RAIDRAIDRedundant Array of Redundant Array of Independent DisksIndependent Disks

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What is RAID?What is RAID? RAID configurations used.

RAID configurations used. Performance of each

Performance of each configuration.configuration. Implementations.

Implementations.Way forward with RAID.

Way forward with RAID.

Wha

t is R

AID?

Wha

t is R

AID?

Stands for Redundant Array of

Stands for Redundant Array of

Independent Disks.

Independent Disks.It’s a technology that enables

It’s a technology that enables

greater levels of performance,

greater levels of performance,

reliability and/or large volumes

reliability and/or large volumes

when dealing with data.

when dealing with data.How??How?? By concurrent use of two

By concurrent use of two

or more ‘or more ‘hard disk drives’.

hard disk drives’.How Exactly??

How Exactly?? Mirroring, Mirroring,

Stripping (of data) and Error

Stripping (of data) and Error

correction techniques combined

correction techniques combined

with multiple disk arrays give you

with multiple disk arrays give you

the reliability and performance.

the reliability and performance.

Mean Time Between Failures Mean Time Between Failures (MTBF)(MTBF)

RAID

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RAID

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Commonly used ones:

Commonly used ones:1.1.RAID 0

RAID 02.2.RAID 1RAID 13.3.RAID 5

RAID 54.4.RAID 10RAID 10Other types used…but

Other types used…but rarely: RAID 2,3,4,6,50……

rarely: RAID 2,3,4,6,50……

RAID 0RAID 0a.a. It splits data among two or more disks.It splits data among two or more disks.

b.b. Provides good performance.Provides good performance.

c.c. Lack of data redundancy means there is Lack of data redundancy means there is no fail over support with this no fail over support with this

configuration.configuration.

d.d. In the diagram to the right, the odd In the diagram to the right, the odd blocks are written to disk 0 and the even blocks are written to disk 0 and the even

blocks to disk 1 such that A1, A2, A3, blocks to disk 1 such that A1, A2, A3, A4, … would be the order of blocks read A4, … would be the order of blocks read

if read sequentially from the beginning.if read sequentially from the beginning.

e.e. Used in read only NFS systems and Used in read only NFS systems and gaming systems.gaming systems.

RAID 0 analysisRAID 0 analysisFailure Rate:Failure Rate: MTBF of RAID0 is roughly proportional to the number MTBF of RAID0 is roughly proportional to the number

of disks in the array.of disks in the array. Pr(disk fail) = 5%, then Pr(disk fail) = 5%, then

Pr(atleastonefails) = 1 – Pr(nonefails) = 1 – [1-0.05]Pr(atleastonefails) = 1 – Pr(nonefails) = 1 – [1-0.05]22 = = 9.75%9.75%

Performance:Performance: The fragments are written to their respective disks The fragments are written to their respective disks

simultaneously on the same sector.simultaneously on the same sector. This allows smaller sections of the entire chunk of data to This allows smaller sections of the entire chunk of data to

be read off the drive in parallel, hence good performance.be read off the drive in parallel, hence good performance.

RAID 1RAID 1•RAID1 is ‘data mirroring’.RAID1 is ‘data mirroring’.

•Two copies of the data are held on Two copies of the data are held on two physical disks, and the data is two physical disks, and the data is

always identical.always identical.

• Twice as many disks are required Twice as many disks are required to store the same data when to store the same data when

compared to RAID 0.compared to RAID 0.

•Array continues to operate so long Array continues to operate so long as at least one drive is functioning.as at least one drive is functioning.

RAID

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Failure Rate:Failure Rate: If Pr(disk fail) = 5%, then the

If Pr(disk fail) = 5%, then the

probability of both the drives

probability of both the drives

failing in a 2 disk array is

failing in a 2 disk array is P(both fail) = (0.05)

P(both fail) = (0.05) 2 2 = 0.25%. = 0.25%.

Performance:Performance: If we use independent disk

If we use independent disk controllers for each disk, then

controllers for each disk, then

we can increase the read or

we can increase the read or write speeds by doing

write speeds by doing operations in parallel.

operations in parallel.

RAID 5RAID 5• RAID 5 is an ideal combination RAID 5 is an ideal combination

of good performance, good fault of good performance, good fault tolerance and high capacity and tolerance and high capacity and

storage efficiency.storage efficiency.

