TopicRAID

Redundant Array Independent Disk

Raid History

S.L.E.D.Single Large Expensive Disks

Single drive used to store data. Capacity: good

Problem: Data cant be read and write quickly If SLED fail then all data loss.

Raid

RAID Overview:The heart of the RAID storage system is

controller card. The task of the controller card is toManage Individual Hard Disk DrivesProvide a Logical Array ConfigurationPerform Redundant or Fault Tolerant Operations

RAID Controller:

Raid

Patented 1987

Built in 1989

Updated several times

That’s all I could find. Until I made these updates….

RAID: Redundant Arrays of Independent DisksHence, the I in RAID now stands for“independent”instead of “inexpensive”.RAID:Multiple disk drives provides reliability via redundancy.commonly used to address the performance and reliability issues.

RAID

Mirroring Duplicate every disk Gives good error recovery

Data stripping A method of concatenating multiple drives into one logical storage unit. the data is split into different parts. Parity: Splitting data onto blocks with the help of XOR operation

Redundancy:

What exactly is a RAID?RAID is basically drives stacked on top of each other like a cake with layers that can share their data together.

RAID 0 – Data StripingRAID 1 - MirroringRAID 2 – Hamming CodeRAID 3 – Single Check Disk per GroupThere are more lavels like 4,5,6 10.

Features Of RAID Levels:

RAID Level 0:

RAID level 0:Simplest RAID implementation

Includes striping but no redundancy Highest-performanceHigh risk of data loss

Multiple drives involvedCould lose all data in array with one drive failure

Block level stripingNotes:

If two different I/O requests are pending for two different blocks of data, in all likelihood the two blocks are located in two different disks, then the requests can be issued in parallel

If a single request is spread across multiple logically contiguous strips, the request can be handled in parallel.

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R̀ecommended ApplicationsVideo Production and Editing Image Editing Pre-Press Applications gaming systems.

Disadvantage:Relaibility problem –no mirroring or parity bits.

Advantage: speed enhancement

Maximum utilization of physical drive storage capacity, because no room is taken for redundant data or data-parity storage

RAID Level1:

RAID Level 1:

Highest level of redundancyEach drive has a mirrored copy in arrayNo striping at this level

Improves read performance over single disks because multiple disks can be read at once

Slower write performance because two disks must be accessed for each modified data item to maintain mirroring

High storage overheadOnly half array stores unique data

Most suitable where reliability is primary concern.

Performance: If we use independent disk controllers for each disk, then we can increase the read or write speeds by doing operations in parallel.

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Application:AccountingFinancial

Advantage: Provide best Performance

Provide Fault tolerance

Disadvantage: High cost.

Requires twice the disk space

RAID Level 2:

RAID Level 2:Implements redundancy via striping

Striped at bit levelUses Hamming ECC to check data

integrityECC data stored on separate drive

Significant overhead in storage and performance .

RAID 2 is the only RAID level that can repair errors, the other RAID levels can only detect them

Read – all disks are simultaneously accessed

Write - all disks are simultaneously accessed

Write penalty – computation of the Hamming ECC

Used when many disk errors occur, but given the high reliability of individual disks, rarely used.

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Advantages:Random Read Performance= FairSequential Read Performance= Very GoodSequential Write Performance= Very Good

Disadvantages:

Random Write Performance= PoorRequires a complex controllerHigh overhead for check disksNot used in modern systems

RAID Level 3:

RAID Level 3:Also stripes at the byte levelUses XOR to calculate parity for ECC

Much simpler than Hamming ECCRequires only one disk for parity

information regardless of the size of the array

Cannot determine which bit contains error, but this information can be gathered easily by inspecting the array for a failed diskHigh transfer rates, but only one request serviced at a time

Cont………!!!In the case of a disk failure,

All data are available missing data can be calculated from the parity bitWrite: just maintain the parity such that later it can be regenerated.Failed disk to be replaced and the data regenerated

BYTE level striping and XOR ECC allows for one check disk: lowest overhead possibleExample… A:0101 XOR B:0011 = Check:0110A is gone?

B:0011 XOR Check:0110 = 0101 (A)

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