striped Mainframe Volumes

Dynamic Sparing

SRDF (Symmetrix Remote Data Facility)– Mirror of Symmetrix logical Volume maintained in a separate Symmetrix

Assigning Symmetrix Logical Volumes to Hosts

Configure Symmetrix Logical Volumes

Map Symmetrix Logical Volumes to Front-end ports– Performed via EMC ControlCenter or Solutions Enabler

Make Symmetrix Logical Volumes accessible to hosts– SAN Environment

Zone Hosts to Front-end ports Perform LUN Masking

Can be performed via EMC ControlCenter or Solutions Enabler LUN Masking information is maintained on the Symmetrix in the VCM Database

(VCMDB)■ LUN Masking information is also flashed to all the front-end directors

Data FlowData Flow

Exercise

Q: Architecture ExerciseIdentify the components of a data storage environment:

Q: Data Flow Exercise – Write Cache HitIn this example, the storage system uses write-back cache.

Identify the operations performed when writing data to disk, then put them in the correct order:

Q: Data Flow Exercise – Read Cache HitThe storage system uses write-back cache. Identify the operations performed when reading data from disk. They are presented in the correct order (1 to 4):

Q: Data Flow Exercise – Read Cache MissThe storage system uses write-back cache. Identify the operations

performed when reading data, then put them in the correct order:

section 2 – storage systems architecture

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