Raid Technology & Troubleshooting

8/9/2019 Raid Technology & Troubleshooting

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INTRODUCTION TO RAID

The redundant array of Independent Disks (RAID) architectureintegrates multiple disk drives into an array to provide higher performance, capacity and reliability as compared to a single largedrive.

This architecture is also known as Redundant array of InexpensiveDisks and comprises various implementations called RAID levels.


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RAID levels

RAID 0

RAID 1

RAID 2

RAID 3

RAID 4

RAID 5


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RAID-0

RAID 0 represents a striped disk array that does not store parityinformation. As a result, it does not provide data redundancy.

RAID 0 represents a striped array that does not store parity information. Asa result, it does not provide data redundancy.

RAID 0 arrays with large stripes are beneficial for multi-user environments

and RAID 0 arrays with small stripes are used in single-user systems thatrequire access to long and sequential records.


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Advantages and Disadvantages of RAID-0

Advantages-

High performance

Handles data redundancy

Disadvantages

Lack of data redundancy

Loss of even one disk in the array result in the loss of the entire volume.


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RAID-1

RAID 1 provides data redundancy by implementing disk mirroring. This isdone by using a pair or group of identical primary and secondary hard disks.

In disk mirroring, data is written concurrently to both the primary andsecondary hard disks.

If the primary hard disk fails the secondary or mirrored hard disk is used

until the primary hard disk is restored. This restoration takes place by usingthe data on the mirrored disk.


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Disk Mirroring

Disk Mirroring offers complete redundancy of data but it is expensive. This

is because twice the disk capacity is required to store the same volume of

data.

Disk mirroring offers better read performance as data can be read from the

hard disks concurrently.

Write operations are slow because the same data is written to both the hard

disks.


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Disk Duplexing

The fault tolerance of disk mirroring can be enhanced by implementing disk

duplexing. Disk mirroring consists of a single disk controller for the mirrored

hard disks in the array.

This makes the data irrecoverable if the controller fails. Disk duplexing

overcomes this problem by utilizing a separate disk controller for the

mirrored disks.

Disk duplexing results in increased costs.


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RAID-3

RAID 3 requires at least three hard disks in which two are striped disks. Thethird hard disk is used to store parity data.

If one striped disk fails, the data can be restored from the from the paritydata.

The write operations in RAID 3 are slow as parity data has to be stored on

the parity disk. The read performance is high because the parity disk is used only when the

data cannot be read from the striped drives.


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Use of RAID-3

The capacity of a RAID 3 array can be increased by adding more hard

disks. This involves an effective increase in the size of the parity disk, which

should be equal to or greater than the size of the individual striped disks.

RAID 3 is most suited for applications that access data sequentially.

It should not be used for intensive and transactional database applications.


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RAID 5

RAID 5 requires at least three hard disks. The parity data is distributedacross all disk drives. As a result, a portion of the total disk space isdedicated for storing parity data.

This portion generally amounts to the size of one hard disk in the array.

RAID 5 enhances read performance as data can be read from all the disk

concurrently. Write operations are slow because parity data is recorded in the parity

stripes.


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Benefits of RAID-5

When a hard disk fails in a RAID 5 array, it can be restored from the parity

information stored on the other hard disks.

Read write operations can continue on these hard disks while the data is

being restored from the information.

This lowers the performance but greatly increases the availability of data in

the array.

RAID 5 very useful for storing critical data.


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RAID 2 and RAID 4

RAID 2 is rarely implemented because it is designed for disk drives

that do not have built-in Error-Correction Code. At present, ECC is

embedded in most disk drives.

RAID 4 is seldom used because it does not support multiple

concurrent write operations.


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Hybrid RAID level

RAID integrates multiple disk drives into an array to provide

optimum performance, capacity and reliability.

By combining different RAID implementations, various hybrid RAID

levels can be created.


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Hybrid RAID level types

1+5

5+1

0+1

1+0

0+5

5+0


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1+5 and 5+1

It is a combination of disk mirroring and block striping with

distributed parity.

A RAID 1+5 array is created when a number of mirrored sets are

striped with parity.

On the other hand, a RAID 5+1 array is a mirrored configuration of

complete RAID-5 arrays.


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0+1 and 1+0

The 0+1 and 1+0 hybrid levels use mirrored sets in conjunction with thestriped sets without parity.

RAID 0+1 is created by mirroring two striped sets, where as a RAID 1+0array is created by striping multiple mirrored pairs.

RAID 0+1 and 1+0 combine the advantages of striping and mirroring to

obtain large arrays that can provide high performance and efficient faulttolerance.


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ZCR Cards

RAID levels are implemented using Zero Channel RAID (ZCR) cards. A

ZCR card is a compact implementation of a RAID controller card and

functions with an onboard SCSI chip. This allows for more space on the

card. The compact design of a ZCR card reduces the cost of RAID controller

card. ZCR provides the flexibility to add a card at lower cost that one-

channel and two channel RAID cards.