• An arrangement of parity and An arrangement of parity and CRC to help rebuilding drive data CRC to help rebuilding drive data

in case of disk failures.in case of disk failures.

• “ “Distributed Parity” is the key Distributed Parity” is the key word here.word here.

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MTBF is slightly better than RAID 0.

MTBF is slightly better than RAID 0.

This is because failure of one disk is

This is because failure of one disk is

not quite a harm. We need more time

not quite a harm. We need more time

if 2 or more disks fail.

if 2 or more disks fail. Performance is also as good as RAID

Performance is also as good as RAID

0, if not better. We can read and write

0, if not better. We can read and write

parallel blocks of data.

parallel blocks of data.One of the drawbacks is that the write

One of the drawbacks is that the write

involves heavy parity calculations by

involves heavy parity calculations by

the RAID controller. Write operations

the RAID controller. Write operations

are slower compared to RAID 0.

are slower compared to RAID 0.

Pretty useful for general purpose uses

Pretty useful for general purpose uses

where ‘read’s’ are more frequent the

where ‘read’s’ are more frequent the

‘write’s’. ‘write’s’.

RAID 10RAID 10a.a. Combines RAID 1 and RAID Combines RAID 1 and RAID

0.0.

b.b. Which means having the Which means having the pleasure of both - good pleasure of both - good

performance and good failover performance and good failover handling.handling.

c.c. Also called ‘Nested RAID’.Also called ‘Nested RAID’.

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Software based RAID:

Software based RAID: Software implementations are

Software implementations are

provided by many Operating

provided by many Operating

Systems. Systems. A software layer sits above the disk

A software layer sits above the disk

device drivers and provides an

device drivers and provides an

abstraction layer between the logical

abstraction layer between the logical

drives(RAIDs) and physical drives.

drives(RAIDs) and physical drives.

Server's processor is used to run the

Server's processor is used to run the

RAID software.

RAID software.Used for simpler configurations like

Used for simpler configurations like

RAID0 and RAID1.

RAID0 and RAID1.

Implementations (contd…)Implementations (contd…)Hardware based RAID:

Hardware based RAID:

A hardware implementation of RAID

A hardware implementation of RAID

requires at least a special-purpose

requires at least a special-purpose

RAID controller.

RAID controller.

On a desktop system this may be built

On a desktop system this may be built

into the motherboard.

into the motherboard.

Processor is not used for RAID

Processor is not used for RAID

calculations as a separate controller

calculations as a separate controller

present.present.A PCI-bus-based, IDE/ATA hard disk

RAIDcontroller, supporting levels 0, 1, and 01.

What’s happening present day?What’s happening present day?RAID 6:RAID 6:

a.a.It is seen as the best way to It is seen as the best way to guarantee data integrity as it uses guarantee data integrity as it uses

double parity.double parity.b.b.Lesser MTBF compared to Lesser MTBF compared to

RAID5.RAID5.c.c.It has a drawback though of It has a drawback though of

longer write time.longer write time.

Road

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ad…

Road

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The expanded use of RAID-6 and other

The expanded use of RAID-6 and other

dual-parity schemes is a virtual certainty.

dual-parity schemes is a virtual certainty.

RAID vendors to support "fast rebuild"

RAID vendors to support "fast rebuild"

features that can restore hundreds of

features that can restore hundreds of

gigabytes in just an hour or so!!

gigabytes in just an hour or so!!

Striping(of data) would extend across

Striping(of data) would extend across

RAID groups -- not just across drives

RAID groups -- not just across drives

within a group.within a group. Improved disk diagnostic features should

Improved disk diagnostic features should

offer more reliable predictions of

offer more reliable predictions of

impending drive failures, allowing the

impending drive failures, allowing the

rebuild process to begin before an actual

rebuild process to begin before an actual

fault occurs.fault occurs.Hot Spares!!

Hot Spares!!

Refe

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http://searchstorage.techtarget.com/generic/0,295582,sid5_gci1196310,

00.htmlhttp://www.lascon.co.uk/

d008005.htmhttp://en.wikipedia.org/wiki/RAID_0#RAID_0

http://en.wikipedia.org/wiki/RAID Paper - A Case for

Paper - A Case for Redundant Arrays of

Redundant Arrays of Inexpensive Disks

Inexpensive Disks (RAID) - David A

(RAID) - David A Patterson, Garth Gibson,

Patterson, Garth Gibson, and Randy H Katz

and Randy H Katz