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RADIOS

To use a ZCR card, you need a RAID I/O system (RAIDOS) chip embedded

on the motherboard.

This chip is used to enable an embedded I/O controller. The I/O controller

and I/O processor are the main hardware components of a RAID system.

The RAIDIOS circuit allows the I/O processor to configure the controller and

service its interrupts.


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Software RAID Vs Hardware RAID

In software RAID, the existing CPU cycles are used. This reduces theprocessing capacity of a server.

In hardware RAID, dedicated circuitry and embedded software are used.

This helps to provide RAID functionality without increasing the load on theCPU.

In both hardware and software RAID, software is used to calculate the arrayallotments and data flow.


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IOC RAID

The common types of hardware RAID implementations are I/O Controller(IOC) and I/O Processor (IOP). In IOC RAID, processors on disk controllerare used to handle RAID functionality.

IOC based RAID systems can run processes in the background and areindependent of operating systems running on servers.

The IOC performance is constrained by the controllers processor speedand memory bandwidth.


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IOP RAID

In IOP, various RAID processes are performed by using a special processor

or subsystem in the array. IOP is a commonly implemented hardware RAID

option and is used in servers and Host Bus Adaptors (HBA).

IOP RAID systems have a dedicated processors and firmware that helps inefficient handling of RAID tasks, such as RAID level migration. However,

IOP is an expensive RAID implementation.


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IOC/IOP RAID Comparison

When compared to IOP based RAID systems, IOC based RAIDsystems are cheaper. However, they lack in performance andflexibility.

For example, IOC does not provide optimum performance whenused with applications such as databases.

Moreover, IOP handles more RAID-management jobs as comparedto IOC.


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Software RAID: Types

Software RAID implementations are of two types:

RAID based on drivers

RAID based on OS


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RAID-System based on drivers

A driver based RAID system is implemented in disk controller

drivers. Driver based RAID is independent of the operating system

but is dependent on the version of the hard disk controller for RAID

functionality.


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RAID-System based on OS

A RAID system that is based on the OS is implemented by including a RAID

engine in the OS.

The RAID system is independent of the hard disk controller but is

dependent on the OS. For proper functioning, both types of Software RAID are dependent on the

systems processor and memory resources.


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Software RAID: Issues

Software-based RAID is cheaper that hardware based RAID.

However, software RAID systems have certain disadvantages when

compared to hardware RAID systems.


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Disadvantages include limited performance as less reliability. SoftwareRAID has components that depend on the OS. This results portability issue.

Software RAID solutions are implemented as kernel mode components.Moreover, in OS such as Linux, the RAID solutions are included in Kernel.Usually, kernel mode components require a scheduler to preempt their

operations when their time quantum expires or a higher priority task isscheduled.


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A kernel mode RAID engine needs to share processor time with other

kernel mode components and applications.

This may lead to an increase in the CPU load, thereby affecting the engines

performance. An increase in the CPU load may be caused due to factorssuch as network traffic, application servers, operating system architecture

and components, and increased I/O processing to and from the secondary

storage.


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Hardware RAID advantages

In contrast, hardware RAID offers various advantages. In hardware RAID,

the associated software or firmware is executed on a dedicated processor.

Therefore, hardware RAID does not share the processor with other kernel

mode components and applications.

This provides the advantages of asymmetric multiprocessing such as

distributed task processing.


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Hardware RAID - Advantages

Hardware RAID is not dependent on the operating system of a

server. Moreover, in the case of a RAID hardware malfunction the

server can continue to operate and alert the user about malfunction.

If the server fails, hardware RAID continues to function with the help

of backup modules.


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RAID DISK concepts

Mirrored Drives

Hot Plug Drives

Hot Swap

Hot Spare Drives

Hot Plug Boards


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Mirrored Drives

Most RAID arrays use mirrored drives where all the data on a serveris written simultaneously to two hard disks.

The RAID Controller uses the mirrored drive when one hard disk in aRAID array fails and this ensures that the downtime is kept at aminimum.

The process of a mirrored drive replacing a failed drive is known asfailover.


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Hot Plug Drives

Hot plug drives ensure that a server is always online. A failed drive

in a RAID array can be replaced with hot plug drives.

A hot plug drive can be installed or removed from a RAID array

without interrupting server operations.


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Hot Swap

Hot swap is the ability of a RAID system to recognize a drive that is

inserted while the system is running.

For example, if a drive in a RAID 1 array fails, the malfunctioning

drive can be replaced with a new drive while the system is still

operating.


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Hot Spare Drives

A hot spare drive is connected to a RAID Controller even when other

drives in the RAID array are not functional. When a drive fails, the

controller automatically rebuilds data on the hot spare drive and

disconnects the failed drive.


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Hot Plug Boards

Hot plug boards are used for hard disk failures in a RAID array. A

hot plug board allows compatible hard disk drives and peripherals to

be installed or removed without shutting down the server.

The advantages offered by hot plug board are scalability, high

uptime and minimal down time.

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Raid Technology & Troubleshooting